Assaying: In Three Parts ...

Charles Howard Aaron, Dewey & Co · 1884 · Historical

Assaying: In Three Parts ... by Charles Howard Aaron, Dewey & Co (1884). Full text and reference in the Mountain Man Mining Library.

Public-domain full text preserved in the Mountain Man Mining Library. Original source: archive.org.

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Assaying

In

Three Parts.

Part ist. — Gold and Silver Ores; Part 2d. — Gold and Silver Bullion; Part 3d. — Lead, Copper, Tin, Mercury.

By C H. AARON, Metallurgist,

author op Testing and Working Silver Orbs," Leaching Gold and Silver Ores,

Part 1.

D E W Ey"'C O.,

Proprietors Mining And Scientific Press,

SAN FRANCISCO, CAL., 1884. [Copyrighted.]

Miss Eugenia Schenk

San Francisco, Cal.,

The Steadfast Friend Of Truth And Progress — An Honest Paper,

This Book Is Respectfully Dedicated By The Author.

Contents.

Pbeface 7

Introduction 9

Implements 12

Assay Balance 21

Materials 30

The Assay Office 36

Preparation of the Ore 38

Weighing the Chairge 41

Mixing and Charging 42

Assay of Litharge 43

Systems of the Crucible Assay 44

Preliminary Assay 46

Dressing the Crucible Assays 48

Examples of Dressing 56

The Melting in Crucibles. 58

Scorification 62

Cupellation 66

Weighing the Bead 71

Parting 74

Calculating the Assay 1 . . 78

Assay of Ore Containing Coarse Metal 81

Assay of Boasted Ore for Solubility 84

To Assay a Cupel 84

Assay by Amalgamation 85

To Find the Value of a Specimen 86

Tests for Ores 90

A Few Special Minerals 95

Solubility of Metals 98

Substitutes and Expedients 100

Assay Tables 102,104,106

Preface.

That faults will be found in this book is beyond a peradventure. I shall find some myself after the printer has immortalized them, and my friends (and enemies) will discover others. But what can be expected of so humble an individual, when a celebrated scientist gives the sanction of his illustrious name to such a statement as this, " by weighing in this manner, by difference, the error's arising from inequality in the equilibrium, or length of the arma, are eliminated." — Croohea* edition of MitchelVs Manual of Assaying, 1868, The italics are mine.

Inequality in equilibrium looks paradoxical, but letting that pass as a harmless error, if error it be, the statement that weighing by difference, in the manner directed, can eliminate an error caused by inequality in the length of the arms of a balance, is itself an error so gross as to suggest the idea that Professor Crookes' may not have given personal attention to the article on weighing in the work quoted.

The present work was written nearly two years ago, but other engagements prevented its publication at that

Preface.

time. This part has been carefully revised. For matter I have drawn to some extent on Mitchell's Original Manual, but mainly on my own experience, observation, and that stock of imbibed knowledge which every one gains more or less, without knowing exactly whence nor how.

I lay little other claim to originality than such as may be based on manner, rather than on matter, and I ask no greater meed of praise or pecuniary reward than has been accorded to my other eflforts in a similar direction.

San Francisco, Sept 188S, C. H. Aaron.

Introduction.

An dasay is an operation performed on a known quantity of matter, for the purpose of ascertaining how much of a certain substance that quantity of that kind of matter contains. A test is an operation performed on an indefinite quantity of matter, in order to ascertain the character of that matter, or to determine the presence or absence of some particular substance. Thus, to test a piece of rock for silver, is merely to try whether it contains silver or not. To assay it for silver, is to find out how much silver is contained in a weighed quantity, as an ounce or a half ounce, and thence, by calculation, in a ton of such rock. Testing may often precede assaying, with great advantage, because it enables us to know what kind of matter we have to deal with, and thus to adapt our method so as to insure a correct assay ; or it may show us that an assay would be useless, because of the absence of the substance sought, or of its presence in such minute quantity only as to be practically worthless.

To avoid confusion hereafter it is as well to mention that, not only the act of assaying, but also the definite quantity of matter operated on, is called an assay the accent being on the first syllable of the word. In the verb to assay, the accent is on the last syllable. A test

(9)

10 Introduction.

is either the act of testing, the thing tested, or the agent by means of which a test is made.

Assays and tests are of two principal classes, the dry, or fire, and the wet, or humid assay or test. In the dry way the substance under examination is usually melted by heat, with the addition of such substances as may be necessary to produce fluidity, and to separate the particular substance sought from the other components. The added substances are called fluxes, reducers, oxidizers, desulphurizers, etc., according to their functions in the operation. In the wet way, the substance, if solid, is acted on by means of liquid solvents, such as acids, etc., which convert it, wholly or in part, into a liquid. By the addition of reagents" to the liquid, the substance sought is separated, or its presence, and in assays its quantity also, is determined by the occurrence of some appreciable phenomenon, such as the production of a precipitate, or of a color. For example, a portion of impure bullion is dissolved in nitric acid, and a solution of common salt is added. The formation of a white precipitate indicates the presence of silver. This is a test.* If the portion of bullion be weighed, and the exact quantity of salt required for the complete precipitation of the silver be also ascertained, the proportion of silver in the bullion is easily deduced. This is an assay. Again, a substance, supposed to contain copper, is dissolved in acid, and ammonia is added ; the production of a blue color indicates that copper is present, and if the portion of the substance be weighed before being dissolved, we can ascertain how

*Thi8 must only be taken as an illustration. Lead or mercory would also give a white precipitate, but in practice they are easily distinguished.

Introduction. 11

much copper it contained, by noting how much cyanide of potassium it takes to destroy the color.

Silver ores are assayed in the dry way. Gold ores by a combination of the dry and the wet way. By ores, is. here meant all mineral, rock, or earth containing the metal sought, in small particles, or in chemical combination. Silver ores very frequently contain gold, and gold rarely occurs quite free from silver. The ore is ground to powder, weighed, and melted in an earthern vessel (cmcible or acorifier) with fluxes, including lead in some form. A glassy mass {slag), and a lump of lead (button) result. The lead contains the precious metal, and is melted on a porous support {cupel) in an oven {muffle), converted into litharge, and absorbed by the support. The precious metals remain in the form of a bead on the support. The two are parted by boiling the bead in nitric acid. The following described implements and material are used, all of which can be obtained from John Taylor & Co., 120 Market Street, or Justinian Caire, 521 Market Street, San Francisco. The prices given are those at present prevailing, but are liable to change.

Assaying Gold

Iron Mortar and Pestle for Pulverizing Ores. — Size and price of each: pint, 75 cents; 1 pint, $1.00; 1 quart, $1.25; i gallon $1.75; 1 gallon, $2.25; gallons, $3.00; 2 gallons, $4.75; 3 gallons, $7.50. It is convenient to have several sizes, but the third or fourth size will answer all purposes. It is also convenient to have a

Circular Grinding Plate and Muller, 21 inches in diameter, Weighing 96 pounds, and made of cast-iron. Price, $11.50. Or a rectangular grinding plate, 28x7 inches, with trunnions and bearings at side, to discharge the ground ore into a pan by tilting the plate. Weight of muUer, 40 pounds; weight of plate, 53 pounds. Price, $7.00.

Buck's Patent Amalgam Mortar. — May replace plate and muUer for grinding. Is also good for the assay by amalgamation. Diameter, 8 J inches; weight, 58 pounds. Price, $7,50.

Wedgewood Mortar. — Price and diameter of each: inches, 60 cents; 4 J inches, 75 cents; 5 inches, 90 cents; 5 J inches, $1.00; 6 inches, $1.25; 6f inches, $1.50; 7J inches, $1.75; 8 inches, $2.25. Chiefly used for powder-

And Silver Ores.

ing fluxes and mixing the assays. One of these is sufficient, and, in case of need, may be dispensed with.

Sieves. — These should be five in number, respectively of 10, 20, 40, 60 and 80 meshes to the running inch. They are formed of brass wire cloth, stretched on circular rims of wood or tin, the latter being preferable. The coarsest sieve is about eight inches in diameter, the finest about five inches, the others intermediate.

Sample Pans. — These are similar to cake tins. There should be two sizes, about 6 and 3 inches in diameter, respectively. Those which are enameled are best.

Spatulas, etc. — Two apothecaries* spatulas, the one about 10, the other about 6 inches long ; and a few large and small common spoons should be provided.

Becker's Pulp Scales. —These scales cost $12.00, will carry several ounces, and are sensitive to onefiftieth of a grain when new. A set of weights from 1 ounce to J grain costs $3.50, or a set from 1,000 grains to 1-100 of a grain, with rider, including all the weights necessary for this scale and the finer assay balance to be described hereafter, may be bought for $18.00. Gramme weights can be had if required.

Crucibles. — Two kinds of assay crucibles are in common use, the French and the Hessian. The former are made of fine clay, and are used a number of times. The most suitable sizes are the numbers 8 and 9, costing $1.25 and $1.50 per dozen. The Hessian pots are triangular,

Assaying Gold

made of sand and clay. There are several other kinds in the market. Covers made of clay cost about 75 cts. per dozen, but for the country it is better to buy number 2 black lead crucible covers, at 10 cents each, as they last very much longer. They can be cut to fit the crucible. Crucible Rack. — This is simply a small portable bench with eight or twelve holes, in which the charged crucibles may be placed in readiness for the furnace, thus avoiding the risk of upsetting, and enabling a number to be carried at once from the mixing table. It can be made by any one who can use carpenters* tools a little.

Muffle and Muffle Furnace. — The muffle is a sort of small oven, of a semi-cylindrical form, made of clay, open at the front end, closed at the back except a narrow slit or two small holes. Some have holes in the sides. It is heated by being surrounded, excepting the front end, by glowing coals in the muffle furnace. In it objects are exposed to heat out of contact with the fuel. The muffle furnace is a sheet-iron cylinder lined with fire tiles, having an opening at the back to connect with a flue or stovepipe, and three openings at the front with doors ; the upper one corresponding with the open end of the muffle, the middle one for introducing fuel under the muffle, the lower one for the admission of air and removal of ashes. Immediately above the lower door is a grate. The open end of the muffle rests in the upper opening, the

And Silver Ores. 15

closed end on a projection of the lining, or, if not extending across the furnace, on a piece of fire-brick, or an assay crucible. The top of the furnace is closed by a movable iron cover. The cover may be suspended by a wire attached to a rope which passes over a pulley above, and counterpoised by a weight, to facilitate removal and replacement, or it may slide to one side on a bracket, which can be bought with the furnace. There is also a round iron pan in which some sand may be placed, and which replaces the cover on occasion, forming a sand-bath convenient for heating and drying purposes. The fuel used is coke or charcoal. Sizes are from 10 to 14 inches inside diameter by 20 to 29 inches high. Weight, from 58 pounds to 214 pounds. Prices, from $12.00 to $22.00. The corresponding mufiies cost from $8.00 to $13.50 per dozen.

The muffle furnace should stand on a solid table, of such a height that the operator may work conveniently in the muffle, standing or sitt ing as preferred. The top of the tabte should be protected by sheet-iron, and large enough to afford room for the assay mould, cupel tray, tongs, etc. Under the table may be a bin for fuel. If the same furnace is used for crucibles, there should be a platform at one side for the operator to stand on.

ScoRiFiER. — A dish or cup formed of clay- There are different sizes, but that most used is 2J inches in diameter, which admits of two being placed in the width of a mint muffle. Price for best, 75 cents per dozen. May be used many times.

Roasting Dishes. — Similar to scorifiers, but shallower, and from four to five inches wide ; as they are used in the muffle, the size should be selected to suit it. $1.25 to $1.50 per dozen.

It)

Assaying Gold

Melting Furnace. — For fusing the assays in crucibles, also for melting bullion. Similar to the muffle furnace, but without the opening for the muffle. Price, $19.00. In a small establishment the muffle furnace is used also for melting, the crucibles being placed by the sides of the muffle, or the muffle being removed for the time. Muffle and melting furnaces may be built of common bricks with fire-brick lining, and suitable iron bands and tops.

Fire Irons. — Comprise a bent and a straight poker, a fire shovel, and scorifier tongs, 29 to 30 inches long, price, $1.00; cupel tongs, 26 inches long, price, $1.00; assay crucible tongs, 30 inches long, price, $1.25. Other varieties of tongs are sometimes used, bjit are not absolutely necessary.

Assay Moulds. — Made of cast-iron. Used to pour the melted assays into from crucibles or scorifiers. The conical kind is the best. Prices, deep or conical, $1.37; shallow or round bottom, $1.25 Pliers, Etc. — Two pairs of flat-nosed plyers, respectively 4 and 6 inches long; prices, 40 and 50 cents; used for removing beads from cupels. A pair of shears, $1.50. A pair of scissors 6 to 8 inches long. A pair or two of

AND SDiVEB ORES. 17

coarse forceps, about 6 inches long, used for handling lead buttons while hammering them, etc. A pair of fine steel forceps, 5 inches long, for lifting assay weights and beads. Some use ivory-pointed forceps for the weights and weighing capsules. It Is well to have a variety, but Hie fine steel " pincettes," rather long and requiring but very little force to close them, suit the author best for handling the smaller weights and beads, which, with stiff or coarse forceps, are apt to be lost; prices 50 cents to $3.50.

Spirit Lamp. — Used for boiling in test tube; price, 50 cents to $1.00. A few very small coal oil lamps will be found very convenient for various purposes, replacing, for most uses, the spirit lamp and gas jet.

Watch Glasses. — To place beads and other small things on.

Hammers, Etc. — One steel hammer, about 6 ounces, for large beads, etc., 50 cents; another, about 1 pound, for beating lead buttons and breaking specimens, etc. ; one of 4 pounds, for breaking coarse rocks a very small one, such as is used by a watchmaker, is very useful for flattening the smallest beads ; a wooden mallet ; a hatchet ; a steel cold-chisel about 8 inches long, and another about 4 inches ; a nailbrush for cleaning buttons, and a toothbrush for beads: a camel's hair pencil and a flat brush of the same, about 2 inches wide.

Anvils. — One about 3 inches square, with shank, by which to flx it in a block, and a flat one to lay on a table; the first to beat and clean lead buttons on, the second to use in cleaning or flattening small beads of precious metal.

Assaying Gold

Cupels and Cupel Mould. — A cupel is a small, thick cup, made of ground bone ashes pressed in a mould. Cupels may be bought ready-made, but it is better to make them. The moulds are of brass or iron, the latter being the cheapest and equally good if not allowed to rust. At least two sizes are required: the larger, 1 J or 2 inches; the smaller, IJ inch in diameter. Price, $1.50 to $2.25, if of iron. The cupel is used in the separation of base from precious metal, by the aid of heat, in the muffle. To make a cupel, moisten the bone ashes sufficiently to make them cohere slightly when compressed in the hand ; till the mould and drive the die down by a few blows with a mallet, and turn it round, to smooth the cupel. The mould, being bottomless, must be placed on something solid — as the anvil. The cupel is pushed out from the mould by means of the die. If this is very difficult to do, it is because the ashes are too dry, or the die has been driven too forcibly. The cupels are placed on a tray to dry. The price of bone ashes is 6 cents per pound. Ten pounds will make about 200 medium-sized cupels.

