α. Cupellation. This operation, the most important of the whole, has been already described. Unlike silver, gold will bear the highest heat of the furnace without ‘vegetating,’ ‘fuming,’ or being absorbed by the cupel. The loss of weight gives the amount of copper in the alloy.
β. Quartation. The cupelled sample is fused with three times its weight of pure silver (called the ‘witness’), by which the gold is reduced to one fourth of the mass, or less, and in this state may be easily removed.
γ. Parting. The alloy, after quartation, is hammered or rolled out into a thin strip or leaf, curled into a spiral form, and boiled for a quarter of an hour, in a small flask, with about 21⁄2 to 3 oz. of nitric acid (sp. gr. 1·3); and the fluid being poured off, it is again boiled in a similar manner with 11⁄2 to 2 oz. more of nitric acid (sp. gr. 1·2), after which the gold is carefully collected, washed in pure water, and dried. When the operation of ‘parting’ is skilfully conducted, and the acid not too strong, the metal preserves its spiral form; otherwise, it falls into the state of flakes or powder. The second boiling or digestion is technically termed the ‘reprise.’ The loss of weight by ‘parting,’ after deducting that of the ‘witness,’ corresponds to the quantity of silver originally in the specimen.
δ. Annealing. This consists in putting the pure gold obtained by the last process into a small porous crucible or cupel, and heating it to redness in the muffle.
ε. Weighing. This must be done with the utmost accuracy. The weight, in grains troy, doubled or quadrupled, as the case may be, gives the number of carats fine of the alloy examined, without calculation.
According to the ‘old French method’ of assaying gold, the following quantities are taken:—For the assay pound, 12 gr.; fine silver, 30 gr.; lead, 108 gr. These having been cupelled together, the (perfect) button is
rolled into a leaf (11⁄2 × 5 inches), twisted on a quill, and submitted to parting with 21⁄2 oz. and 11⁄2 oz. of nitric acid, sp. gr. 1·16 (20° Baumé). The remainder of the process is similar to that above described. Two assays are made in the same manner, with a third on pure gold or gold of a known fineness; and no conclusion is drawn, unless the assay of the latter comes out accurately, and that of the first two correspond to each other.
For alloys containing platinum, which usually consist of copper, silver, platinum, and gold, the method of assaying is as follows:—The alloy is ‘cupelled’ in the usual way, the loss of weight expresses the amount of copper; and the button, made into a riband and treated with sulphuric acid, indicates, by the portion dissolved, that also of the silver present. By submitting the residuum to quartation, the platinum becomes soluble in nitric acid. The loss after digestion in this menstruum expresses the weight of that metal, and the weight of the portion now remaining is that of the pure gold. Gold containing palladium may be assayed in the same manner.
2. In the wet way:
The richness in gold of any substance, whether liquid or solid, when the quantity is small (and indeed in all other cases), is most simply and economically performed by the common method of chemical analysis. The gold may be thrown down from its solution by adding a solution of protosulphate of iron; the precipitate, after being washed, dried, and gently heated, may be weighed as pure gold.