In gilded copper goods the quantity of gold is very trifling, still, however, it is of consequence to separate it from the other metals ; this is done in two ways : 1st, The gilded metal is fused and granulated, after which it is again fused, and flowers of sulphur are gradually added, till the whole becomes dark-coloured. When cold, it is reduced to powder, and amalgamated, as al ready described when treating of the ores of gold.--By -this means the mercury combines with the gold, and leaves sulphuret of copper.
2d. The metal is brought to a state of fusion, and about an equal quantity of a mixture of litharge and sulphur, previously fused together, is added to it, and the whole is well stirred. Charcoal in fine powder is then thrown in, in small portions at a time, the mixture being constantly stirred with a copper rod. By this means the litharge is deprived of its oxygen, and the lead falls to the bottom, carrying the gold along with it.
The alloy of these metals is then subjected to the process of cupellation, by which the gold is obtained pure.
Tile quantity of silver on plated copper goods is often so great, as to make it an object of considerable im portance to be able to separate this metal from the copper.
The process for separating silver and copper, when the former is in large quantity, has been already ex plained, but the proportion of the latter metal in plated goods is so great, that the silver cannot by these means be obtained from them, because they will not bear to be alloyed with a sufficient quantity of lead ; recourse must therefore be had to other methods.
The best mode of separating copper and silver in plated goods was recommended by Mr. heir. This gentleman discovered, that an acid composed of sul phuric and nitric acids did not act on copper, while it dissolved silver ; he therefore applied this to the sepa ration of these metals. For this purpose, 1 lb. of nitre is dissolved in 8 lb._ or 10 lb. of sulphuric acid, with the aid of a gentle heat, in a glazed earthenware dish. Pieces of the goods are then thrown in, and the heat is raised to about the 200dth degree. When the whole of the silver is dissolved, the fluid is poured off, and the metal is precipitated by muriate of soda, and is obtained in the usual way ; or the solution is diluted with water, and pieces of copper are immersed in it, by which the silver is precipitated in the metallic form, the diluted acid act ing on the copper.
There is still another method of separating silver from copper, by means of the black oxide of manganese, when the silver is in large proportion.
The process consists in subjecting thin pieces of the alloy, surrounded by black oxide of manganese, to an intense heat, by which the metal is melted, and a black powder is procured. This is mixed with three times its bulk of pounded green glass, and again exposed to a strong heat, after which the silver is obtained pure at the bottom of the vessel. In the first part of this process the silver and copper are oxidated, and in the latter, the oxide of silver is reduced by heat alone, whereas the oxide of copper requires the presence of inflammable matter for its reduction.
Platinum.
Owing to the infusibility of Platinum, it is with great difficulty obtained in mass, so as to have it in a state fit to be wrought into different articles. The simplest and the cheapest method of working Platinum, is that prac tised in France. For this purpose, the grains of Pla tinum are mixed with an equal weight of white arsenic, and one-third of pearl-ashes. This is thrown, in suc cessive portions, into a red-hot crucible, and well stir red with a platinum rod. When the whole is in a state of fusion, the crucible is removed from the furnace, and the mixture is allowed to cool. It is then taken from the pot, and bruised, and again subjected to a similar process. If after the second fusion the metallic matter is magnetic, it must be a third time treated in the same way.
After this it is mixed with an equal weight of white arsenic and a small quantity of pearl-ashes, and again fused in a flat earthern dish, in which it is allowed to cool.
It is next exposed to a heat in a muffle, sufficient to expel the arsenic, but not so high as to fuse the mix ture. It is then, when hot, plunged into oil, and the exposure to heat and immersion in oil repeated, till the whole of the arsenic is driven off ; the heat being gra dually increased as the metal becomes less fusible. After the arsenic is expelled as completely as can be effected in this way, and the charcoal of the oil is con sumed, it is digested in nitric acid, and then boiled in water to remove impurities. Several of the pieces of metal thus obtained are put together, and then exposed to a high temperature ; in which state they are struck so as to make them adhere. The mass is then heated in a forge and beat on an anvil, by which one compact piece of metal is procured.