Test Tubes. — In nests, 8 to 6 inches, per dozen, 50 cents ; single, 3, 4, 5, 6, 7, 8 inches, per dozen, 50 to 75 cents. Made of glass; used for boiling acids, etc.

Test Tube Rack. — To set the tubes in ; also has pins for draining tubes by inversion. Price, $1.

And Silver Ores. 19

Test Tube Holder. — Wooden tongs, with round jaws, usually lined with cork. To hold the test tubes while hot.

Dry Cups (annealing cups), — Small crucibles, very thin ; made of clay, unglazed. Used for drying and annealing the gold from an assay. Price, for best, $1.50 per dozen.

Water Bath. — A dish having a cover consisting of a series of rings. By removing more or less of the rings a suitably-sized opening is made in which to rest a vessel to be heated by boiling water. Used for drying ore, etc., at heat of boiling water.

Lens, or magnifying glass, for examining smaU beads of metal to see if they are clean ; also, for inspection of minerals.

Cupel Trays. — Made of sheetiron, to carry cupels or watch glasses on ; 6 holes, $1.50 ; 9 holes, $2.00 ; iron trays, 5x7 inches, 75 cents.

Coal Oil Stove. — This is a great convenience in a country office where gas cannot be had. A pan containing some sand, placed on the stove, forms a sandbath for heating liquids in glass or porcelain vessels, drying samples on, etc.

Glass Funnels. — Price (each): 1 ounce, 10 cents; 2 ounces, 15 cents; 3 ounces, 15 cents; 4 ounces, 15 cents; 8 ounces, 20 cents; 16 ounces, 25 cents; 32 ounces, 40 cents; 64 ounces, $1.00; 128 ounces, $1.50.

Assaying Gold

FiLTEB Paper. — Used for filtering liquids to separate solid matter. Cut in circular form, folded twice across, and applied as a lining to a funnel into which the liquid is then poured. The clippings of the paper are excellent for cleaning glassware; 30 cents to $1.50 per quire. Swedish is the best; imitation Swedish is very good; cheap gray answers for filtering rain-water for assaying, and for some other purposes.

Filter Stand. — Also used in heating a vessel over a lamp. Price, with 2 rings, $1.00; 3 rings, $1.25; 4 rings, $1.50; light made, 3 rings, all wrought-iron, $1.25; iron filter stand, finer made, 8 rings and light, $1.25.

Wa s h I n g Bottle. —

Used in washing objects

in vessels .or on filters.

The bottle is to be nearly filled with water. On blowing into one of the tubes, a jet is expelled from the other. Price, 60 cents.

Assay Balance. — This is the most costly and delicate apparatus used by the assayer. The cut represents a first-class Oertling balance, costing $190. Assay balances are of many grades of price and quality, down to TrcBmners No, 3, at $40.00> with weights. It is by no means advisable to buy the cheapest grades, unless in case of absolute necessity. Becker's No.5, costing $95.00, is a good, reliable instrument. A very fair balance can be got for $80.00, but much below this price they have no apparatus for riders, which, in the

And Silver Ores.

UNlVEP-f7 r )

Assay Balance.

writer's opinion, is a very grave defect. A good assay balance is able to carry a load of twenty or thirty grains in each pan, and turn distinctly with one-thousandth grain. It is inclosed in a glazed case, the front of which is arranged to open by an upward sliding panel, counterpoised like a

22 Assaying Gold

window, by means of cords and weights. The case is supported by legs which are adjustable in length by screwing into brass sockets. Within the case are two spirit levels at a right angle the one to the other. The beam is poised on knife edges of agate, resting on agate plates. The pans are suspended by means of stirrups with agate plates, resting on knife edges or points fixed to the beam. A slender finger of steel descends or rises from the center of the beam, indicating its slightest movement upon a graduated arc of ivory. The distance from the center of the beam to the points on which the pans are suspended, is divided into a certain number of equal parts, marked on the front of the* beam. These divisions are subdivided by shorter marks. A brass bar extends across the case, behind the beam. It supports a sliding rod which extends to the outside of the case, and terminates in a milled head. The sliding rod is called a carrier, and is furnished, at its inner end, with an arm extending at a right angle to the rod, over the beam. On the arm hangs a small bifurcate weight, formed of fine wire, in such a manner that it can be placed astride upon the beam like a man on horseback. By means of the carrier, operated from without the case, the rider is placed upon the beam at any desired point, or removed at pleasure. The rider acts upon the principle of the pea of a steelyard, and is used to complete the weighing of an object on the balance, without the necessity of opening the case. In order that the instrument may not be injured by the weight of the beam and pans resting constantly on the delicate points of suspension, nor by any shock occasioned by the placing or removal of weights or other objects during the operation of weighing, an apparatus is pro-

And Silver Ores. 23

vided, by which, on turning a key or milled head, the beam is lifted off its bearings, and two small tables rise under the pails and receive their weight. The stems of these tables are screws, working in brass sockets, by which the height of the table is adjusted.

Above or below the center of the beam, working on a screw-thread on the index, or on a separate slender stem, is a little ball or "cheese** of brass, called a gravity bob. By screwing this upward or downward, the center of gravity of the beam is raised or lowered, and its sensitiveness increased or diminished. Around this stem is twined a slender bit of wire, with one end projecting in front. By turning this promoting end a little to the one side or the other, the beam may be balanced.

The balance must be placed on a solid table, the legs of which, when possible, should pass through holes in the floor, and rest on the ground- The holes should be large enough to allow the legs to pass without touching the floor ; pieces of sheep-skin with the wool on, tacked around their edges, will exclude wind and dust. The legs of the scale case should rest on pieces of plate glass, to prevent sinking. In a moist climate, an open vessel containing some strong sulphuric acid or quicklime should remain constantly within the case. In a very hot and dry climate, Oertling recommends keeping a shallow dish containing water on the table under the balance. The acid or lime is to prevent rusting, by absorbing the moisture of the air. The dish of water is to prevent warping of the woodwork by too great dryness. A light cover of canvas, paper, or wood, in the form of a box, should be placed over the case when the balance is not in use, in order to exclude dust. The balance should not be

24 Assaying Gold

in the same room in which ore is pulverized, or acid boiled, nor in which is any furnace. The weights used with this balance are either grains or gramme, corresponding with those used in weighing the ore for the assay. If grains, the set is from ten grains to one one-hundredth grain. If grammes, from one gramme to one milligramme. In either case the smallest is one-thousandth of the largest, and is considered as the unit or one of the system. (Where riders are not used the set includes smaller weights, but these are regarded as fractions of the unit.) The weight of the rider must agree with the other weights, and with the number of divisions on the beam. In general it weighs ten units of the set, and the beam has ten principal divisions on one or on each of the arms, and fifty subdivisions. In other balances the beam has twelve divisions, and the rider weighs twelve units. In either case the rider, if placed on the mark 1, will counterbalance one unit o.f the set on the opposite pan, and accordingly for the other marks. On a subdivision it is equal to a corresponding fraction in addition to the preceding whole number. Some beams, however, are divided and numbered in twenty parts, and the rider weighs two units; its value on 1 is then one-tenth of a unit; on two, two-tenths, etc.

The weight of a bead or piece of precious metal obtained by an assay is usually reported" in units of the set, not in grains or grammes. Thus a bead weighing 1.156 grains is reported as 115.6. A bead weighing 0.0765 grammes is reported as 76.5. One gramme or ten grains is therefore called 1,000. In gold bullion assaying the set is generally one-half gramme equals 1,000, consequently one-half milligram is the unit, and the rider weighs ten of these units.

And Silver Ores. 25

The balance is adjusted and tested as follows: The case is accurately leveled, first in one direction, as from left to right, and then in the other, as from front to back, by means of the spirit levels and the adjustable legs. (Becker's balances have three legs and a single universal level, of which the bubble must be brought to the center.) The beam is then placed, not resting on the knife edge, but on the extra supports, which are thrown up for the purpose. The pans are then suspended from the ends of the beam. On each stirrup will be found a mark; near the corresponding end of the beam a similar mark. The marks on the stirrups are placed toward the front, and each stirrup is hung on its proper end of the beam. The tables under the pans are adjusted so as to take the weight of the pans without lifting the stirrups off the beam. The supports are then lowered and the index observed. It should swing slowly and steadily from side to side, and, on coming to rest, should point exactly to zero, or the center of the ivory arc. If the index swings unequally, and stops finally on one side of zero, the little wire which projects in front of the center of the beam must be carefully moved, and the trial repeated, and so on until perfect equilibrium is attained. The small capsules or dishes which accompany the balance are then placed, one on each pan. They must balance each other exactly, or else equilibrium must again be produced by means of the wire, and each capsule be thenceforward used invariably on the same side.

It may be here remarked, once for all, that nothing must be placed on the pans, nor in the capsules while they are on the pans, nor must the beam or pans be

Assaying Gold

touched in any way until the extra supports are raised. Neither must the capsules or the weights be touched by the fingers. If it is necessary to clean the capsules, a fine soft handkerchief should be used. Dust is removed from the beam and other parts by means of a camel's hair pencil. The weights and capsules are moved by means of forceps. Occasionally the balance may be dismounted, and the parts cleaned by rubbing with a piece of soft chamois leather, and, if necessary, a very little fine coal oil.

The balance having been so far adjusted, must be tested. The weight 1 of the set should cause the index or pointer to stand at or near ten divisions from zero, and one-tenth, obtained by the riders, should cause a distinct deviation. The deviation should be the same on either side with the same excess of weight, otherwise the balance is badly made. The sensitiveness can be increased to the required degree by screwing the gravity bob upward, but if the bob is raised too hih, the beaii' will fall on either side, and so remain, and no weighing can be done. Some scales are not provided with a gravity bob, being adjusted once for all by the maker. In raising the supports, advantage should be taken, whenever possible, of the moment when the swing brings the pointer to ero, when they may be raised quickly. If this cannot be done, the supports must be raised very gently. In releasing the beam, the supports should be lowered at first very slightly, until it is seen that the beam is near enough to equilibrium m to swing both ways, then promptly to the full extent; but if the load on one side preponderates so much as that the beam will evidently fall on that side too far to swing freely, the

And Silver Ores. 27

lowering of the support must not be continued, but on th contrary it must be raised again, and a nearer approach to equilibrium established. These cares are necessary to prevent shocks which would injure the delicate bearings.

The next point is to ascertain if the two arms of the beam are of equal length. This is done by placing counterpoising loads on the pans, and changing the loads from the one pan to the other. If the exchange of loads produces no disturbance of the equilibrium, that is if an object weighs the same on either end of the beam, the arms are equal in length. In first-class balances there is not much likelihood of a defect in this respect, the rectification of which, by means of screws provided for the purpose, is a work of such delicacy that a person who is not familiar with such apparatus should not attempt it.

It must be understood that the condition of equilibrium, and equality in the length of the arms, are two distinct things, either of which may exist without the other. The beam must be balanced by some means before any weighing is undertaken. If it is then found that the arms are of unequal length, by testing as above, recourse must be had to double weighing, in order to obtain the true weight of an object. The best method of double weighing, called the method by substitution, is to place the 1,000 weight on one pan of the balance, and counterpoise it by a special weight m ade for the purpose, of any suitable material, as gold or platinum. The weight is then reihoved, the object to be weighed is put in its place, and taller weights added until the counterpois3 is balanced. The difference between the 1,000 weight and the sum of the weights required to restore equilibrium is the weight of the object.

28 Assaying Gold

It may here be observed that if , as in assaying, only relative weights are required, the inequality in length, of the arms is of no consequence provided that all the weighings of the assay are made on the same pan of the same balance, because then they are all affected in the same ratio, and the relative proportions remain correct. This is easily done in bullion assays, and may be so in ore assays if a chemical balance is used which is at once large and strong enough to weigh the ore, and delicate enough to weigh the gold and silver obtained; but where> as in general, the ore for an assay is weighed on one balance, and the precious metal on another, double weighing becomes necessary if the arms of either are unequal.

Another method of double weighing is by reversal. The object is weighed first on one pan and then on the other, and the sum of the apparent weights divided by two is taken as the true weight. This is not strictly correct. The true weight is the square root of the product of the apparent weights, but if their difference is small, the error is inappreciable. When it is required to weigh off a certain quantity of a substance on such a balance, the weight corresponding to that .quantity is placed on one of the pans, and counterpoised by any means. The weight is then removed and replaced by the substance. The quantity of the latter required to restore equilibrium is equal to the weight which it replaces.

A careful assayer will also test the weights in order to ascertain if they agree among themselves. It is not important that they be absolutely standard, but oily that they are correctly proportioned th one to the other, for an assay is simply a question of the proportionate weight of precious metal, as compared with the weight

And Silver Ores. 29

of ore from which it is extracted; hence the weights used in weighing the ore for the assay must accord with those used for weighing the metal obtained, and all must agree among themselves, according to their marks. If 1,000 parts by weight of ore, no matter what kind of weights are used, contain one such part by weight of silver, it is clear that 1,000 pounds of similar ore contain one pound of silver. The proportion once ascertained, it is easy to deduce the absolute quantity of metal in a ton of ore. On the one hand, then, the ten-grain or 1,000 assay weight should counterbalance the ten-grain ore weight, and, on the other hand, it should also equal the 400, 300, 200, and 100 assay weights together. The 400 should equal the 300 and 100, and so on down to the smallest. The ore weights must be checked in a similar manner on the pulp scales, working from the ten grains, after that has been found to agree with the similar assay weight. In case the ore weights are not a complete set, the assay weights may be combined so as to make twenty grains, and that quantity of granulated lead may be weighed on the assay balance as many times as may be necessary to make a quantity equal to the ore weight as marked, and placed on one pan of the pulp scales. The ore weight being placed on the other pan, it will be seen whether the two agree or not. A difference of less than one per cent, is not of great importance. The riders may also be tested by weighing them against the ten assay weight, if the beam has ten main divisions, or against the ten and two if it has twelve divisions, etc., and the divisions on the beam by placing the rider on each mark in succession, and the corresponding weight on the opposite pan.

30 Assaying Gold

Materials.

Litharge (lead oxide). — Twelve and one-half cents per pound ; composed of lead and oxygen ; a flux, an oxidizer, a desulphurizer, and a source of lead.

A flux, because it causes refractory substances to melt at a comparatively moderate heat. It fluxes mast rocks, earths, and metal oxides, hence is very destructive to the crucible if used in excess, for which reason other substances are used instead, to a certain extent.

