The refining of gold is performed in different ways, according to the metals with which it is mixed. Of these the most common is culiellation, or testing, the former term being employed when the operation is performed on a small, the latter, when it is practised on a large scale.
Gold is one of the metals which is not oxidated by heat and air, while the other metals, with which it is usually alloyed, except silver, pass into the state of oxide, at a high temperature. This constitutes the pro f ess of cupellation, which is merely the separation of the precious from the baser metals, by means of oxida tion. When, however, the base metals are in small quantity, compared with that of the gold, the oxidation is not complete ; but if a metal, which is easily oxidated, and the oxide of which is very fusible, be added to the alloy, the oxidation of the base metals is promoted, and they arc completely separated. The metal always employed is lead, and the quantity added depends on the proportion of gold which the alloy contains. (For a particular account of the process of refining, see in this article, p. 77.) If lead be the only metal with which the gold is al layed, the process of cupellation is sufficient for its purification ; if, however, which is usually the case, sil ver and copper be also present, other methods are ne cessary to free it from these metals. When copper only exists, the gold is mixed with silver and lead, and then subjected to cupellation, by which the base metals are removed, but the gold still retains the silver ; it is necessary, therefore, to have recourse to means for separating these two metals. When the gold amounts to about one-fourth of the alloy, the process is car ried on by means of nitric acid. Silver is easily acted on by nitric acid, while gold is not ; if, however, the gold exceed what is stated above, the whole of the silver is not dissolved ; it is necessary, therefore, for this process, that the alloy employed do not contain more than one-fourth of gold ; if it do, it must be fused with poorer alloy, i. c. alloy which contains a great deal of silver. When in this state, it is poured into cold water, by which it is reduced to powder, or granulated, as it is called. The granulated alloy is then put into which are vessels of a pear shape, about 12 inches long, and 7 wide at the bot tom. Into each of these about 40 ounces of the alloy are put along with nitric acid, so that the acid cover the alloy to about the depth of two or three inches. These
are gently heated on a sand bath, and when the action has ceased, the solution is poured off, and another quan tity of acid is added, and the process is repeated a third time, which is, in general, sufficient to dissolve the whole of the silver. The last portion of acid, as con taining little metal, is used as the first quantity, in the next purification ; the two first are decomposed by the —immersion of copper plates, by which the silver is de posited in the metallic state. The gold left after the action of the acid is washed with warm water, till the fluid which comes off does not alter the colour of a piece of copper, and is then fused in a crucible with nitre and borax, by which all impurities are removed.
When the proportion of gold in an alloy is much smaller than that mentioned, a different process is some times followed for separating it from the silver. For this purpose, the alloy is melted and granulated. About seven-eighths of it are then mixed with about onc eighth of the flowers of sulphur, and the mixture heated in a covered crucible, first slowly, and afterwards till the whole is brought into fusion, in which state it is kept on the fire for about an hour ; one-third of the alloy kept out is then added, and thoroughly stirred with it by means of a wooden rod, and the whole is again heated for about an hour. Another third of the alloy is afterwards mixed with it, and after a similar operation, the remaining quantity is added, and the mix ture kept fused for about three hours, during which it is frequently stirred. It is then, when the surface has become quite white, poured into cones greased in the inside. When it is solid, it is found to have separated into distinct parts, the upper is sulphuret of silver, the lower is a metallic button, composed of gold and silver. If the former contain gold, it is exposed to heat in an open crucible, by which part of the sulphur is dissipated, and the silver set free, unites with the gold, and col lects at the bottom of the vessel as the matter cools. The metal thus obtained is repeatedly subjected to the same process, till the alloy procured contain a large enough quantity of gold to admit of its separation from the silver, by the process of parting already de scribed.