DRY OR FIRE ASSAYING Gold and Silver Ores.—Gold and silver are bought by the ounce (Troy), and, for convenience in the evaluation of ores, the contents of precious metals are not reported in percentages, but in oz., or decimals of an oz. (Troy), per ton of ore. The ton (Avoirdupois) contains 32,666.6 oz. (Troy), and, to save calculation, the weight of ore taken for assay is 32.67 grammes, or some multiple of this. This weight is known as the Long Assay Ton (A.T.), and has the advantage that every o.00i gramme of metal found represents i oz. (Troy) per ton of ore. In some countries it is more usual to employ the Short Ton of 2,000 lb., and the Short Assay Ton of 29.17 grammes is used in assaying. In many of the Latin countries the Metric Ton of i,000 kilos is the standard, and the values are reported in grammes per ton, no Assay Ton being required. Sometimes the gold or silver present is reported in its money value (e.g., $42.50 per ton). The assay consists of three operations: (i) fusion, (2) cupel lation, and (3) parting.
(I) Fusion.--This may be carried out by two methods.
(a) In the "Pot Fusion" method, one A.T. or more of the crushed ore is mixed and fused in a clay crucible, or pot, with lead oxide (about 3o grammes), a reducing agent such as char coal, argol, or flour (up to about 5 grammes), and a suitable quantity of reagents called "fluxes" which combine with the "gangue," or waste matter in the ore, forming a "flowing," or fusible slag. The fluxes used depend upon the nature of the ore. If the gangue contains silica, tin 'r zinc, soda is required; for iron, lime, or other basic materials, borax, crushed quartz, or similar fluxes are introduced. The quantity of these fluxes is varied to suit the requirements of the ore under examination, and will be nearly twice the weight of the ore. The crucible is placed in a coke, oil, or gas furnace at a fairly low temperature, and kept at a dull-red heat for about ten minutes, to allow the chemical reactions to be completed. By the action of the reduc ing agent, metallic lead is formed, which sinks through the charge to the bottom of the pot, alloying with, and so collecting the gold and silver. The temperature is then raised until the whole charge is molten and thoroughly fluid, and the contents of the pot are poured into a dry, iron mould. When cold, the brittle slag is broken away with a hammer, leaving a "button" of lead which contains the precious metals.
Certain types of ore require special treatment. Sulphide ores may be given a preliminary roasting at a red heat to burn off the sulphur, or the ore is desulphurised during fusion by adding suitable reagents to the charge. Cupriferous ores are treated with acid before fusion, to dissolve away the copper. Arsenical ores are roasted before fusion, and antimonial ores are oxidised by adding nitre to the charge.
(b) In the "Scorification" method, about io grammes or A.T. of ore is mixed with 30-35 grammes of granulated lead in a "Scorifier," which is a shallow fireclay dish. Another 30-40 grammes of granulated lead are placed on top of the charge, and about i gramme of borax is added as a thin cover. The scorifier is placed in a muffle furnace at a very high temperature, when simultaneous oxidation of the lead and of the ore takes place. Some impurities volatilise, and the remainder combines with the oxidised lead and the borax, forming a slag. A ring of this slag soon forms round the surface of the molten lead, and, as oxidation proceeds, the ring extends towards the centre and finally covers the whole surface. At this point the oxidation ceases, and the charge is poured as before. The quantities of granulated lead and borax required will depend upon the nature of the ore, and are variable. It will be seen that, as a smaller quantity of ore is used, the method is only suitable for rich ma terials, and hence is practically confined to the assay of rich silver ores, very high grade gold ores being rarely found.
(2) Cupellation.--By this process the gold and silver are isolated from the lead obtained in the first operation. A cupel is a shallow cup made of bone-ash or some other absorbent matter, which, when hot, is capable of absorbing any molten material which wets its surface. The lead button is fused upon a red-hot cupel in a muffle furnace. The lead melts and oxidises, forming molten litharge, which is absorbed, together with any oxidised impurities, into the cupel. The gold and silver do not oxidise, but remain on the cupel, forming a small bead of "Bullion," which is weighed.
(3) Parting.—The bullion bead is attacked with nitric acid which dissolves, or "parts," the silver from the gold. Before commencing this operation, it is necessary to ensure that the alloy is of such a composition that the whole of the silver will be dissolved away from the gold. The proportion, by weight, of silver to gold should be at least 4:1 if the gold present exceeds 0.01 gramme, and this ratio is increased for smaller amounts of gold. If there is insufficient silver present, more must be added by the process of "Inquartation." This consists of adding a sufficient weight of silver to the bead, either by melting the metals together with a blow-pipe flame, or by recupelling them in a small piece of pure lead. The name "Inquartation" was introduced in early times, when it was believed that the proportion of silver to gold should always be 3:1. The inquarted alloy is flattened under a hammer and dropped into boiling nitric acid. The silver dissolves, leaving a brown residue of gold. This is washed with distilled water, dried, and annealed by heating it to redness, when it takes on the familiar yellow colour of pure gold. The weight of this residue gives, by calculation, the gold content of the ore, and the silver is determined by difference from the weight of the bullion.