An oxidizer, because it gives up oxygen to combustible or oxidizable substances, causing them to bum, or become oxidized. It thus bums sulphurets in an assay. It is a common error to suppose that the substances thus burned are necessarily burned out On the contrary, while some are burned out others remain in the assay, but their condition is changed; they are converted into the respective products of their combustion.

A desulphurizer, because it burns the sulphur of sulphurets, as well as oxidizing their metals, except lead and noble metals.

A source of lead, because when it gives up its oxygen the lead remains in metallic state.

To prove these things, melt some litharge in a crucible and throw in about one-fiftieth as much iron sulphurets in powder. A piece of lead will be got from a portion of the litharge ; the rest fluxes the iron oxide made by burning the iron sulphuret. Or, instead of iron sul-

And Silver Ores. 31

phuret, throw in some galena. The sulphur will be burned and the lead from both litharge and galena will remain.

Soda (sodium bicarbonate). — Six and one-half cents per pound ; a flux, an oxidizer, and a desulphurizer.

Fluxes quartz, quartzose rocks, and some metal oxides. Oxidizes some metals. Desulphurizes galena, and some other sulphurets, especially,if charcoal or flour is added. The sodium combines with the sulphur. Mix some powdered quartz with thrice its weight of soda, and heat to bright redness in a crucible. The result will be a glass which can be poured out. Melt some soda with iron filings and dissolve in water; the iron will be found to be oxidized. Melt some galena with several times its weight of soda; lead will be got.

Borax (sodium hihorate). — Twelve and one-half cents per pound ; a flux.

Fluxes clay, lime, magnesia, slate, etc., and metal oxides generally ; also quartz, but not so well as soda. Melt some and add a little powdered ore containing sulphurets. The gangue (rock, etc.) will be dissolved; the sulphurets will be found at the bottom.

Borax swells when heated, giving off* water. This is sometimes inconvenient in an assay. To prepare it for Use, heat it gently in an iron pan until it swells no more, then cool and grind it. It should not be melted, as that makes it hard to grind. It should be kept in a close vessel if in a moist climate. Sometimes it is used undried for assays which can be heated very rapidly, and in the dry climate of Arizona or Nevada it will dry pretty well of itself when ground.

32 Assaying Gold

Borax Glass. — One doUar per pound; a flux, same as borax, but does not swell. To prepare it melt some borax, cast it in thin plates and grind to powder. One part, by weight, is equal to two of undried borax.

Glass (common bottle or window). — A flux. Acts similarly to borax, but is less easily melted. Used in assays containing much lime, clay, etc. ; not with quartzose ores. Also useful in assays made with much nitre or litharge, to protect the pot. A quartzose ore fluxed with soda makes glass, which then fluxes the metal oxides, lime, etc., in the ore. Sometimes used in pieces, but is better ground. Heating to redness and quenching in water facilitates grinding.

Nitre (potassium nitrate). — Fifteen cents per pound. First a powerful oxidizer, then a flux ; also a desulphurizer. When heated gives off a large quantity of oxygen, leaving potash, which is a flux of much the same nature as soda. It is a desulphurizer in two ways ; firstly, by giving ofl' oxygen to bum sulphur ; secondly, by the potassium combining with sulphur, as the sodium of soda does, but if enough nitre be used all the sulphur is burned, being converted into sulphuric acid. Nitre can oxidize all metals except gold and some of the platinum group. Used to counteract the eflect of too great a quantity of sulphurets in an assay, which it does by burning a part of them, which would otherwise produce too much lead from the litharge.

Sulphur. — Six cents per pound. A sulphurizer. Used in a certain class of assay, in certain cases, to prevent copper from entering the lead button, which it does by converting the copper into a sulphuret.

And Silver Ores. 33

Iron (nails or wire). — A desulphurizer for galena and for compounds of gold or silver with sulphur, but not for other metal sulphurets, as copper, zinc, etc.* Used in a certain class of assays to free lead and precious metals from sulphur. Can also combine with antimony and arsenic, keeping them out of the lead. It reduces litharge to lead, itself being burned, but is not used for that purpose. Melt some borax, add some galena and a large nail ; lead will be got.

Salt (aodiuTn chloride). — Used because it becomes very fluid, floats on the assay, and serves as a cover to exclude air and furnace gases; also to wash the side of the pot. It must be dried and ground.

Granulated Lead. — Fifteen cents per pound. Used in the assay by scorification,in which the litharge required is produced from lead, instead of lead being produced from litharge, as in the crucible assay.

Sheet Lead. — Twenty-five cents per pound. Tea lead will answer. Must contain no gold.

Flour. — Used in the crlicible to produce lead from litharge. One part, by weight, of flour generally produces fifteen parts of lead, but this varies a little according to the quality of the flour. Powdered charcoal is also used for this purpose. One part of charcoal produces about thirty parts of lead. Any substance which produces lead from litharge is a reducer.

*Iron can take all the sulphur from lead or silver compounds, setting those metals free. It only takes a part of the sulphur from pyrites, sulphuret of copper, and some other sulphurets, leaving the metals still combined with a part, in the form of mcUte.

34 Assaying Gold

Test Silver. — Two dollars and fifty cents per ounce. Used in the gold assay. Must contain no gold.

Acids. — Nitric; sixty-five cents per pound; hydrochloric (muriatic), and sulphuric acids. The first only is strictly necessary. It must contain no chlorine. The others are convenient for certain tests.

Distilled Water. — Filtered rain-water will answer. Common spring water is often used. It must contain no chlorine. Used in the gold assay for washing the gold.

For the sake of brevity only the more important propei*ties of the foregoing substances have been mentioned. Most writers on assajdng give determinations of the reducing powers of the flour and charcoal, and of the oxidizing power of the nitre, which vary slightly with their purity. The student will find that these are needless, as the figures given are near enough for all practical purposes.

The water and nitric acid may be tested for chlorine thus: Dissolve a few grains of silver by boiling in rather dilute nitric acid, or a few grains of lunar caustic in water. Add a drop or two of the solution to the suspected water or acid in a clear glass vessel, and observe if it produces a white cloud or a milkiness. If it does so in the water, that will better be rejected, but not so the acid. Stir the acid with a glass rod, or, if in a bottle, which is best, shake the bottle well, then let it stand until the acid is clear, and add another drop of silver solution, and so on until no further milkiness is produced, then, . after settling, pour the clear acid off into another bottle for use.

The water may be treated in the same way in case of

And Silver Obes. 35

need. In a mill, good condensed water can be got from the boiler (not from the exhaust of the engine). The assays of litharge and lead for gold and silver will be found in their appropriate places. Litharge and lead always contain some silver, and this is allowed for in the assays of ore. If they contain gold also they are unfit for use. Any silver that is nearly pure will answer for test silver if it contains no gold. To test it, dissolve about ten grains in nitric acid. It should leave no residue. All the fluxes should be ground to powder, for which purpose a coffee-mill is convenient, except for glass, for which the iron mortar, covered with a cloth, is better. Borax, nitre, and salt need not be finer than ground coffee. Some use undried borax in lumps placed at the bottom of the crucible.

36 Assaying Gold

The Assay Office.

The office should properly consist of three adjoining rooms; one in which the pulverizing, fire work, and other rough and dirty work is done; another in which to weigh and mix the assays, boil the acid in separating gold from silver, and keep the fluxes, acids, crucibles, spare muffles, etc. ; and a third in which to keep the assay balance, books, and papers. The first need not be more than ten feet square. The last may be little more than a welllighted, and dust-tight closet; the middle one should be a moderate-sized room ; but it often happens that only one room can be had, and all that can be done is to make the best of it. The muffle furnace should be so placed that when the operator works in the muffle the light comes from behind him. The sun should not shine into the room. There should be a strong bench or table on which to use the pulverizing implements, and it is better if it rest on the ground, and not on the floor, so that the entire office may not be shaken by the blows of the pestle or hammer. A block, similar to that used by a butcher, will answer well instead of a table. Another table on which to place the pulp scales and mix the assays will be required. Above this table, or in any convenient situation, should be shelves for packages of material, and upon the table, which should be of a good size, a long box divided across into five or six compartments about six inches square and deep, and lowest in front,

And Silver Ores. 37

with a hinged cover, to contain the several fluxes, etc., also ihe test tubes in their rack, the spirit lamp, smaller anvil, etc.

The assay scales should be so placed that the light from a window may strike them obliquely over the left shoulder of the operator while seated at his work. It is still better to have light from two sources obliquely behind the operator, in order that no shadows may be cast on the balance, either by the person or by the case frame. The sun should not be allowed to shine on the balance at any time. The table on which the balance stands should be reserved for that purpose only; there should be another on which to work with pen and paper.

38 Assaying Gold

Preparation Of The Ore.

All ore assays must be finely powdered, but not the whole of a large sample need be so. A large sample is broken by hammer and block, pestle and mortar, etc., so that no one piece shall be a considerable fraction, say more than the one-thousandth of the whole, then mixed on a suitable cloth or smooth table. The sample is then spread in a layer and divided, by the back of a saw-blade, a trowel, or spatula, into quarters. Two opposite quarters are rejected and swept cleanly off the table. The other two are again mixed, after further pulverizing, if needful, and again divided. When reduced to a pound or so, the sample is passed through the coarser sieve, and now a sheet of strong paper or of oilcloth is suitable to mix on, by lifting an edge and drawing it forward so as to roll the sample upon itself, and to heap it in the middle; then again spreading by the spatula, and so several times. The sample is thus gradually reduced in quantity, being more finely ground step by step, until several ounces have been passed through the finest sieve.

For mixing a sample, after it has reached a certain fineness, no implement is equal to the finger. Heap the ore, then, with the finger, beginning at the center of the heap, trace a spiral to the edge and back to the center. Again heap by lifting the edges of the sheet, and again spread by the finger, and so on.

Without perfect mixture at each stage of division, the

And Silver Ores. 39

final sample will not be a true one, and this final sample must again be well mixed, in order that each assay taken from it may also be a true sample of the whole. An idea of what is required may be got by adding a little flour to a finely powdered sample of dark-colored ore, or charcoal to a light-colored one, and mixing until the tint is uniform without streaks or spots.

Each time that a sample is ground and sifted, the whole of it must pass the sieve unless there are particles of tough matter, such as metal, or silver-glance, etc., which cannot be ground to a powder. In this case the tough matter must be kept, and no further division of the sample can be made, as it must all be weighed and treated as directed further on.

Damp ore must be dried. In particular cases all samples, whether they appear damp or not, should be dried after being powdered, then allowed to cool before weighing the assay. Many substances, such as clay for instance, lose more and more water at successive degrees of heat up to whiteness, wherefore there must be a standard temperature for the drying. That standard is the heat of boiling water ; hence the sample is dried on the water bath.

All implements used in powdering and sifting must be cleaned, lest the sample be contaminated by some remains of a former one. In many cases simply wiping the mortar, etc., with a cloth will suffice, but after rich ore, the tools must be cleaned by grinding some barren quartz, sand, or glass. Sieves are cleaned by brushing and tapping.

The prepared sample is put in a sample pan with a tag on which is marked the number of the sample, and

40 Assaying Gold

any other desired particulars, such as the general character of the ore, as a guide in dressing, or the metal for which it is to be assayed,

The character of the ore can, in general, be ascertained sufficiently, and most conveniently, by an examination of it in the lump, before grinding. When this cannot be done, as in the case of samples which come already ground, recourse must be had to washing (panning) a little of the powder in a dish, by which the character and proportion of sulphurets may be judged if there are any. The color and general appearance also furnish indications, or special tests may be applied, some of which are given further on.

And Silver Ores. 41

Weighing The Charge.

Sit or stand squarely in front of the scales. See that the scales balance, with the weighing capsules on them. Never weigh anything on the naked pan ; use the capsule. If you have none, take a small basin or a piece of smooth paper. Put the weights on the left-hand side. Never weigh the ore until the fluxes arie laid down; then put the ore on top.

Grains are used by some, grammes by others. Half an ounce (240 grains) or twenty grammes, are the usual quantities for crucible work; some prefer "assay tons," sign A. T. With rich ores these standard quantities may be halved or quartered. For scorification much smaller quantities are taken. For poor ores, double, treble, etc. , quantities may be employed.

After a little practice, it will not be necessary to weigh the fluxes, etc., except in some cases nitre and flour. In general these things can all be measured closely enough.

42 Assaying Gold

Mixing And Charging.

The crucible assay may be mixed on a sheet of writing paper (glazed paper is better), and poured from that into the pot, care being taken to leave nothing behind. A more usual way is to put first the fluxes, or some of them, and then the ore, directly from the weighing capsule into the pot as it stands in the rack, or on a sheet of paper. The mixing is then done with the handle of a teaspoon. The pots must not be more than two-thirds full, on account of the swelling of the charge when heated. Pots which have been used for rich ores should not be employed for poor ones; and for important assays, whether rich or poor, new ones should be taken. This applies also to scorifiers. The scorification assay is mixed in the scorifier.

And Silver Ores. V 43

Assay Of Litharge.

The litharge frequently contains a notable quantity of silver. Each lot, as opened for use, must therefore be sampled and assayed. The ore assays made with it must be corrected by the result. Where, as in a mill, the same quantity of litharge is constantly used in the assays, the readiest way of making the correction is to assay that quantity, and place the resulting bead on the weight pan when weighing the beads; but where varying quantities of litharge are used, it is better to treat a larger quantity, note the weight of silver got, and correct each ore assay according to the litharge used in making it.

In general, only the assays of very poor ores or of tailings need be corrected, because the others lose more in the assay than they gain from the litharge.

Place in a crucible this mixture: —

Litharge 2 ounces or 50. grammes.

Flour 20 grains or 1.5 "

Soda i ounce or 10. "

Cover with salt, and melt quickly. When solid, clean and cupel the button. Make two such assays, and part one of the beads for gold.

44 Assaying Gold

Systems Of The Crucible Assay.

In all fire assays of gold and silver ores, the preeious metals are collected by means of lead, from which they are afterwards separated. In crucible assays the lead is produced from litharge by the action of reducers. The crucible assay is divided into two systems, either of which may be applied to any ore but is best adapted to certain cases.

In the assay by the first system, more litharge is used than is requisite for the production of lead. The excess assists in fluxing the assay. All base metals, except the required lead, are oxidized and dissolved in the slag. This assay is quickly made, and generally gives accurate results. It has the disadvantage of requiring considerable modification for the various ores, as to the fluxes proper, and to the reducers or oxidizers by which the proiustron of lead is controlled. Sometimes a preliminary assay is necessary.

