One of the principal conditions of success in the process of cupellation is an accurate propor tionment of the ore and the flux. Black flux is preferable to charcoal, but soap is better than either, as it penetrates every ramification of the mass, and its carbon is brought into close contact with every particle of the ore. Alkaline fluxes are objectionable, so far as they cause an ebullition of the mass ; and if sulphur be present, it will retain a portion of the gold.
When the gold is in fine grains, such as wash-gold or amalgam, it may be purified by mixing it with a little calomel or corrosive sublimate, heating it at first gently, and then raising the heat until it melts the gold. This operation should be performed in a new crucible ; the silver and other metals are then lost by evaporation. The separation of gold from some metals, such as platinum, iridium, and copper, cannot be effected by the above method of cupellation. Indeed, it is useless to attempt to separate them by a dry method ; the alloy must be subjected to an elaborate chemical analysis, which it is out of our province to describe.
Assay of Iron.—The assay of iron ores by the dry method is not reliable, and, moreover, requires a much greater heat than is usually at the command of the explorer. We shall describe here a method by the wet way, known as the bichromate of potash method, which for accuracy, simplicity, and quickness leaves nothing to be desired.
The standard solution of bichromate of potash is made by dissolving about 305 grains of the pure salt in 4 pints of distilled water. The operator now takes 10 grains of pure iron and dissolves it in pure, weak hydrochloric acid. He then dissolves 3 or 4 grains of red prussiate of potash in half a pint of distilled water, to form the test solution. The remaining apparatus required is an ordinary grain burette, and a square white porcelain slab. He next dissolves in strong, boiling hydrochloric acid io a flask 10 grains, or, if it be poor, 20 grains, of the iron ore to be assayed. The solution of pure wire is then placed in a porcelain basin and diluted with water. A small quantity of the biohromate solution is run into it from the burette, and the contents of the basin are stirred meanwhile with a glass rod; the test is made as followe :—Dip a glass rod in the solution of prueaiate, and make several small drops with the adhering liquid upon the slab, which must be perfectly clean and dry. Wipe the rod, and, after each addition of solution from the burette, stir up the contents of the basin, and take up a single drop upon the end of the rod, letting it mix with one of the drops of teat solution on the slab. When not the faintest trace of a blue coloration appears when the two drops come into contact with one another, the test is finished.
Read off the number on the burette showing the number of grains of solution used, and note it down carefully. The solution of the ore is now diluted with water, and about 30 grains of crystallized sulphite of soda are added to it. It is next heated gently until quite colourless, and then well boiled to expel the sulphurous fumes. The remainder of the process is precisely the same as iu the case of the solution of the wire ; the liquid is transferred to a porcelain basin, and the standard solution is run in until a drop of the contents mixed with a drop of the prussiate solution produces no blue coloration. It is at first rather difficult to sea the last few tints of blue on the slab, but after a few trials it becomes perfectly easy. The calculation is simple in the extreme. If 10 grains of pnro iron required the number of grains of the bichromato solution, previously noted down, then a simple proportion sum will give the amount of iron represented by the number of grains of biohromate solution just employed to neutralize the assay solution. When the strength of the standard solution is correctly known, any number of teats may be made with it in an hour, and the degree of accuracy attained after a little experience with this method is surprising.
Assay of Lead.—The assay of lead ores is not a very difficult operation, but it requires care, inasmuch as the metal is very volatile, and if the heat employed in smelting be raised too high, a considerable quantity may be lost by evaporation. It is always conducted in the dry way. The commonest and best method is that recommended at the School of Mines. The ore is finely powdered and a sample carefully taken. A weighed quantity is then mixed with its own weight of carbonate of eoda, one-fourth of its weight of borax, and a small quantity of argol ; most of the borax and a portion of the carbonate should be reserved to place on the top of the mixture in the cruoible. Tho latter should be an iron pot, of convenient size, and shaped like an ordinary crucible. Having heated it first to a low red heat iu the furnace, the above mixture is carefully introduced, and after a short time the heat is raised, keeping the top of the furnace closed. When the contents of the pot are in a etate of complete fusion, the heat must be raised as high as poasible for a few seconds, in order to cause all the reduced lead to run together in a globule. This done, it is allowed to cool for a moment and then poured out. The bottom, when cool, is carefully cleansed from adhering impurity and then weighed. There is always a certain amount of lead lost by volatiliza tion, but with careful work the loss is less by this method than by any other.