To determine the amount of gold metal in ore, there are two kinds of gold assay adopted:
- The dry assaying method (i.e. by fusing the powdered ore with or without fluxes).
- The wet assaying method (i.e. by the agency of liquids).
In the principal wet assay, the ore is thoroughly dissolved in acids, and, by the addition of reagents, precipitates containing the metals are thrown down.
In some assays, particularly those of copper, iron, zinc, and silver, a standard solution of known strength is added to the original solution by allowing it to drop gradually from a graduated burette, and when a certain change of colour has been produced, by reading off the graduated mark at the top of the liquid column in the burette the amount of metal in the ore can be accurately determined by a slight calculation. At the same time more simple methods will, if not strictly accurate, give good results, and are more likely to be adopted by the prospector.
Then there is the assay by mechanical means (for instance, the separating of the lighter portions from heavier by means of water, as in the “ panning out ” of gold in a deposit).
In dry assays, crucibles or scorifiers capable of standing very great heat, without breaking, are generally used for conducting the operations, and in these the powdered ores, with or without fluxes, are exposed to heat in a furnace, the temperature varying according to the nature of the ore.
The principal fluxes employed are:
Carbonate of Soda, or Potash, which forms fusible compounds with silica.
Borax, which forms fusible compounds with lime, oxide of iron.
Glass, Silica, Fluorspar, Litharge, and others.
Reducing Agents are used, such as charcoal powder, cyanide of potassium.
Oxidizing Agents, such as atmospheric air (removing sulphur, in the roasting process), nitre (which is very rich in oxygen), litharge, salt.
Desulphurizing Agents (for removing sulphur), such as air (in the roasting process), iron nails, carbonate of soda.
Agents to remove Arsenic, such as atmospheric air (in roasting process), nitre.
Collecting Agents (for collecting silver or gold), such as lead, mercury.
Various methods.— Fluxes, reagents.—General treatment of ores. — Preparation of the sample.—Weighing.—Assay ton.—To construct a simple button balance and to use it.—Dry assay for gold and silver.—Apparatus and procedure.—Fusion in a crucible.—Scorification.—Cupellation.—Indications of the presence of metals known from cupel stains.—To make cupels.—Dry assay for lead in galena, tin, antimony.—Wet assays for gold, silver, lead, copper, iron.—Roasting.—Mechanical assay of ores.
Preparing a Sample for Gold Assay
Specimens to be assayed should not be chosen to elicit a “ good assay ” only. They should represent dressed ore ready for shipment. When an average portion of rock has been selected, it should be carefully powdered, if possible, in a mortar, or, in the absence of a mortar, broken up into a few pieces; and these, rolled up in cloth or paper, should be powdered between two hard rocks. To prevent fragments from flying out of the mortar, a loose paper cover, with a hole in the centre for the pestle to pass through, will suffice. Some substances, especially those of a quartzy nature, will be rendered easier to crush by first being heated and thrown into water. If the ore does not contain metallic particles, the operation of powdering and sieving is comparatively easy; when, however, metallic fragments are mixed up with the bulk of the ore, they are very apt to become flattened out by hammering, and do not always present a metallic appearance. In this condition they may refuse to pass through the sieve, and an inexperienced person, not understanding that they may be the most valuable fragments of the sample, is inclined to throw them aside. In reality, they should be collected together and most carefully examined.
When fragments of the ore adhere to the mortar, a little powdered coke or charcoal should be stirred about in the mortar.
When a dry assay or analysis is intended, the best sieve to use is the one of sixty meshes to the inch ; when an ordinary wet assay, the eighty-mesh one ; but for the separation of heavy metals, such as gold, tin, from the lighter matter, by means of water and motion, the ore need not be powdered very finely. A piece of fine muslin will, in the absence of a sieve, answer ordinary purposes tolerably well, if, when the powdered ore be placed in it, the muslin be gathered together at the corners and shaken gently. After the specimen has been thoroughly powdered it should be put back into the mortar and stirred a few times by the pestle in order to evenly distribute the light and heavy particles, and then by a quick overturning of the mortar deposited on a piece of dry paper (glazed if possible). The powder may then be gently mixed by a knife or spatula, and if there be too much in quantity divided into quarters, and one or more divisions selected for the assay. The ore can then be weighed very accurately on the ore balance, after which it is ready for assaying. If the assay is one for gold and silver, the resulting button of precious metal is naturally very small (and to weigh which the very delicate button balance is used), so that great accuracy in the original weighing of ore is necessary, as the following calculation has to be made :—If a weight of ore yields a certain weight of metal, what weight of metal in ounces will a ton of similar ore yield ? If the ore is assayed for ordinary metals, such as lead, then weight of resulting metal/weight of sample of ore x 100 = percentage of metal in the ore.
For weighing gold, silver, or platinum, the troy weight is sometimes used ; for weighing other metals, avoirdupois. The decimal system of grammes and decimals of a gramme is convenient for both.
The management of the button balance requires very great care, and should never be used except for the precious metals, as the ores, fluxes, must be weighed on a less delicate balance. To adjust and thoroughly understand the reading of the button balance needs instruction, and no one should use one until the working of it has been explained. It may be well, however, to mention that the glass slide should always be kept down except during the weighing operation, and that the apparatus should never be by any means exposed to acid or other deleterious fumes.