In the assay by the second system, only enough litharge is used to furnish the required lead. All base metals except lead remain combined with sulphur, or, as arsenic and antimony, are volatilized, or combine with iron. This assay requires a longer time than the other, bui presents the following advantages: The right quantity of lead may nearly always be got at once, for, although any lead which the ore may contain will inevitably comer

And Silver Ores. 45

down together with that from the litharge used, yet this can be allowed for by reducing the quantity of litharge or omitting it. As litharge yields ninety-three per cent, of lead, it is not difficult to make the adjustment nearly enough. Galena contains eighty-six per cent, of lead, hence if the ore is nearly pure galena, but little litharge is needed. The method requires but slight modification for different ores, and may with little disadvantage be made universal. The button is never much contaminated by copper, as it often is in the other* system unless a very large proportion of litharge is used, which is disad- Arantageous in some ways. The crucible is but little Attacked, and the assay is not liable to boil over. The method is especially useful for ores carrying much galena or other sulphuret, and when copper in any form is present. The writer has found it accurate for all ores, except one case of concentrated sulphurets containing gold and silver. All the gold was obtained, but scorification gave a little more silver. All crucible assays failed in this case, as also with an arsenical matte or speiss from a smelter, which also yielded only to scorification.

46 Assaying Gold

Preliminary Assay.

Is the crucible assay by the first system, ores which contain a great proportion of sulphurets would reduce too much lead. Others containing less would reduce the right quantity; others aain too little or none, and an ore containing a large proportion of the higher oxides of iron, manganese, or chromium would prevent the reduction of lead by a reducer, unless the latter were present in sufficient quantity to counteract their oxidizing effect and leave a surplus for reduction. An excess of reducing power in the ore is counteracted by means of nitre.

An experienced workman can generally judge what to do, for although the weight of lead got ought not to be much less than ninety per cent, of that of the ore used, yet there is a considerable margin for variation. The writer rarely makes a preliminary, but in doubtful cases dresses the assay according to his judgment, with more or less of the various fluxes according to the nature of the gangue, and with an addition of nitre or an extra dose of flour as the case may be; but in case a preliminary is required, it may be made as follows: Consider whether the ore is likely to be a reducer (that is it contains a considerable proportion of sulphurets), or an oxidizer (that is carries much red oxide of iron, black oxide of manganese, black oxide of copper, red lead, or a chromate). If it is a reducer, take one-twentieth of the

And Silver Ores. 47

weight to be used in the assay proper, which will be twelve grains or one gramme, according as half an ounce or twenty grammes of ore will be taken. Mix with fifty times as much litharge, and melt. If the resulting lead button is lighter than the ore used, multiply the difierence by 20 and divide by 15, or, which comes to the same, the difference multiplied uy 1.3 is the quantity of flour required for the assay. If tir lead button is of nearly the same weight as the ore, no flour will be needed; if heavier than the ore, five times the difference is the quantity of nitre to be added to the assay. If no lead is got the ore is not a reducer and may be an oxidizer. At least the usual quantity of flour, 16 grains to the half ounce or 1.4 grammes to 20 grammes, will be needed.

In case the ore is supposed to be an oxidizer, mix and melt the same quantity of it with half the former quantity of litharge and l-15th as much flour as ore. This will be for 12 grains of ore 0.8 grains, or, for 1 gramme, 0.07 grammes nearly. (Equal to 16 grains to the I ounce, or 1.4 grammes to 20.)

If no lead is got, at least twice that proportion of flour will be needed. If the button is too light, 1.3 times the deficit will be the weight of extra flour required, in addition to l-15th the weight of ore; and if too heavy, 1.3 times the excess must be deducted from the standard quantity. The figures given are sufficiently approximative, and are based on the supposition that 1 part of flour reduces 15 parts of lead, 1 part of nitre oxidizes 4 parts of lead, or an equivalent of the sulphurets. If charcoal instead of flour be used, it is only necessary to reduce the quantities one-half; 1 part of charcoal brings down 30 parts of lead.

48 Assaying Gold

Dressing The Crucible Assays.

From what has been said of the properties of the different fluxes, it will be seen that the proportions in which they are used may be varied to suit the character of the ore. Mitchell gives one general formula for all ores of gold and silver, as follows: —

Ore 1 part.

Litharge 5 "

Soda 1 ''

Borax glass 1 ''

Salt to cover, and reducer (flour) or oxidizer (nitre) if needed, in accordance with the indications given by a preliminary.

This formula is almost if not quite universal, admitting the variation in respect of flour or nitre, and it is well to keep the proportions in mind. But the same objections apply in a less degree to this as to the exclusive use of litharge as a flux, with flour or nitre to suit. These objections are the expense for litharge, which is comparatively costly, and the rapid destruction of crucibles, which is an important item in* districts remote from a city, and where, as in mills, a great many assays must be made daily. In a majority of cases two parts of litharge suffice; in some, one part is enough; but

And Silver Ores. 49

when the litharge is thus spared, it may be necessary to alter the proportions of the other fluxes.

This, it is thought, the student will be enabled to do by a careful study the action of the fluxes. It should be remembered that soda is a flux for quartz; borax for earths and metal oxides generally, as is also the glass formed by soda with quartz; litharge for all.

As to making a preliminary every time, it is out of the question. The workman must, in many case, rely on his judgment, or have recourse to the second system or '''nail assay.** For ordinary ores, containing little or no ulphuret, some quartz, clay, lime, iron oxide, etc., this formula will in general suffice: —

Ore 1 part.

Litharge 2

Soda 1

Dried borax . 1

Flour 1-15

Salt to cover.

Fuse quickly ; keep in furnace 5 to 10 minutes after subsidence.

If the ore is nearly all quartz, the borax may be reduced one-half and the soda increased; or, as much soda may cause the assay to boil up tocf much, the litharge may be increased instead. On the other hand, if earthy matter or metal oxide predominate, the soda fhay reduced one-half, or more, and the borax increased -even to doubling, or in place of so much borax some glass may be added, in which case it may be better to retain all the soda.

If the button got is too large, it may be scorified to a suitable size; but if the assay is very important, it will .better to make another, with less flour, in the ratio of

50 Assaying Gold

1 part to 15 of lead in excess. If the button is too small, repeat with more flour in the same ratio to the deficit. Thus the assay answers all the purposes of a preliminary, while, if the result is satisfactory, time is saved.

If the ore contains sulphurets less flour must be used, or none may be needed; or, again, nitre may be necessary.

The proportion of some sulphurets which will bringdown the proper quantity of lead are: —

Iron pyrites 11 per cent.

Copper pyrites 12

Zinc-blende 13

Sulphuret of antimony.. 16

Grey copper 15

Galena 34

Sulphuret of copper 23 Sulphuret of manganese . . 13

Any excess must be met by means of nitre.

It is important that the slag should be thoroughly oxidized, retaining no vestige of a sulphuret. The different kinds of sulphurets require different proportions of litharge to effect complete oxidation, as follows — only a small part*of the litharge is reduced to lead: —

1 .Ironpyrites 50 litharge, giving 8 J lead.

1 Copper pyrites 30 " '' 7

1 Zinc-blende 25 " 6 "

1 Sulphuret of antimony . 25 " '1 Sulphuret of copper 25 " 1 Sulphuret of manganese. 30 "

From this it would seem that an ore carrying 10 per cent, of iron pyrites would require fully five parts of litharge, while one with 23 per cent, of sulphuret. of

And Silver Ores. 51

copper would need nearly 6 parts. But the presence of the other fluxes, and probably quartz also, makes a difference, and in practice it is found that 2 or 3 parts suflSee in most cases. If, however, the button is hard, or shows any signs of containing sulphur, or if any matte is seen, the need of more litharge is indicated.

Some workmen use nails with these ores in this system of assay. No doubt the iron .will prevent the formation of any lead matte, but as iron is a reducer, it is liable to throw down too much lead.

The simultaneous use of nitre and flour or charcoal, in certain assays, is a relic of ancient practice, based on certain somewhat misty theories of reaction at different temperatures, and is retained by some who might be expected to know better, but who seem to occasionally prefer the rule of thumb to the rule of reason. The student may rest assured that, in an assay for gold and silver, when flour is needed nitre is not.

Some, even chemists, put nitre into an oxidized coppery ore, with the insane idea of " burning out the copper." Every chemist ought to know that, in the first place, the copper is never burned out but remains in the slag; and, secondly, that it must be slagged in the form of red suboxide, not as a green glass. The black oxide is an oxidizer in an assay, and so far from requiring nitre it calls for an extra quantity of reducer to bring it to the required condition. The red oxide of copper in an ore is already in the proper condition, and for finely divided metallic copper, such as can pass the assay sieve, litharge in liberal quantity is the safe and appropriate oxidizer wBich will convert it into the suboxide with no danger of forming a green slag.

52 Assaying Gold

That a coppery button is sometimes got from an assay by this system, is due to the fact that the affinities of copper and lead for oxygen are nearly equal at a red heat, as is shown by experiments detailed in Mitchell's Manual, where it also appears that the final result, as to which metal shall be thrown down and which entirely or almost entirely oxidized, depends on which of the oxides preponderates in presence. Hence, if the button is much contaminated by copper, the need of more litharge in the assay is indicated.

One way in which to get rid of all difficulty as to the proportion of both nitre and litharge is to use so much nitre as to completely oxidize every constituent of the ore, decomposing any excess of nitre by heat, and throwing down a Ifutton by means of charcoal. For concentrated pyrites the formula is: —

Sod 2 " Ibubblmg ceases under a good

Borax ... 2 Nitre 2

red heat, add part of charcoal in small lumps.

When the assay again flows quietly, shake the pot in the fire, and after a few minutes pour.

This formula has been used by the writer with good

Note. — In an assay with much nitre, two periods of ebullition or frothing are observable. The first occurs at a moderate red heat; the second at a higher temperature. At least this is the case with some ores, and it seems that ignorance of the fact my cause failure, the operator being led to think the fusion finished when it is not so. The author ascribes the second ebullition to the decomposition of nitrites formed earlier. The phenomenon appears to be independent of the preliminary melting of the nitre in its water of crystallization.

And Silver Ores. 53

results, as compared with all other methods. It now appears unnecessary to use so much litharge. The proportion of nitre may be reduced according to the ore, and, for almost any other ore than this, much less charcoal will be needed. Pyrites require parts of nitre other sulphureis less, galena f part. Nearly pure silverglance requires only to be melted with 3 parts of litharge and a little borax.

The following is a general formula for assays by the second system : —

Ore 1 part. ""

Litharge 1

Soda 3

Borax . . J

Sulphur 1-10*

Flour 1-10'

Iron 3 nails.

Glass.

Salt to cover.

Melt, and leave in )- strong fire about 20 minutes after' fusion.

In this none of the substances will do any harm, even if not required, yet if the ore is known to contain a good deal of sulphur that may be omitted. Also if the ore is quartzose the glass may be left out. On the other hand, if the ore carries much oxide of iron or of copper it will be proper to increase the quantities of flour and sulphur. If it were pure red oxide of iron, or black oxide of copper, 50. per cent, of sulphur and 40 per cent, of flour would not be too much. One of the merits of this system is that no copper gets into the button, if enough sulphur is present, and the writer is almost sure that nickel and cobalt would also be retained in the slag or matte. The system is excellent for gold-bearing sulphu-

54 Assaying Gold

rets, and when properly made gives higher results than are often got by the other system, especially when the latter assay is roasted. (Assays of gold and silver ores need never be roasted.)

The purpose of the nails is to free the lead from sulphur, arsenic, or antimony. If the nails are entirely dissolved, or sink below the slag, there is too much sulphur, etc., or too few nails present. Twelvepenny nails are suitable; if too long, cut them; thick wire is equally good.

When the ore contains a great quantity of pyrites a little nitre may be used. It assists by removing a part of the excess of sulphur which pyrites contain. The flour should then be omitted. If the button is too small (supposing the assay to have contained the proper quantity of litharge or lead in some form), or if it contains sulphur, arsenic or antimony, the nails have been removed too soon (see article on melting), or too little soda has been used. If it is coppery, not enough sulphur has been present.

And Silver Ores 55

Examples Of Dressing.

The borax was only air dried. ' Eureka Mi/ne, Arizona.

Ore 10 grammes.

Litharge.. 20

Soda T '' Y

Borax 15 Salt to cover.

The flour was varied according as the ore contained more or less oxidizing matter.

This ore consisted of oxide and carbonate of iron, black oxide of manganese, carbonate of lime, clay, and a sprinkling of molybdate of lead and horn silver. The assay fused very quietly, and gave excellent buttons and a glassy slag.

Silver King Mine, Arizona.

Ore 10 gr9,mmes.

Litharge 40

Soda 7

Borax 5

Flour f

Salt cover.

The ore contained quartz, calcite, porphyry, blende, galena, a little pyrites, heavy spar, and much native silver.

56 Assaying Gold

Tailings from the concentrators : —

Tailings 20 grammes.

Litharge 60

Soda 15 ''

Borax 10

Flour li

Salt to cover.

Concentrations 10 grammes.

Litharge 50 "

Soda 10 '' '

Borax 10 "

Nitre 1

Scorification worked well with 8 parts lead and some borax glass.

The same ore by the second system: —

Ore 20 grammes.

Litharofe . . 20 xr x j on

-r> lA ic minutes after subsi-

Borax. ... 10 "

Flour.

Nails 3.

dence.

Concentrations 10 grammes.

Litharge 20

Soda ' 30

Borax 5 "

Flour X "

Nails 3

The ores of the Blind Spring District, consisting of partzite, mixed with quartz, oxide of iron, and manganese, gave good results, with threa or four parts of litharge, flour, and either soda or borax, or both. With soda and

And Silver Orbs. 57

not borax the slag was as liquid as could be desired, but when cold was granular. With borax the slag was glassy. The results were equal. Although partzite contains a great deal of copper, some of the ore carrying as high as thirty per cent, of that, no copper was ever got in the buttons. Whether this was due to the large proportion of litharge, or to the fact that the partzite contains the copper as an antimoniate, is not known.

58 Assaying Gold

The Melting In Crucibles.

The furnace should have a damper. The fuel should be in pieces not larger than an egg, nor much smaller than a walnut, that is for the general purpose of melting in crucibles. The finer portion can be used advantageously when working the muffle, as less draft is then required, and by placing fine fuel on the top of the fire the muffle is more evenly heated in all parts. The fine fuel is also useful when the fire burns unequally, as by it the draft can be checked in any particular part. When the muffle furnace is used for the crucibles also, it is sometimes desirable to use a rather short muffle, not extending entirely across the furnace, the rear end being supported by a piece of fire-brick or an old crucible. This allows of crucibles being placed at the rear, as well as along the sides of the muffle. If the muffle is removed in order to save it and give more room for crucibles, it should not be replaced without first removing a portion of the glowing coals, and placing a bed of cold fuel on the remainder. The removed hot coals can be put above the muffle, together with fresh fuel. In this way a fracture of the muffle by intense and sudden heat is prevented. When the muffle is removed for the melting, the opening in the front part of the furnace which it occupied should be closed by a plug of fire-clay, which may be con-

And Sii.Ver Ores. 59

veniently made by cutting off about two inches of the closed end of an old muffle. This is easily done by means of a coarse file. When many meltings are to be made, it is always better to have the muffle out.

