VALUATION OF ORES AND MINEBALs.—Having ascertained by examination the nature of the mineral, the assayer now proceeds to examine it further with a view to determining its value for smelting or other extracting purposes. The methods employed to effect this are very numerous. In many cases, the whole of the metal contained in a weighed quantity of the powdered ore may be obtained by subjecting the sample to a strong heat in a furnace with the aid of reducing agents, or flukes. In others, it is necessary to employ the longer and more tedious operations of analysis by the wet way. Only the simplest and reediest methods will, however, be described here. It will be assumed that the operator understands the use of the balance, and is, to some extent, accustomed to the manipulation of apparatus, reagents, &c.
For the operations which have to be performed on a large scale in the dry way, the following apparatus is usually required :— Crucibles.—These are of different kinds, according to the use for which they are intended. A full description of each will be found in the article on Crucibles. It is sufficient to say that the most useful to the assayer are those made of a mixture of fireclay and sand, commonly called Hessian crucibles. Those made of plumbago, porcelain, and platinum are also sometimes indis pensable.
Furnaces.—A description in full of the ordinary forms will be found in the article on Furnaces, but those of special construction, used exclusively for assaying purposes, will be described under the metal for the extraction of which they are employed.
Cupels.—Tliese are small, round dishes, made generally of bone-ash or other porous material, which is capable of resisting high temperatures. They are made in the form shown in Fig. 280, and vary from A in. to 2/ in. in diameter.
Balance.—Two balances will be found sufficient for ordinary purposes, one for rough, and one for delicate work. The former should carry 3 or 4 lb. in each pan, and should turn easily with a quarter of a grain. The other should carry 100 grains in each pan, but should turn quickly and distinctly with of a grain.
Other necessary implements are crucible-tongs of all sizes, stirrers, a sand-bath, drying-oven, iron mortars, &c., &c.
Assay of Antimony.—There are two methods of assaying the ores of antimony in the dry way, one being employed when the ore contains sulphur, as is generally the case, and the other when no sulphur is present ; these will be described in turn. Owing to the ease with which antimony is vaporized, its assay must rather be considered as a commercial approximation than as being rigorously exact. When the pure sulphide is operated upon, its examination will be of but little value, since every 100 parts of that mineral correspond to 72.8 of antimony. According to Mitchell, the best method of assaying the crude sulphide, which is by far the most important ore, is to mix it inti mately with 4 parts of cyanide of potash, and then to heat the mixture gently in an earthen crucible. Only a low heat should be employed in order to avoid as far as possible sublimation of the metal. A bead of pure metallic antimony is found at the bottom of the crucible after the operation.
The assay of antimonial ores containing sulphur may be performed by roasting them, and then fusing the oxidized residue with black flux, a substance formed by deflagratiog 1 part of saltpetre with 2 or 3 or more parts of argol (crude bi-tartrate of potash); generally 1 part of saltpetre and 2/ parts of argol are taken. Or the assay may be performed by directly fusing the crude mineral with the addition of black flux and finely divided metallic iron or with iron scale. Owing to its fusibility, and the readiness with which it is sublimed, sulphide of antimony must be roasted with much care. The operation should be conducted at a very low heat, and tho assay should be constantly stirred with a thin iron rod until all the sulphur is burnt off. The residue being then fused with 3 parts of black flux, yields a bead of metallic antimony.