A very good plan is to use the conventional assay ton weights in weighing the ore, as, by this conventional system, the number of ounces of precious metal in a ton of ore may be known according to the amount of milligrammes, the button of precious metal weighs.
Thus, in America, a conventional assay ton (A.T.) weighing 29·166 grammes may be used (where 2,000 lbs. = 1 ton); or in British countries one weighing 32·667 grammes (where 2,240 lbs. = 1 ton). Still, there is no occasion to know the exact weight of the piece of metal used as an A.T., so long as the operator knows how to read a balance where A.T.’s are made use of.
If 1 A.T. of ore yields a button of 1 milligramme, a ton of ore yields 1 oz. troy of precious metal.
One-tenth A.T. is a very convenient quantity of ore to take ; for if the button weighs x milligrammes, this represents 10 x oz. of precious metal per ton of ore.
In the absence of a proper balance, the following may be of servicc :—
Procure from a carpenter a very thin strip of pine wood (about one foot or fifteen inches long and one-third of an inch wide). Place a fine needle across by means of wax, or through the middle. Next obtain a piece of sheet tin or other metal (one inch by half-inch), and bend its edges up perpendicularly one quarter-inch on each side. On these upturned portions place the needle ends. Should the beam not balance properly, trim either end by shaving off very thin pieces until it does. Now divide the strip into twenty equal parts, i.e. ten on each side of the middle, and mark them 1, 2, 3, so that the 1 marks may be nearest the middle and the 10 marks at the ends.
Three weights are required:
One grain: Can be obtained by weighing out a piece of thin brass wire (ends bent together) on a chemist’s balance
One tenth grain: To obtain this, place the one-grain weight on the 1 mark of the wooden balance and place such a smaller piece
of wire, bent at the ends on the 10 mark on the opposite side, as will cause the beam to balance properly.
One-hundredth grain: To obtain this, place the one-tenth grain weight on the 1 mark, and a piece of thread or such like material on the 10 mark on the other side as will cause the beam to balance properly.
Place it on the 10 mark and see if 1 grain on 10 mark (opposite side) exactly balances it ; if it does, the button weighs 1 grain. If the wire weight be too much, move it towards the middle of the beam to a division, until it is a little lighter than the button. Leave it on this mark. Then take the one-tenth grain, and, commencing from the end of the beam, move it towards the middle until the division reached is that one where this weight together with the first weight is just lighter than the button. Then proceed with the one-hundredth grain in the same way.
Suppose, now, that the one grain weight be at 8, the one- tenth grain at 7, and the one-hundredth at 3, the weight of the button is .873 grains, that is, a little more than eight- tenths of a grain. A rule of three sum then determines the amount of precious metal per ton of ore.
If a certain weight of ore yields eight-tenths of a grain, how many grains will there be in a ton of similar ore ? (N.B. There are 32 666 troy ounces in one ton.) The number of ounces of precious metal in a ton will be known.
DRY ASSAY FOR SILVER AND GOLD
In a gold and silver assay, the precious metals in the sample, either by the scorification or “fusion in a crucible” method, have to be absorbed by lead, and the resulting button of lead containing the gold and silver has to be cupelled in the muffle; the final result being that the precious metals are left on the top of the bone-ash cupel as a shining globule.
As an assaying apparatus or “outfit” is to be obtained complete in a chemical apparatus shop, there is no occasion to enter into too much detail, the portable furnaces manufactured for cupellation in a muffle being made expressly for prospectors and assayers. The most necessary articles are the following:
An ore and button balance with weights, two or three muffles, Hessian crucibles, scorifiers, cupel mould, crucible, scorification and cupel tongs, pokers and scrapers, an iron pestle and mortar (or a plate and rubber), box sieve (80 mesh), spatula, hammer, bone-ash for making cupels, litharge, borax, carbonate of soda, iron nails, nitre, coke, charcoal, test tubes, acids, brush for cleaning the buttons.
How to light the fire
First, place some dry twigs and paper or wood shavings or chips, and above this slightly larger wood round about the outside of the muffle, and set light to it. Then throw in pieces of charcoal, coke, or anthracite coal broken into small pieces about the size of hen’s eggs. Shut the mouth of the muffle and the grate door. Raise the temperature as high as possible for the scorification process.
Though fusion in a crucible is very convenient for poor gold and silver ores, inasmuch as a greater charge can be used at once than in a scorifier, the scorification process is however, the usual one for ordinary ores.
Assay of Gold and Silver Ores by Scorification:
Half the lead should be mixed with the powdered ore and placed in the scorifier; the other half should be spread over this, and the borax on the top. The scorifier may then be placed in the muffle and the door closed until fusion is complete. Then the door may be partly opened and the temperature raised until the surface is covered with litharge, the whole time being about half an hour. The scorifier can then be taken out by the tongs and the contents carefully poured out into an iron cup or mould. When cool, the button of lead (which contains the gold and silver) should be detached from the slag, cleaned by hammering, and then, in the shape of a cube, is ready for cupellation.