To place a crucible in the fire, make a nest for it among the glowing coals, seize it with the bent tongs in one hand, place and sustain it, while with tne cupel tongs in the other hand, the coals are packed around it. If the fuel is charcoal, a cover must be used to prevent bits of the coal falling in, but with coke this is not necessary, instead a glowing coal may be laid across the top of the crucible.

Place the pots in one unvarying order, so that any given one may always occupy the same place, according to its number. Thus it will not be necessary to mark them, though this may also be done by means of reddle.

If more assays are to be made than the furnace can contain at once, they should all be prepared before commencing to melt; then, as soon as number one is ready to pour, remove it carefully from the fire, preserving the nest which it occupied, pour it, and in its place put the first of the new set, which will be number 7, if the furnace holds 6 pots, and so on with the others.

If time is an object, it is a good plan, while weighing and fluxing the assays, to let an assistant place in the fire a full set of old crucibles as dummies, to be removed, one by one, and replaced by charged pots when ready. This will save time and trouble in making the first set of nests. If any of the assays fail, note the numbers and make them again after all the others are done. For certainty, celerity, and convenience, system is indispensable.

The assays will swell when heated, hence it is proper to

60 Assaying Gold

partly or wholly uncover the pots when fusion begins, and to watch that they do not boil over, which is more likely to occur if they are closely covered. This precaution may become needless when by practice the exact manner of fluxing a given ore so as to avoid the danger is known.

The tendency to boil over proceeds from several causes, as the use of too much soda, or of undried borax, the latter especially with very slow heating, so that if undried borax is used the assays should be put at once into a strong heat, but this cannot be done when much nitre is eraployed, as that causes foaming. The boiling up of an assay may be checked by throwing in a teaspoon ul of salt, which cools it. It sometimes happens that an assay is heated too much at the bottom before the upper part is hot, producing fusion below, while a crust remains above, and is pushed out of the pot by the expanding gases. To guard against this, place the pot well down near the grate, and pack hot coals closely about the upper part.

If the slightest overflow occurs, or if the fusing mass comes in contact with the cover, the assay must be rejected. If the slag appears thick, try the addition of a little borax. If that does not help it, soda will. In adding soda look out for boiling up. A certain proportion of soda may sometimes make a thick slag, when less would work nicely. Borax, glass, or litharge are the remedies.

When the fusion is finished, in from five to twenty-five minutes after subsidence, the pot is uncovered, with care that no coal may fall into it, seized in the bent tongs, removed from the fire, the sides rinsed in the slag by a cir-

And Silver Ores. 61

cular, swinging movement, tapped on the edge of the furnace, to settle the lead, and the still liquid contents poured into the mould. The pot is completely inverted and tapped against the mould. The overflowing of the slag in the mould is of no consequence. The pot should be examined. It should retain only a glaze of slag, and if any pasty lumps or globules bf lead remain in it the assay is defective.

If nails have been used in the assay they must be removed before the pouring. With the bent tongs in the left hand hold the pot; with another pair, usually the cupel tongs, seize a nail, wash it in the slag and tap it against the pot, then examine it. If it is free from adhering lead reject it and take another in the same way. If lead is seen to stick to tha niil so th ifc it cannot be washed, it shows that the assay is not finished, either for want of heat or of time, and it must be returned to the furnace. When cooled the assay is turned out of the mould and the slag examined. It should contain no globules of lead. The button is then beaten to a cube, brushed, marked, if so desired, and placed in a tray in readiness for cupellation.

The buttons must be soft and malleable, and separate easily from the slag when cooled. The color of the slag varies with the constituents of the ore, and the proportions of the fluxes. The slag is not always glassy, nor is this important, but it must be liquid, not pasty, when poured.

62 Assaying Gold

Scorification.

Is this method the litharge is made from lead during the operation. Scorification is the surest and simplest method of all. It is well adapted to rich ore, but not to that which is poor, on account of the smallness of the quantity treated, or the time and fuel consumed. However, ores containing nickel, tellurium, or tin, are better scorified, and buttons containing tin or nickel cannot be cupelled until scorified. Ores are scorified with an addition of granulated lead, and generally borax.

Granulated lead, like litharge, contains silver, and must be assayed. Take

Lead, 2 ounces or 60 grammes. Glass, 60 grains or 4 "

Mix in a scorifier. Place in the heated muffle and close the door until the lead is melted, then open the door and maintain a moderate red heat, keeping the lead fused. When the lead is covered by the slag, remove from the muffle and pour into the assay mould. Return the scorifier to the muffle, and as soon as the assay is cool enough to separate the slag, return the lead to the scorifier, and so on until the lead is of a suitable size for the cupel ; weigh the resultingbead, and note its weight and that of the lead used, in the assay book.

And Silver Ores. 63

Ordinary ores require from 8 to 12 parts of lead. Galena, or silver glance, 2 parts. Those containing much iron, zinc, or tin, or of which the gangue is lima, require more lead, sometimes as much as 32 parts to one of ore. Tellurium may require 100 parts. A small quantity of borax is beneficial unless the gangue is quartz. With an unfamiliar ore it is best to make veral assays, each with a different quantity of lead. If that one in which the greatest quantity of lead was used gives the highest result, another should be made with yet more lead, and so on until two assays agree. The following is a general formula for ordinary ores: —

Ore, 60 grains or 4 grammes, Lead, 1 J ounces or 48 grammes.

And a little dried borax, unless the gangue is quartzose. Mix one-half of the lead with the ore and borax in the scorifier. . Cover with the remaining lead. Place in the darkly red-hot muffle. Close the door and increase the heat very gradually* until the lead is thoroughly melted. Open the door and moderate the heat. The ore will be seen floating upon the lead. It soon takes the form of a ring, leaving the lead exposed. This is called the "bull's eye." If the bull's eye does not appear, the assay will fail. This is sometimes caused by the ore not being finely ground. When the bull's eye appears, maintain a cupelling heat by managing the damper and the door of the ash-pit. When the bull's eye. is covered by

*Some writers advise the rapid heating of the assay; with some ores this may do, but it would seem that it must always be dangerous, and with the ore of the Silver King Mine in Arizona it caused sputtering.

64 Assaying Gold

the melted slag, add a couple of lumps of borax, by means of the cupel tongs; close the door, and give a higher heat for about 5 minutes. Meanwhile, heat to redness the suitably bent end of a thick piece of iron wire, then with it stir the contents of the scorifier. If any pasty lumps be felt, add a little more borax and close the door. The stirring must not be prolonged, especially with arsenical ores, or the wire will melt away, and the iron will form a crust on the assay. When the slag is perfectly fluid, the assay may be poured. The scorifier should be returned to the muffle (unless it is much corroded, when a new one must be taken), because the button may be too large, and must then be re-scorified. There is less loss of silver in scorifying the lead than in cupelling it; therefore the more the weight is diminished, within reasonable limits, the better. The button is cleaned and prepared for cupellation as in the crucible assay. The slag should be glassy ; the color depends on the kind of ore. The lead button must be soft.

When very sulphurous ores are assayed, it is best to omit the borax entirely until the bull's eye is covered. When 32 parts of lead must be used, only 30 grains of ore, or 2 grammes, can be treated in a 2-inch scorifier. The scorifiers are handled by means of the scorifier tongs, and may be used until too much corroded, though the same rules apply to these as to crucibles in regard to re-use. As a scorifier may occasionally be pierced by the litharge formed, it is well to have an extra bottom in the muffle. Such a bottom may be bought, or made by cutting another muffle. The muffle bottom, extra or not, should be protected by a thin layer of bone ashc s, which

And Silver Ores. 65

will absorb the spilling in case of accident, and may be scraped out. The scorifiers may be in a measure protected by rubbing the inside with reddle. Scorification furnishes a convenient means of purifying as well as lessening the weight of a button got in the crucible. By this means any substance, except the noble metals, may be removed from lead.

66 Assaying Gold

Cupellation.

By this process lead is separated from precious metal. . The cupel should be somewhat heavier than the button. It is placed in the brightly red-hot muffle by means of the cupel tongs, and the door is closed. When the cupel is thoroughly heated, the button is placed upon it. Again the door of the muffle is closed, until the lead is well melted, when it is opened to admit air. The lead now appears luminous. The heat and the draft through the muffle must be regulated by means of the damper and the ash-pit door, so that the lead smoke rises to about the middle of the muffle, and a portion of it escapes by the front. The button gradually becomes smaller, and colored spots are seen moving upon its surface. These spots are litharge, which sinks into the cupel. The heat should now be reduced ; not so much as that the button shall seem to swim in litharge, but so that a crop of small crystals shall be formed, principally on the front side, where the cool air strikes the cupel. After a time the button appears covered by rainbow- colored rings, which seem to move rapidly. It becomes a *'bead/* which seems to rotate upon its axis. At this instant the cupel is pushed farther back, where the heat is greater, or a glowing coal is placed in front of it. Suddenly the bead becomes still, and in a few seconds more the cupellation

And Silver Ores. 67

is finished. The finishing is called the brightening or bUck.

If the bead consists largely of gold, which is known by its greenish color while fused, or if it is not larger than the head of a large pin, the cupel may now be removed from the muffle to the cupel tray ; not so a larger bead consisting principally of *silver. Melted silver absorbs oxygen from the air, and expels it while cooling. If a large bead is cooled suddenly on the surface, while the cupel is still intensely heated, the gas, when expelled from the interior, breaks forcibly through the solidified crust, carrying with it some molten silver, a part of which is liable to be lost. Such an accident may be prevented by causing the bead to solidify very slowly, or from below upward. This is done in either of two ways: Firstly, by inverting over the bead on the cupel another cupel heat 3d to redness: secondly, and more surely, by drawing the cupel toward the front of the muffle, watching it, tongs in hand, and at the instant when thi baad ceases from tremb ing on the cupel being lightly tapped on one side by the tongs, pushing the cupel back to the hotter part of the muffle. The surface of the bead is thus prevented from solidifying, or is again melted, while the partial ly cooled cupel, absorbing the heat, causes the solidification to proceed from below, theas escaping quietly.* The disruption of the bead as described is called sprouting or vegetation.

At the instant of solidification, the bead suddenly

*Thi8 method was discovered by Win. Trelan, Jr., of San Francisco, assayer. The explanation is that of the writer.

68 Assaying Gold

omits a flash of light, which is very perceptible in those which have been previously removed from the muffle.

The signs of a successful cupeilation are as follows: Some crystals of litharge are seen on the cupel. The bead is well rounded, bright on the upper surface, unless when slightly frosted by incipient sprouting, crystalline below, adhering slightly to the cupel. A bead which does not adhere a little to the cupel contains lead. Too strong adherence indicates that the button contained other base metal, and not enough of lead for its complete removal ; the bead then probably contains copper. This does not often happen in ore assaying. In either of these cases the bead must be enveloped in a small piece of sh6et lead, and recupelled. , As several cupellations may be conducted at once, the cupels may be marked by vertical scratches on the sides, in order to avoid mistakes.* Each

Tho writer never marks the cupels, but places them on the tray and in the muffle in one unvarying order, according to their numbers. When many assays are made at once it is best to work so that those in front shall be finished first, in order that the finishing may be managed, and the cupels removed easily from the muffle at the proper time, for if left in long after the brightening a large loss of silver may result. If the buttons are all of one size there can be no objection to charging them all on to the cupels as soon as the latter are hot enough; otherwise, small buttons which may from their numbers fall to the rearward cupels, should be Reserved until the larger ones nearer to the front have made some progress.

In important assays of rich ore or bullion only so many cupels should be worked at one time as can be moved back or forward, etc., at will.

Finally, in poor and comparatively unimportant assays, extreme particularity is neither requisite nor feasible. The main point is to guard against excessive heat. Practice makes perfect, and rules are for tyros; so take some test silver, and lead, and work away on cupeilation, noting the losses, until you find out what you can do.

And Silver Ores. : 69

set of assays is numbered 1, 2, 3, etc., irrespective of the numbers' of the samples or deposits from which they are made, which, hoiY©ver, must not be lost sight of. On removal from the muffle, the cupels are placed on the cupel tray and carried to the small anvil. The larger beads are seized by the flat-nosed pliers, laid on the anvil, again grasped well within the jaws of the pliers, and forcibly compressed. This loosens any adhering litharge or bone-ash, which is then removed by brushing. Very small beads are removed by the pincets, laid upon the anvil, turned on the side and flattened by a blow from the small hammer, which has the same effect as the compression in the pliers. These should be examined by the aid of the lens, to see that they are clean. Each cleaned bead is placed on a watch-glass bearing the set number of the assay on a disc of paper pasted to the convex side, so that it can be read through the glass. The glasses are ranged on a tray similar to the cupel tray.

Some assayers replace the cleaned beads on the cupels, and thus carry them to the assay balance. It is a questionable practice, and leads to the occasional loss of a bead. If the cupel is placed within the scale case the result is dirt. If not, the bead is usually carried some distance in the forceps, and is liable to be dropped to the floor, where a very small bead cannot be found. The watch-glass containing the bead is placed close to the pan of the balance, and loss of the bead is scarcely possible, or the beads are removed from the glasses and placed in a row within the scale case, then weighed in succession.

The weight of each bead is noted, in a book kept for

70 Assaying Gold

the purpose, opposite to the set and sample numbers. Under the weight is noted the correction due for ilver in the litharge or lead used in the assay, which, being subtracted, leaves the net weight of precious metal obtained from the ore.

Note. — There is always some silver lost ia cupelling, but it is not usual to make a correction unless in the case of assays of very rich ore for the market. The loss may be approximately ascertained by recupelling the bead, before parting, with a weight of lead equal to that of the original button. The loss of weight in the second cupellation will give an idea of that in the first ; but as the heat used and time occupied may be different, the loss may also differ materially in a large bead. A better way is to smelt out another button, add enough lead to the first bead to make a button of equal weight with the new one, and cupel the two side by side. They will then lose equally, as nearly* as may be, and the difference of their weights added to that first got will be the truth, and will be equal to the weight of the second bead plus the loss sustained by the first in recupellation. Thus: —

Weight of first bead 347

Weight of second bead 346

First bead recupelled 342 342

Differences 4 5

Old bead 347 new :Uo

True weight 351 351

And Silver Ores. 71

Weighing The Bead.

When about to weigh on the assay balance, the operator must first see that it is in equilibrium. If this is not the ease, dusting by means of the camel-hair brush, or wiping the capsules with a fine handkerchief will frequently make it so. The extra supports may also require to be cleaned occasionally, as they sometimes adhere slightly to the beam or pans, drawing one side down when lowered. When the weight of a bead or other object is required, it is best placed on the left-hand pan, because the right hand is then conveniently employed in adjusting the weights on the right-hand pan, while the left hand operates the turnkey by which the supports are raised or lowered. (In some balances the extra supports are fixed, and the planes on which the beam rests when in action are movable. The principle is the same.)

