A regular fire assay is not within reach of many prospectors, for the necessary apparatus cannot, as a rule, be carried in the wilderness. Whenever possible, however, a fire assay gives the truest results, especially in the case of gold and silver. The operation includes testing the ore, sampling and pulverizing, weighing the ore and reagents, calcination and roasting, reduction and fusion, distillation and sublimation, scorification and cupellation, inquartation and parting the gold and silver, weighing and tabulating.
"Notes on Assaying" by Dr. Ricketts is a very useful manual to have at hand. A pair of scales for weighing ore and buttons of base metal. It should take 10 ounces in each pan, and show I -20 of a grain. A bullion scale to be kept strictly for the precious metals. Loaded with one gram, it should show 1 -20 of a milligram. Weights. Avoirdupois; troy, metric and "assay." Assay weights save much calculation. The unit of the system is a weight of 29.166 gram. Its derivation is as follows: 2000 tbs. : I A. T. : : I oz. Troy : I milligram. To use this system, weigh out one A. T. of the ore and whatever number of milligram of gold and silver the assay gives indicates an equal number of Troy ounces to the ton of 2000 lbs. Avoirdupois.
A muffle and a melting furnace, portable and of medium size, are handy, though furnaces may be built of ordinary brick, lined with fire brick, that would be better for permanent use. The fuels may be coke, anthracite or bituminous coal, charcoal, oil or gas. Crucibles of black lead, French clay, Hessian sand, and quicklime are necessary to hold the assay. Roasting dishes, scorifiers and cupels are required. The cupel is made of the ashes of burnt bone, and it is better to make them on the spot, as the bone ash may be carried anywhere without damage, whereas the cupels are very fragile. The bone ash is moistened with water, stamped in a cupel mould, and allowed to dry slowly. A good one will absorb its own weight of lead, but it is better to calculate on its absorbing but three-quarters of that amount. The crucible, scorification and cupel tongs, a couple of hammers, iron pestle and mortar, sieves from 20 to 100 mesh, and scorification mould complete the requisite tools.
In addition, however, the assayer will require quite a bulky lot of apparatus, reagents and chemicals. All dealers keep lists of assayers' supplies on hand, and a full and complete assortment will cost about $200 in New York or Chicago. Quart bottles, with glass stoppers; ordinary corked bottles, ring stands, alcohol lamps, wash bottles, test tubes, horn spoons, iron pans, parting flasks, annealing cups, glazed black paper these will suffice, provided the assayer has, as well, the outfit recommended for blow-pipe work. Dry reagents, such as litharge, borax (crystallized), silica, cyanide of potassium, yellow prussiate of potash, argol, charcoal, starch, metallic iron, pure lead, nitre, powdered lime, sulfur, carbonate of ammonia and common salt are necessary. As solvents and precipitants, distilled water, sulfuric, nitric and hydrochloric acids, chloride of sodium, nitrate of silver and sulfideted hydrogen are also indispensable.
This will seem rather a formidable list, and so, under certain conditions, it may be; indeed, where means of transport is limited, all regular assay work must be postponed until the return to civilization. Assaying is not, however, difficult, being mostly a matter of rule of thumb, and correct results may be arrived at without a deep knowledge of chemistry, although such knowledge will never come amiss. A preliminary examination will show what the ore probably is. The blow-pipe is especially useful, though to the skilled assayer often unnecessary. The ore is first powdered, and any metallic flakes picked out and tested separately. A fair sample must be selected, otherwise all the work will be thrown away and the result be valueless. The next step is weighing the ore and the reagents. Moisture is drawn off by heating in a crucible, a low heat being sufficient. Roasting will eliminate sulfur, antimony, arsenic, etc., and must take place in a "flat dish, so that the air may have free access. The powder should be stirred frequently. Reduction is the operation of removing oxygen, and it takes place usually in a crucible or scorifier. Scorification consists in placing the ore in an open dish with proper reagents, and collecting all the volatile ingredients in the slag. Cupellation, on the other hand, collects them in the bone ash, of which the cupel is composed.
When silver must be separated from gold, it is sometimes convenient to increase its proportion by the addition of some known weight of the inferior metal. After fusing, the globule is placed in nitric acid, and the silver parted from the gold, which may then be weighed. This result subtracted from the weight of the original globule gives the amount of silver. To test an ore for gold, take a pound of it, crush in mortar and pass through a fine sieve. Take one fourth ounce Troy of the powder. Place in scorifier with an equal amount of litharge. Cover with borax that has been melted and powdered, and put the scorifier in the muffle of the furnace. A blacksmith's forge might do at a pinch. Heat until the mass has become a fluid, possibly twenty or thirty minutes. Next pour into the scorification mould, and, after the slag has set, remove it with a hammer. Hammer the button into a cube and place it in the cupel, which must first have been thoroughly heated. Heat until all the base metal has been absorbed by the cupel and the button has "brightened," or flashed; when this occurs, remove the cupel to the front of the muffle, cool, and remove the button with pincers. Weigh it, and you have the amount of gold and silver in Bounce Troy. A simple sum in proportion gives the amount in a ton. All ores containing sulfur, arsenic, antimony, or zinc, should be roasted.
There are three stages in the scorification process; roasting, fusion, and scorification. During the first, the heat should be moderate until fumes cease to be given off; during the second, the heat is raised and a play of colors is seen on the surface of the lead; in the closing stage, the heat is lowered for a time until the slag covers the lead, when it is again raised for a short time and the scorifier removed. Brittle buttons may be due to arsenic, antimony, zinc or litharge, and must be re-scorified before cupellation, with more lead. Take the cupel slowly from the fire to avoid "spitting," by which portions of the buttons are lost. Watch closely for the brightening.
Silver is volatile at a high heat, but when the muffle is almost white, the metal well fused and clean, the fumes rising slowly, and the cupel a cherry red, all is going smoothly. If the fumes rise rapidly, the muffle is too hot. On the other hand, dense, falling fumes show the temperature is too low. Lead that is poor in silver stands the highest heat without vitiating the assay. When the material in the cupel "freezes," i.e., the absorption by the cupel stops, reject the assay and try again, giving more heat or more lead.
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Gold and Silver Prospecting Basics