If Fusion in a Crucible be desirable, the following formulae are to be recommended :—
For ore, chiefly of rock—
The more quartz in the ore, the more carbonate of soda should be used; the more iron and other metallic bases,
the more borax. The ingredients should be well mixed together and a little borax placed on the top. The crucible should be heated, though not too rapidly at first, until the contents are quite liquid. This will take about twenty minutes. After which it may be removed and the contents poured into the iron mould. When cool, the lead button should be detached from the slag, cleaned, and beaten into the shape of a cube; it is then ready for cupellation. Fusion for silver and gold bearing copper ores and sulphides. Weigh the ore and roast it before fusion is commenced:
While the muffle is in the process of heating; place the empty cupel inside, and when the proper temperature of the furnace is reached, known by the cherry-red colour, gently, by means of the cupel-tongs, place the lead button (containing the gold and silver) obtained from the scorification or “fusion in the crucible” method into the concave hollow of the bone-ash cupel. Close the door of the muffle until the temperature of the fused metal is the same as that of the muffle. The behaviour of the assay can be observed through a slit at the side or top of the door. The assay must not be allowed to “freeze” (“freezing” is known by the fumes ascending right to the top of the muffle), nor must it be too hot (being too hot is known by the fumes scarcely rising at all, and the outline of the cupel being indistinct). If inclined to “ freeze,” a piece of charcoal may be put into the muffle to increase the heat, and the fire stirred. When the proper temperature is attained, the fumes from the cupel should reach about half-way up the height of the muffle, the cupel should be red, and the metal very luminous, while a stream of fused matter circulates about on the surface of the molten liquid. The button gradually becomes more convex, and at last a mirror-like speck of bright silver or gold, or both, is left, The cupel should then be gradually drawn by means of the cupel tongs to the muffle door, so that the metal may not “spit,” which it might do were the cupel to be too suddenly cooled in the cold air. In form the little button should, if a proper one, be well rounded, crystalline below, and easily detached from the cupel. As the button may contain both silver and gold, it should, after being cleaned by brushing with a paint brush and weighed, be removed and subjected to the action of nitric acid, in order that the silver may be dissolved and the gold left in the form of a dark powder; after this the gold may be weighed, and the original weight of the button, minus the weight of the gold, will represent that of the silver.
N.B.—To separate the two metals in the button, place the button in a test tube with about ten times its weight in nitric acid (dilute), and boil for about a quarter of an hour ; the silver will be dissolved and the gold left. The liquid should be decanted, a little pure nitric acid poured on the gold powder to make sure that no silver remains, and the liquid poured off and the gold washed and dried. If the appearance of the button suggests that it is rich in gold, some silver must be fused with it before acid is poured on, as unless there be three times the amount of silver as gold, the “ parting,” as the above process is called, will be incomplete.
Indications of the presence of metals in the ore known by cupel stains:
Antimony—pale yellow to brownish red scoria; sometimes the cupel cracks. Arsenic—White or pale yellow scoria. Cobalt—dark green scoria and greenish stain. Copper —green or grey, dark red or brown. Iron—dark red brown. Lead—straw or orange colour. Manganese—dark bluish black stain. Nickel—greenish stain; scoria, dark green. Palladium and Platinum—greenish stain ; the button will be very crystalline. Tin—grey scoria; tin produces “ freezing.” Zinc—yellow on cupel; the cupel is corroded.
How to prepare Bone-ash Cupels
The ash of burnt bones (that of the sheep or horse is preferable) should, in not too fine nor too coarse a state, be mixed with water (about an ounce of water to a pound of one ash), so that it may, when of the proper consistency, adhere together when pressed, although not stick to the fingers. Place a metal disc—a coin if it fits well—into the bottom of the cupel mould, and then fill the cavity with bone-ash ; place the hammer with the convex base on the top of the ash and give it a smart blow by a mallet or other hammer. The cupel can then, by means of the finger, be pushed uppermost and out of the mould.
Assay for certain Metals other than Gold or Silver
To find the amount of lead in Galena, the usual lead ore.
—powdered ore, two or three times the weight of carbonate of soda, three iron nails (tenpenny) placed in , the top for taking up the sulphur, and a cover of salt or borax. The assay may be conducted in a muffle or other furnace.
—two-thirds full of ore and fluxes—should be heated to redness, and the temperature gradually raised until the operation is finished, which will be in about twenty or twenty-five minutes.
The contents of the crucible are to be poured into a mould, and, when cool, the lead button separated from the slag.
Weight of button/Weight of ore sample x 100 = percentage of metal.
As galena always contains more or less silver, the resulting button ought to be assayed for the precious metal in the cupel. As a cupel does not conveniently absorb much more than its own weight of lead, the button may have to be divided into two or more portions, and each of these cupelled separately.
Galena may be roughly assayed for lead by placing the powdered ore, without fluxes, in an iron dish, and exposing it to the heat of a blacksmith’s forge.
To assay Copper ores by the crucible method, including the refining process, requires much practice, and for this reason the “ wet assay ” is the more suitable for obtaining an approximate estimation of the amount of metal in copper ore.