When equilibrium is nearly attained the case must be closed at each trial, because the slightest current of air will disturb the action of the balance. It is here that the great convenience of the riders and carriers is realized. The weights having been adjusted to the nearest 10, or lower if so preferred, the case is closed and the operation finished by placing the rider on the beam at that point at which it produces equilibrium. It is better that the beam should swing a little, as that proves it to be

72 Assaying Gold

free. It may be caused to do so, when necessary, by a gentle touch of the carrier. An object should not be weighed while it is hot, as currents of air would be produced which would make it appear too light.

It may be known when the balance is in equilibrium without waiting for the beam to come to rest, which with a very sensitive instrument would be tedious. The oscillations diminish continually so that no two will be exactly alike, but if the mean of two swings to one side is sensibly equal to the intermediate swing to the other side of zero, the balance is in equilibrium. When riders are not used, the fractions of the unit may be estimated by the oscillations of the pointer. They will be of greater amplitude on one side of zero than ©n the other, and the middle point between any two consecutive extremes is very nearly that at which the pointer would come to rest if allowed to do so. More accurately, the middle point between the mean of two deviations on one side and the intervening deviation on the other side of zero, is that at which the pointer would rest. If, for example, the pointer swings 5 divisions to the right then 3 to the left, and again 4 to the right, the mean between 4 and 5 is 4, and the middle point between to the right and 3 to the left is f ths to the right. Or the beam may be allowed to come to rest and the deviation noted.

The value of this amount of deviation must be found by calculation from the observed deviation caused by a known preponderance of weight. Thus, if it has been found by trial that the 1 weight causes the pointer to stop at 5 divisions from zero, the value of 1 division is 0.2 and of f ths of a division is 0.15. In any case the swing

And Silver Ores. 73

is useful as an indication of the weight required, and the assayer should study the action of his instrument under different loads. It occasionally happens that the beam appears to stick, not moving freely. A slight tapping of the case by the finger nails will often remove this difficulty. The bead having been weighed must be parted, unless the ore is known to carry no gold.

74 Assaying Gold

Parting.

Silver is soluble in nitric acid, gold is not. Hence if .we boil an alloy of gold and silver in nitric acid the silver is dissolved, the gold remains. But, in order that the separation may be complete, it is necessary that the alloy shall contain at least twice as much silver as gold, and that it be in the form of a thin sheet unless it contains a much larger proportion of silver. With from 2 to 3 parts of silver to one of gold the separation is perfect (practically speaking), and the gold remains in a single piece. With a larger proportion of silver the gold remains as a dark powder. The beads obtained from an ore assay frequently contain a very large proportion of silver, and are then at once boiled with nitric acid in a test-tube until no more red fumes are seen and the particles of gold have collected together, when the acid is carefully poured off. The tube is then nearly filled with pure water, the gold allowed to settle, and the water poured off (usually twice). The tube is now filled with water and held between the fingers of the right hand ; a dry-cup is inverted over it and retained by the thumb. By turning the wrist the tube is inverted, the gold settles through the walpr to the bottom of the dry-cup, which is now right side up, the inverted tube within it. When the gold has all settled, the dry-cup is held by the left hand, the tube by the right, and the latter is carefully

And Silver Ores. 75

raised, allowing air to enter and water to escape, nearly filling but not overflowing the cup. A s%ht pause, to allow the disturbed gold to again subside, and by a dextrous movement, like breaking it off, the tube is removed from the cup, at the same time being turned mouth uppermost. The spilling of some water from the tube is of no consequence provided it does not overflow the cup, by which gold would be lost, but a skillful operator will scarcely spill a drop. The cup now contains water and the gold. It is gently tapped and manipulated until any separated particles of gold are gathered together. The water is then poured off. In each pouring off of acid or water from the gold, a glass rod, test-tube, or dry-cup, is held to the lip of the vessel containing the gold. Down this the liquid flows steadily without danger of loss of the metal. The gold remains in the cup, is drained as much as possible, and any remaining water is absorbed by the porous dry-cup. The dry-cups, marked with reddle if so preferred and containing the gold, are placed on a tray and taken to the muffle, in which they are placed by means of the cupel tongs (or a special pair with circular jaws), and heated to redness, again placed on the tray, and when cool carried to the assay balance and the contents of each weighed in its regular order. The gold usually adheres slightly to the dry-cup. It must be loosened by the point of the fine steel pincets, and transferred to the weighing capsule by inverting the dry-cup over that and tapping the cup with the handle of the pincets. The weight of the gold is noted under the net weight of the appropriate bead, from which it is subtracted, leaving the net weight of the silver. To those beads which do not contain enough silver for

76 Assaying Gold

the parting, more must be added. The eases in which this addition must be made are as follows: —

1st. If the bead is yellow it requires the addition of 2 or times its weight of silver, according as it is paleyellow or darker.

2d. If, on attempting the parting, the bead is found to be blackened but not much attached, it requires the addition of 2 parts of silver.

3d. If, after parting, the weight of the gold is found to be more than one-third of that of the bead, it must have as much as times its weight of silver added, and be again parted.

This addition of silver is called inquartation. For this purpose some test silver cut in small pieces is kept on a watch-glass in the scale case. For small beads the silver is not always weighed, but a piece is selected which is known to be sufficient. It is better to weigh it ; the gold can then be kept in one, or at most two or three pieces, and there is less danger of loss in the washing.

The silver and the bead are placed in a cornet, or capsule, made by folding a small piece of thin sheet-lead in the shape of a hollow cone, which is then closed and folded as a little packet. This is cupelled, and the bead cleaned as before, flattened on the anvil, annealed by heating to redness, parted, washed, " cupped," heated to redness and weighed. If the gold does not break up in the parting it should be boiled a second time for several minutes in fresh acid before cupping. The acid need not be stronger than 32° Beaum, and will be less liable to bumping or breaking the gold than if stronger. The spirit-lamp is generally used for the boiling in test-tubes. The flattened bead can be annealed on a piece of charcoal

And Silver Ores. 77

before the blow-pipe flame or on a clean cupel in the muffle.

Some assayers prefer to use a small glazed porcelain basin for the parting. Cupping is then unnecessary, the gold being washed apd dried in the basin. As the glazed basin does not absorb the moisture it must not be heated strongly until the gold is almost dried, otherwise the boiling of the water may scatter the particles of metal or even throw them out. The method is convenient when many assays are to be made, because a number can be parted at the same time on the sand-bath. However small flat-bottomed mattresses might be used equally well on a sand-bath, or an arrangement for supporting a number of test-tubes in an oblique position, which is better than the vertical.

Note. — lu case of assaying gold-bearing sand, which, however, is not often done, at least by fire process, the workman must be on his guard againtit platinum and irridium. The first would be dissolved in the parting if not in too great proportion, and would thus appear to be silver. The second rri'ght find its way also into the bead and be reckoned as gold. A b3ad containing platinum does not brighten well nor round up nicely on the cupel.

In suspicious cases take more than the usual quantity of silver for the inquartation. This will cause the platinum to be dissolved and the gold to fall to powder ; then, after heating to redness, any grains of irridium may be distinguished by their color. The silver in this class of ore may generally be disregarded, but to determine that also, the parting acid and wash water must be saved and hydrochloric acid added, drop by drop, as long as it makes a white precipitate. The diluted liquid must then b9 filtered, the filter washed with distilled water, dried, placed in a crucible, some litharge, soda, and a little borax added, and the whole smelted. The filter paper will reduce enough lead for cupellation. From the weight of the bead must be deducted the weisrht of silver which was added for the inquartation; the remainder must be ascribed to the ore.

78 Assaying Gold

Calculating The Assay.

The weights of gold and silver are reported, as already said, in units and decimal fractions of the unit of the system of weights in use. A.s a ton or 2,000 pounds of ore contains fourteen million grains, it is easy to calculate by proportion that, if 240 grains contain 1 -100th grain or 1 unit of the system, a ton would contain 1.215 ounces, which, if gold, would be worth $25.12; if silver, $1.57. These figures are therefore constant factors for half -ounce assays, and multiplying the weight of metal obtained from such an assay in units of the system by these factors gives us the number of ounces in a ton, or the value in dollars. If only a quarter of an ounce of ore be taken for the assay, the value as found by the above factors must be doubled, for it is clear that a half-ounce would have yielded twice as much metal. For a 60-grain assay the result must be multiplied by 4, because 60 is one-fourth of 240, the number of grains in half an ounce. On the other hand, if an ounce of ore is taken, the result must be halved. Oq the same principle factors are obtained for assays of 20 grammes, etc.

And Silver Ores. 79

The following are factors by which to multiply the bead weight reported in units of the respective sets and fractions thereof

Assay of i ounce (240 grains) oz. ton, 1.215

Dollars gold, 25.12 silver, 1.57

Assay of 20 grammes, oz. ton, 1.458

Dollars gold, 30.15

" silver, 1.89

If 10 grammes of ore be assayed and the bead be weighed with a set of weights of which J gramme= 1000, the factors are the same as for 20 grammes when the bead is weighed in milligrammes, because milligramme is in the same proportion to 10 grammes as 1 milligramme to 20 grammes.

An ounce of gold is calculated as worth $20.67; an ounce of silver, $1.2929; but this is only the assay value of silver, the value which it had some years ago, as compared with gold. At present it is worth less, and the value thus found must be discounted from 10 to 20 pr cent., according to the market, to get the commercial value. However, assays are still reported on the basis of $1.2929 per ounce for silver. The best way, though not

*Some assayers state the weight in decimal fractions of the gramme or grain. It is then only necessary to make a corresponding change in the place of the decimal point in the factor. For example: If an assay on half an ounce of ore gives 15-lOOths of a grain of silver, that is 15-lOOOths of 10 grains, and it be written 15, it must be multiplied by 1.57 for the value in dollars per ton of ore =$23.55 ; but if it were written as a decimal fraction of a grain, thus 0.15 grain, ic would have to be multiplied by 157. giving the same result.

so ASSAYING GOLD

customary in California or Nevada outside of the chief cities, is to report the ounces of each metal in a ton of the ore.

*If 291.66 grains of ore be used, 1-lOOth grain 1 ounce per ton.

If 377 grains, 1-lOOth grain=$1.00 per ton silver, or $16.00 per ton gold (nearly).

f-In 29.16 grammes, 1 milligramme 1 ounce per ton.

In 37.8 grammes, 1 milligramme =$1.00 per ton silver, or $16.00 per ton gold (nearly).

One per cent, is 291.66 ounces in the ton. If gold, it is worth $6,029.27; if silver, $377.10.

Following is an example of an assay worked out in dollars per ton : —

No. 1— Sample 297 — concentrated . sulphurets, 240 grains, nail assay.

Bead weight, 7.2

Silver in litharge. .0.5

Factor. . .25.12 Factor. .1.57 6.7

Gold 2.3 Silver... 4.4

7536 628 Gold. 2.3 $57.77 p. t.

Gold. . . .57.776 Silver, 6.908 Silver, 4.4 6.91

Total $64.68

This is the "assay ton " by grains.

t This is the assay ton by grammes.

Thi sign for an assay ton is A. T. As the quantities are rather large it is usual to take an aliquot part of an A. T. of ore and multiply the bead weight accordingly.

And Silver Ores. 81

Assay Of Ore Containing Coarse Metal.

When ore contains coarse metal which cannot be passed through the sieve, the entire sample must be weighed. The coarse metal which remains on the sieve must also be weighed. Its weight deducted from that of the entire sample leaves the weight of the powder. These weights must be noted. The sample then consists of 2 parts, each of which must be assayed. If the coarse metal is not in too large quantity, it may all be scorified with its weight of lead and a little borax, and the button cupelled. Then, as the weight of coarse metal is to the weight of fine metal in the bead, so is the weight of an ordinary assay (240 grains or 20 grammes) to the weight of fine metal which that quantity of coarse metal would give. From this the value of the coarse metal per ton is deduced. The powder having been assayed also we find that the whole sample contained so many grains (or grammes) of ore worth so much per ton, and so many (the coarse metal) worth so much per ton. This is just the same as if we had so many tons of each quality, instead of grains or grammes, and all we have to do is to find the average value per ton of such a mixture. To do this we multiply each quantity by its value per ton, and add the results together. This gives us the total value of the two lots, which, divided by the united weights, gives the value per ton of the mixture.

82 Assaying Gold

Suppose the total weight of the sample to be 565 grammes, consisting of 560 grammes of powder assaying $40 per ton, and 5 grammes of coarse metal assaying $30,420 per ton. Then—

560 tons at $40 ton contain $ 22,400.00 and 5 $30,420 " " 152,100.00

and 565 '' '' $174,500.00

Dividing the total amount by the total number of tons we get $308.85 as the value of one ton of the ore. This is a simple example. If the ore contained both gold and silver it would be necessary to calculate for each, in order to get the average value of the ore for each.

In the case of silver ore containing native silver or silver glance etc., the method generally used is as follows: The entire sample, or a suitable quantity thereof being weighed, is finely powdered and sifted. The matter remaining on the sieve is not weighed but is either scorified with lead or melted in a crucible with litharge, borax, and soda, adding a .little flour; the resulting button is cupelled and the value of the fine silver got is computed for the entire sample at so much per ton. The powder is also assayed in the usual manner for value per ton and the two values are added. The quantity of ore to be used for the assay depends on the coarseness of the grains of metal. It should be sufficient to give a good average of the lot. If the sample submitted is a piece of moderate size the whole must be weighed and ground. In other cases, as a sample of ground ore or concentrations, a convenient quantity is 600 grammes. The weight of fine silver got from the coarse silver then needs only to be divided by 30 in order to find how much of it corresponds

And Silveb Ores. 83

to 20 grammes of the ore, from which the value per ton is computed and added to that found in an assay of the powder. This method is simpler than the other, and, though not mathematically correct, it is considered that the cupellation loss fully counterbalances the error of calculation, which by this method gives the result too high. In the example given, 565 grammes of ore gave 5 grammes of coarse metal which we will suppose to have been silver. As this coarse metal was worth $30,420 per ton the 5 grammes must have yielded 4.0334 grammes, or 80.668 per cent, of fine silver, and if 565 ammes of ore contained 4.0334 grammes of fine silver, 20 grammes would contain 0.1428 grammes, which, as before explained, would be reported as 142.8 $269. 25 per ton, to which add the assay value of the powder, $40.00 per ton, and we have a total value of $309.25, which* in this instance, is but slightly above the result got by the true calculation, and the difference would undoubtedly be compensated by a proper allowance for cupellation loss. . Another example: A sample of concentrations known to contain native silver. Weigh 600 grammes, grind and sift. Refine the coarse metal. It weighs 6 grammes, which, divided by 30 gives 0.2 grammes; 20 grammes of the powder give fine silver 0.342 grammes — total, 0.542 grammes —which would be written 542 and would correspond to 790.42 ounces per ton of the concentrations. As gold is not subject to sensible loss in cupellation we cannot admit this method of calculation when that metal is in question.

84 Assaying Gold

Assay Of Roasted Ore For Solubility.

This is to ascertain what percentage of the silver in roasted ore can be extracted by leaching, which is generally equal to that which can be got by amalgamation. From a true sample of the roasted ore weigh out two assays. Leach one on a filter of coarse filter paper with a cold solution of sodium or calcium hyposulphite of density about 5° Beaum6, until the leach gives no dark precipitate on addition of a drop of solution of a polysulphide. Dry the assay (filter and all) on a roasting-dish in the muffle, raising the heat so as to burn the filter.

Dress and melt the leached and the unleached sample. Cupel and part the buttons and calculate the percentage of the silver extracted from the leached sample.

To make a polysulphide, boil quicklime or concentrated lye with ground sulphur as long as sulphur is dissolved.

To Assay a Cupel. — When it is desired to know how much silver has been absorbed by a cupel during cupellation, grind the saturated part of the cupel very fine and scorify it with borax glass and a little granulated lead. A clear slag will be formed. Cupel the button.

And Silver Ores. 85

Assay By Amalgamation.

This is for working results. Take any convenient quantity of the ore powder, add water enough to make a pulp, and heat it. If gold ore, add about 1 -40th per cent, of cyanide of potassium. If silver ore (not roasted), add 2 per cent, of salt, and such other cheu icals as may be desired in such proportion as can be profitably used on the large scale. Grind with pure quicksilver for several hours in iron mortar or Buck's amalgamator. Separate the pulp from the quicksilver by washing (panning). If gold ore, distil the quicksilver or dissolve it in dilute nitric acid. The gold will remain. If silver ore, strain the quicksilver through a piece of wet buckskin. (Twist the buckskin so as to press the quicksilver through.) Tie the amalgam in a bit of cotton cloth, put it in a dry-cup and heat to redness in the muffle. Collect the metal, gold or silver, wrap it in sheetlead, cupel, part, weigh, and calculate results by percentage.

86 Assaying Gold

To Find The Value Of Specimen.

Handsome specimens of gold in quartz are in great demand. There are many rules for ascertaining the proportion of the gold in such specimens, all based on the different densities of the specimens as a whole, of the gold, and of the quartz. The specific gravity of the gold and of the quartz is generally assumed, the former at from 17 to 19, the latter at 2.6. That of the specimen is found by weighing it in air and in water, and dividing its weight in air by the difference. In this case the easiest rule to remember is this : —

Divide the specific gravity of the gold by that of the quartz and by that of the specimen. From the greater quotient subtract the lesser; the remainder is the proportion of gold. From the lesser quotient subtract 1; the remainder is the proportion of quartz.

Then, as the sum of these proportional quantities is to the proportion of gold, so is the weight of the specimen to the actual quantity of gold in it. Suppose a specimen weighs 32 ounces in air and 28 ounces in water. The difference is 4, and 32 divided by 4 gives 8, which is the specific gravity of the specimen. If the specific gravity of the gold is assumed to be 17 and that of the quartz 2.6, we have 17 divided by 2.6 gives 6.539 nearly, and 17 divided by 8 gives 2.125. Subtracting the lesser quo-

And Silver Ores. 87

tient from the greater leaves 4.414, and subtracting 1 from the lesser leaves 1.125, and the proportion is 4.414 of gold to 1.125 of quartz. Adding these together we have 5.539. Now it is a mere question in the rule of three. If 5.539 ounces of the specimen contain 4.414 ounces of gold, how much does the whole specimen weighing 32 ounces contain ? Answer, 25.5 ounces. This is correct, provided the assumed specific gravities of the gold and gangue are correct, but if greater accuracy be desired proceed thus: —

Weigh the specimen, a piece of the metal, and a piece of the gangue, each in air and in water. Divide the diiSerence between the respective weights in air and in water by the weights in air. From the greater quotient subtract the next less and from that the least. The first remainder is the proportional weight of the metal, the second is that of the gangue, the sum of these is that of the specimen. Having the actual weight of the specimen that of the metal is easily found as before by the rule of three.

Suppose a specimen of native silver in spar weighs 84 pounds in air and 73.5 in water. A piece of the silver weighs 27 grains in air and 24 in water. A piece of the spar 20 in air and 12 in water, then —

Spar in air 20

Spar in water 12

Difference 8-:-20= 0.400

Specimen in air . . 84 Specimen in water 73.5

Difierence 10.5-:-84= 0.125

88 Assaying Gold

Silver in air 27

Silver in water. . . 24

Difference 3-:-27= 0.111

And 400 — 125= 275= proportion of silver " 125—111= 14= " spar

289= specimen

Then 289 : 275 : : 84 : 79.93 pounds of silver in specimen.

The difference between the weight in air and in water of a piece of rock, metal, etc., may be found in several ways, according to the facilities at hand, as follows: —

Weigh the object on scales or steelyard of suitable delicacy. Suspend the object by means of a hair, a fine thread, or a wire, according to its weight, under the pan of the scales or from the hook of the steelyard ; submerge it in water and again weigh it. Subtract the second weight from the first.*

Or, weigh the object; take a suitably-sized vessel full of water, or in which the water stands at a certain mark, and weigh that and the object together if convenient, or if not weigh them separately and add the results. Now remove some of the water, place the object in the vessel, replace so much of the water as to raise it again to the mark and weigh again. The difference between the last weight and the sum of the weights of the object and of

Some scales are provi'ied with a from which the object can be suspended above the pan. A little bench of sheet-brush or tin is then placed across the pan, leaving the pan free to move beneath it, and a vessel containing water in which the object is submerged is placed on the bench.

And Silver Ores. 89

the vessel of water, is the weight of the water displaced by the object, which is equal to the diiSerence of the weight in air and in water. This method is suitable for sand. Special bottles, called specific gravity bottles, graduated to hold a certain weight of water, may be bought with counterpoise. Price, $1.75 to $3.50; capacity 100 to 1,000 grains; also, 100 grammes.

The weight of the object in air, divided by its difference in water, is the specific gravity. The difierence, divided by the weight in air, is the specific displacement. From either of these the proportional quantities of two diiSerent substances composing a mixture can be determined bv the rules given for the valuation of specimens.

90 Assaying Gold

Tests For Ores, Etc

Silver. — Powder, mix about a grain with twice as much fine lead (free from silver), soda, and borax. Moisten with water, place in cavity scooped in piece of charcoal, smelt by blowpipe flame, cupel the bead, examine bead with lens, if large enough to handle, part for gold. Or, powder, roast, boil in glass or earthen vessel with a clean strip of copper, bluestone, salt and water; gives a white coat on the copper. (Nearly all silver ores will do so if boiled with bluestone and salt, without roasting.)

Gold. — Powder, roast if sulphurets are present, grind very fine and wash in pan or horn, examine with lens ; yellow flattened particles not dissolved by nitric acid.

Copper. — Powder, moisten with salt brine and throw into fire; an intensely blue flame. Or, moisten with muriatic acid, and direct blowpipe flame on it; the same. Or, roast, steep in ammonia; a blue solution. Or, boil in acid and add ammonia; the same.

Lead. — Powder, mix about one grain with 4 grains of soda, a little borax, and water to moisten, place on charcoal and melt under blowpipe flame; gives a malleable, metallic globule, which when melted on a cupel gradually disappears, leaving a yellow stain and perhaps a bead of

And Silver Ores. 91

silver; or a coat on the charcoal which is dark lemonyellow hot, sulphur-yellow cold; disappears on being heated but not touched by the blowpipe flame.

Bismuth. — Powder, treat on charcoal same as for lead, coat darker than that of lead, coat heated but not touched by flame, melts into brown globules; globule of metal, if got, is brittle and reddish in color. It becomes covered by brown crust which cracks during solidification, showing the bright metal, or the globule gives birth to a smaller one which sprouts from its side and remains bright; best seen on a cupel. This is the only metal besides lead which can be cupelled alone.

Zinc. — Powder, throw into fire, brilliant white flame; treated on charcoal same as for lead, coat yellow hot, white cold; coat heated but not touched by flame, remains and is luminous, touched by flame disappears. Black zincblende is often mistaken for galena. The two may be distinguished by this infallible sign — the powder of galena is hlach, that of blende brown or yellow.

Tin. — Powder, mix with soda (and cyanide of potassium if at hand), heat strongly on coal, covering with blowpipe flame; white metallic globules which do not sink into a cupel when melted on it, but form a crust which is yellow hot. Nitric acid converts the globule into white powder, insoluble in water. Coat on coal similar to zinc but closer — does not disappear when heated whether* touched by flame or not. Rare in United States.

Antimony. — Powder treated as for tin, white coat on coal extending a long way — easily driven about by flame ;

92 Assaying Gold

globule, if obtained, thrown on table while hot rolls along, giving off dense fumes. Nitric acid converts the globule into white powder, which vanishes in smoke if heated on charcoal.

Black Oxide of Manganese. — Powder, add muriatic acid or sulphuric acid and salt; effervesces and smells of chlorine, which if abundant and the test is made in a testtube or small bottle, may be seen as a yellowish-green gas. Melt a trace with borax on platinum, beyond the point of the flame; an amythistine bead indicates manganese, and as it evolves chlorine from muriatic acid it is the bin-oxide or " black oxide,'* and an oxidizer.

Chromium (in chromates, etc). — Powder, melt a very little with borax on charcoal in blowpipe flame; an emerald-green glass.

Iron. — Powder, boil in muriatic acid and a of nitric acid, add a drop of solution of potassium f erro-cyanide (yellow prussiate of potash) ; a deep blue color shows iron present. But treat another portion with muriatic acid alone and add the ferro-cyanide; if the same color is not now produced the 'iron is not in a form to be an ooddizer.

Tellurium. — Heat a fragment, or a little of the powder moistened, by blowpipe, on a piece of white porcelain; moisten the still hot porcelain with strong sulphuric acid; a redcolor — tellurium.

Arsenic. — Powder, throw on live coah or heat on charcoal within the blowpipe flame; a garlic smell — arsenic.

And Silver Ores. 93

Selenium. — Test as for arsenic; a smell of rotten horseraddish — selenmra.

Sulphur. — Melt with soda on charcoal, then moisten and place on a piece of silver; a dark stain indicates sulphur. But heat to redness in a scorifier or on an iron spoon ; if a smell like that of burning matches is not perceived, the sulphur is not in a form to be a reducer.

' Quartz or hard silicate. — Not scratched by knife, not affected by acid; melted with 2 parts soda froths and makes clear glass.

Earthy Carbonates (lime, magnesia, baryta). — Scratched by knife, foam if touched by acid, dissolve with effervescence in muriatic acid to a colorless solution; burned in fire become caustic like quick-lime, and no longer foam with acids.

Earthy Sulphates (gypsum, heavy spar, etc.). — Scratched by knife, not affected by acid; show sulphur by the test with silver, but give no smell on spoon.

Nitrates (Chile nitre, etc.). — Flash when heated on charcoal ; dissolved in water with sulphuric acid and salt can dissolve gold.

Chlorates. — Flash on coal like nitrates; with water sulphuric acid and nitre can dissolve gold.

Borates (borax, borate of lime or cotton balls). — Moistened with glycerine and held in blowpipe flame give green color to it. Or, treat with sulphuric acid in porcelain dish, add alcohol and set fire to it; a green flame. No copper must be present in either test.

94 Assatinq Gold

Alkaline Carbonates (of soda or potash). — Dissolve in acids witii effervescence, and do not lose this property by being melted; dissolved in water, and lime-water added, give a white precipitate.

Clay. — Breathed on gives a peculiar smell, easily recognized when once known.

Blowpipe. — Plain brass tube 7 to 10 inches, 25c to 36c

With moisture-bulb, 7 to 9 inches 50c

Black's, all Japanned , 50c

Brass, with trumpet mouth-piece, condensing chamber and platinum jet $2 00

ISame, only all nickel plated 2 25 Fletcher's, with hot blast (the tube encircles the flame of the lamp, and becomes heated . and dries the air) 1 00

By placing the tip within the flame of a candle or lamp, and blowing into the larger end, a pointed flame is produced. To make a cupel for use with the blowpipe, scoop a circular cavity in a piece of charcoal, fill it with very fine moistened bone ashes, press with a large bullet or other convex object, and dry. A button to be cupelled should be heated but not touched by the flame.

To make test lead free from silver. — Dissolve acetate of lead in water and add a piece of zinc. Wash the resulting lead with water and a little sulphuric acid, then with water. Dry and test for silver, which will not be found unless the zinc contained it, in which case the zinc is not fit.

OF THf

And Silveb Ores. 95

A Few Special Minerals.

SiLVEB Glance (Silver and Sulphur, 87 per cent silver). — Black, heavy, soft, malleable, sectile (can be cut in slices). Alone, BB* yields nearly pure silver.

Horn Silver (Silver and Chlorine, 75 per cent silver). — Pearly, yellowish, or greenish; exposed to light turns dark; heavy, waxy, malleable, sectile. With soda BB yields pure silver. Iodide and bromide are very similar.

Ruby Silver — light and dark red silver — (Silver, Sulphur, and Arsenic or Antimony). — Red to black, powder red, brittle, tender. BB with soda yields silver.

Brittle Silver (Silver and Antimony). — Black, powder black, brittle, and very tender. BB with soda yields silver which may contain a little copper.

PoLYBASiTE (Silver, Copper, Sulphur, Antimony and Arsenic). — Black, brittle, rather hard. BB with soda yields an alloy of silver and copper.

Stromeyerite (Silver, Sulphur and Copper) — Gray, rather hard. BB with soda yields silver and copper.

Galena (Lead and Sulphur, 86 1-2 per cent lead). — Nearly black, shining, heavy, rather tender, breaks in

BB means before blowpipe, on charcoal.

96 Assaying Gold

square blocks. The fine-grained kind generally carries antimony. BB with soda yields lead.

Carbonate op Lead {Lead Oxide and Carbonic Add, 66 per cent lead), — White, yellow, or nearly black, greasy looking, rather hard, froths with acid. Alone, BB yields lead.

Iron Pyrites (Sulphur and Iron). — Yellow to bronze, shining, not scratched by knife, strikes fire with steel. Alone, BB gives abundant fumes of sulphur. When roasted, before, not in the flame (or in the muffle) until it smells no more, is a red powder. Arsenical pyrites contain also arsenic ; color silver- white. Often carry gold.

Copper Pyrites (Copper, Iron and Sulphur). — Yellow, shining, often irridescent on surface, scratched by knife. BB with soda (after roasting) gives grains of copper and iron. Purple copper is very similar. Sometimes carry silver.

Copper Glance (Copper and Sulphur). — Nearly black, rather soft, sectile. BB with soda (after roasting) yields copper. This mineral is sometimes mistaken for silver glance. The two may occur combined.

Gray Copper (Copper, Zinc, Antimony and Arsenic, and sometimes silver and mercury). Dark gray. (See tests for silver, etc.)

Copper Carbonates (Copper Oxide and Carbonic Add). — Bright blue or green, of ten crystallized, froth with acid, dissolve in ammonia with beautiful blue color. BB with soda yield copper

And Silveb Orbs. 97

Red Copper {Copper cmd Oxygen). — Deep red, tender, ' does not froth with acid, dissolves in ammonia, and turns blue by exposure. BB with soda yields copper.

Zinc-blende (Zinc and Sulphur), — Black, red, yellow, green, or colorless (rarely), powder always light colored. (See test for zinc and sulphur.)

SuLPHURET OF ANTIMONY (An4:imony and Sulphur), — Gray, shining, usually fibroids or in stars, tender, and will melt in candle flame. Alone, BB flies away in smoke, leaving white coat on coal. (See test.)

Assaying Ooid

Solubility Of Metals.

The following-named metals act as stated in the different liquids with the aid of heat. It must be understood that not the ores of metals, but the metals themselves, as smelted out by blow pipe or otherwise, are meant: —

In moderately strong nitric acid —

Dissot/ve, Silver, copper, iron, lead, bismuth, zinc, mercury.

In strong sulphuric acid — Dissolve,

Silver, copper, bismuth, zinc, tin, mercury, antimony. Lead is converted into sulphate, and dissolved if enough of acid is present.

In dilute sulphuric acid —

Dissolve, Iron, zinc, tin.

Not, Gold, platinum. Tin and antimony are converted into white powder.

Not, Gold, platinum, iron. Lead is converted into sulphate, and partly dissolved.

Not, Gold, platinum, silver, copper, lead, bismuth, mercury.

And Silyeb Ores.

M

In hydrochloric acid — Dissolve,

Iron, zinc, bismuth, antimony in powder. Tin slowly in an abundance of strong acid.

Not, Gold, platinum, silver, copper (unless in very fine powder), lead, mercury.

In mixed nitric and hydrochloric acids —

Dissolve, Gold, platinum, copper, iron, bismuth, lead (with water), zinc. Tin and antimony with excess of hydrochloric acid.

Silver and mercury are acted on, but not dissolved. Antimony remains as a white powder if nitric acid is in excess.

In solution of caustic potash — zinc and tin dissolve.

100 Assaying Gold

Substitutes And Expedients.

If litharge cannot be conveniently got, pulverized galena may be used in its place in crucible assays by the second system. It should contain no gold and but little silver, and the assays must be corrected for that which it does contain. The weight used should be at least equal to that of thi ore, unles> the latter contains lead, when less will answer. Lead may be obta ned from galena by fusion in a crucible with pieces of iron, iron ore, or soda. It may be granulated by means of a rasp.

Ground glass, mixed with a little soda, will answer in place of borax.

The baking soda sold in the stores will serve as well as any for making assays; also soda ash.

Bone ashes may be made by burning and grinding bones. The soft bone from the horns of cattle is best.

A common table-knife will answer for a spatula.

A coarse sieve may be readily made from a tin pan by suitably puncturing its bottom. A fine one by stretching a piece of gauze on a frame or hoop of wood or metal-

A flat rock will answer for a mortar, or gr nding-plate, a large pebble or small bowlder for a pestle or muUer.

A crucible laid on its side will serve as a muffle. A quicksilver flask, of which one end has been removed, is better. A hole should be made in the rear ♦ nd of the

And Silver Ores. 101

A furnace may be built of rocks and mud, or of mud alone, or adobes. A blacksmith's forge may be used for both melting and cupelling, the cupel being placed on a piece of brick or iron in the midst of the fire, or in any kind of muffle.

A scorifier or a roasting-dish may be formed by hand from good white clay, or carved from soap-stone.

A cupel may be made by pressing moistened bone ashes into a hole bored in a piece of wood, and then burning the wood. Or, a square wooden or metal clamp which can be opened may be used for a mould.

Pincets may be made by bending a strip of sheet metal pointed at both ends. Or, for coarser ones, a piece of stout wire, the ends flattened by filing.

For an anvil, any flat piece of iron will do; the head of an axe or hatchet struck into a block.

A strip of paper folded lengthwise forms as good a testtube holder as need be, by being doubled, the tube in the loop and the ends held injthe hand.

A teacup or part of one will serve for parting in, being warmed with care. A Chinesesmall porcelain basin is better, and is useful in other ways.

A water-bath is any vessel containing boiling water, on which a pan or dish can be placed as a lid.

Reliable pulp and assay scales may be improvised by very cheap and simple meansjif the workman understands the principles of such things, not otherwise. Any attempt to impart a knowledge of these principles here would carry this work too far.

Assay Table.

Fob Two Hundbed and Forty Grains of Orb.

Bead .

OcKCES Ih a Ton OF THB Orb.

Vaidb (in dollars)

/ OF SlLYBB.

Value (in dollars) OF Gold.

u.t.h.th.

u.t.h,th.

u..

u. t. h th.

100488269

125610336

160732403

u. t.h.th.

176854470

200976687

u.t.hXh.

226098605

Find the right-hand figure of bead weight in the column of "Bead Points/* and the figures for ounces or value under the corresponding head. Set the figures down, or as many of them as may be wished, pointing off:—

For tenths u, etc.

For units . . . . . all to the right of u

For tens " " t

For hundreds... " " h

For thousands.. " " th

AND SILVER ORBS. lOS

Proceed then with the next figure, setting down this value with the decimal point under that of the first, and so on, then add all together. Example — a bead=42.7 required the ounces in a ton : —

2- units... 2.43

tens 48.61

42.7 51.89 ounces.

It will be found convenient to take a pointer (a dry pen or a pencil), and when inspecting the table place it between the figures where the decimal point will fall, and then read and transcribe the value. In this way the superfluous figures to the right need not be noticed, and all risk of confusion will be avoided. A little practice will be needed to give facility.

Assaying Gold

Assay Table.

Fob Twbnty Grammes of Ore.

Bead

Poikts.

Ounces In a Ton OF THB Orb.

Value in Silver, (dollars.)

Value in Gold, (dollars.)

u.t.h.ih.

u.t.h.th.

u.t.h.th.

u.t.h.th.

u. t. hik.

u.t.h.th.

.24102493

This table is used in the same maimer as the preceding one. There is also another use to which it can be put which the author believes to have been first noticed by himself. It is finding the equivalent, in troy ounces, of any number of pounds and decimals of a pound avoirdupois. Multiply the pounds by 10, consider the product as milligrammes of metal got from a 20-gramme assay,

And Silveb Obes. 105

and find by the table the corresponding number of ounces. To find the value of a bar of which the weight is given in pounds, multiply the pounds by the fineness and that product by 10. Consider the last product as milligrammes of gold or silver, as the case may be, and take the values from the table as though for an ore assay.

Assay Table.

Fob one Assay Ton op Ore.

Ouncbs.

Gold— Valuk.

Silver — Valuk.

u.t.hM. $ 2067183

u.t.h.th. $ 1292929

4

u.t.h.th.

t.h.th.

10343434.

This table is used in the same way as the others, but as the ounces per ton are got at once from the bead weight, only values per ton are required. It is also applicable to finding the values of bars, and the value per ounce of bullion of any given fineness, as will be explained in the second part of this work. All these tables have been extended so that two figures can be operated on at once, one great advantage of which is that, whenever two or more figures are to be operated on, the work can be done in two wars, and both it and the table checked, in less time than by calculation. It is the author*s intention to have all the tables printed in extenso and in large type on cards, for convenient use.

, Nevin'S .

Roasting Furnace.

This Is A

Compound Cylinder Furnace,

Expressly adapted to the roasting of highly sulphuretted and refractory ores for leaching or amalgamation. It acts continuously, receiving the ore from a battery or pulverizer, and discharging it ready for the pans or leaching vats. It roasts ore on the old and approved plan of oxidizing before chloridizing thus effecting a great saving of salt, and leaving the ore in the best condition for the after-treatment. This has not heretofore been done in any furnace of this class. The furnace is also adapted to the deadroasting of sulphurets for chlorination.

One of these furnaces is in use at the Navajo Independence Mill, at Tuscarora, Nevada. The Superintendent of the Navajo says in his report that 3,955 tons of ore were worked to 92 3-5 per cent, of the battery assay. The Superintendent of the Independence reports: "The Navajo Mining Company milled our ore for us, and worked it to 93 per cent, of the battery assay. Settlement was made at that percentage."

Send for Circular to

San Francisco, California.

OAEE fTNA IBOV WOSES 217 Fremont Street.

The Mining and Seientifie Press

If th leading mining ioanutl in America, and enjoys a large drcalation among the more intelligent operators and workers in the gold and silver fields of the world. Bitablished in 1B00 it has firmly Tnaintained its ixMition as a reliaUe, standard joomaL The mformation given in its oolamns has saved millions of dollars to the practical miners, metallarsts, mill men, mine and share-holders on the western side of our continent. Among its contributors are the ablest and most experienced mining engineers, superintendents and practical miners in this country. The gives weekly a condensed summary of mining operations in the most important mining districts of the country. New processes and methods of mining are described in its weekly issues. New mining machinery and improx'ements are frequently illustrated and explained in a manner to be df grt value to all interested in mining. Subscription price, $4. a year in advance. DEWET & CO., Publishers, No. 262 Market Street.

Our

Wm anl Scientific Fresss

PATKlffT AKM€Y

Was Established in 1860.

It has kept step with the rapid march of

mechanical improvements. The records in

its archives, its constantly increasing library.

the accumulation of information of special

importance ro our home inventors, and the

experience of its proprietors in an extensive

and long continued personal practice in patent business, affords them combined ad-

vaiftages greater tlian any other agents can possibly offer to Pacific Coast inventors.

Circulars of advice free. Address, DEWEY & CO., No. 252 Market Street, S. F.

V/m. D. JOHNSTON, Assayer and Analytical Chemist,

113 Leidesdorfif Street, Between California and Sacramento Streets, San Francisco.

; "Personal attention insu res Correct Retums."

ANDBEWS* IMPBOYED AKAIiGS'AIIIIATOR.

For durability and execution, it cannot be excelled. It <mly requires tlie action of the water to collect or amalgamate the finest gold; and the product is fiir in advance of any and all machines. Knowing Uie great loss of fine gold and quicksilver has led the inventor to place before the public a machine that is a peer to all otiiers. The capacity according to size, from fifty to one thousand tons per day.

E. O. Andrews.

Vo be seen at MeeliaBles' Fair or 747 Karket titreet.

JOHN rr. YLOR.

H. R. Taylor.

KSTABT.ISHKr) ISGS.

John Taylor & Co.,

mroBniu and dbalbes ik .

Assayers' Materials,

And General 8Uppue8 For Mimes And Mills,

Including A Fuu Supply Of Chemicals.

OEBTIiIlffCt'S, BKCKBB'S and TBOi:aK9lfEB't AtSAlir and BVIiUON BA£iAltf€B8 and WfilOHT.

Sole Agents For

BArrrBBSBA (London) MUFFIii:*, CBUCIBIillt and tftCUBIFI-

BBS. Also for CBBMANIA £iBAD and IiFTHABOB, made expressly (or Assayerf ' ase, and free from silver. We have the lead in pigs and small bars, also rolled and granulated, for sale in quantities to suit. A liberal discount when taken in quantity

lilTHABOB put up in tin cans and bags or sold in smaller quantities, as de. sired.

CjffiBMICAIi APPABATU8 in aU its variety. We are always prepared tc make, at short notice, new apparatus for any special purpose.

Agents for OBBTXIItfO'S (London) BECKBB'S and TBOBMSBB'S AAir and BVLLtlOlff BAIxAlirCEtS and WBIHTS.

JOHN TAYLOR & CO., 118 & 120 Market Street, and 15 & 17 California St.

San Franoisoo, Cal. Our Gold and Silver Tables, showing the value per ounce Troy at different degree i of fineness, and valuable tables for computation of assays in grains and granunes, wil sent free on application. Also our new illustrated Catalogue, with prices.

416 Montgomery Street, San Fraiicisco.

Gold and Silver R refinery a nd Assay Office.

HIGHEST PRICBS PAID FOlt

O-old SilTTer SLZidLXiesidL Ores sizidL SvLlpli-vureta.

Manufacturers Of Bluestone.

A.1SO LK-AU iPIPK, SHKKT LK-AID, SHOT, B2TO.

This Company has the best facilities <hi the Coast for working

GOLDs, SILVEK and

IN THEIE VAEIOUS FOKMS. PRENTISS SELBY, - - Superintendent.

Tnstin's Ore Pulverizer

Works Ore Wet Or Dry.

Awarded Silver Medals in 1882 and 1883 by Mechanics' Institute.

Manofactured at the Tustin Windmill, Horse-power and Pnmping Machine Works,

308 Mission Street, San Francisco, Oal.

By W. I. TUSTIN, Inventor and Patentee. Send for Circular.

F. P. BACON. Prehidbnt,.

C. L. FOUTS, .SKRBTARy

THE ULOBE IRON IfORKS CO.,

Manufacturers and Repairers of all Kinds of

Machinery And Iron Castings.

— And Builders Of —

LOCOMOTIVES, HOISTING and

Mining Machinery.

Portable, Stationart And Marine Engines.

Dyer Cannon Ball Quartz Mill.

— Hamufacturebs Op Thb—

Dybr Gannon Baz.I.

OFFICE AND WORKS: SSS and 8S4 Fremont Street. SAN FRANCISCO. CAZb

$1,000 Challenge!

Th[ Fru[ Ore Conoentbiitor.

Or

Over &DD&rfi duw Lu ii§e, uWiag antiru HatlafaiJtbEL. S.itvfla from 40 io 100 per cent, man than n; other CaijcifsiitriiitaT jn nae, anfi pDntntratJotiB afe dean from the tirst wurkintf. The wear o-niX tear ate moroly nditimaU

A maihiine can iA Fieen hi workhiff order, &ni rerndf tu make tests, at the olflaa of Hjnisklej-j tpiera it Hayas, 20 Fremont Htreut-

To tliose intendmg to manufacture or pnroliase tlie so-oallei Triumph*" Concentrator we herewith state:

That ial aJvl.jH h&: been flven that aU ahitkinj j/mtiitn ap])lJtid to liu diitjCtfJU (fcDvfiJi belt tbioJ For eunuentiiLtiQn of ores iM va intrhigstiient un patfitj huli an 1 dwn bathe True Vannizi Mo-chine Compftny.

That has bien cfmmuaiiced in New York ag'&Enat an end-shake mauhms similar tu the Triumph, and tFiat aaonnas ilec-idon U reached tn thecourti there proceed hiMK will be taken ogaJLuHt aU Western JnFrinienioiits.

Adams & Carter,

Agenta Frue Vanning Machine Company, Rcom 7, 109 California Sp!?lTp"B?4H FRANCISCO, CAL.

'i!Z9J

SPP 39 1921 DEC SS III f

Yb 12357