| Finely powdered ore | 60 | grains. |
| Small pieces of iron | 12 | ” |
| Black flux | 180 | ” |
| Common salt | 50 | ” |
Put the powdered ore into the crucible, and place upon it the iron, which should not be in the form of filings or dust, but in small pieces; upon the ore and iron should be put the black flux, and lastly the common salt must be placed above all these substances as a protection against the air. The crucibles, as many as convenient, may now be introduced into the furnace, and slowly raised to a strong red heat, at which temperature they should be kept for about half an hour; at the end of that period they should be removed from the fire, slightly tapped to settle the contents, and then placed aside to cool. When this has taken place, a few blows with a hammer near the base of the crucibles, each in turn, will soon expose the button of silver attached to the undecomposed iron; the latter substance may, however, be easily detached by a few well-directed blows with the hammer.
| Fig. 5. Iron Crucible for Assay. | Fig. 6. Fire-clay Crucible for Assay. |
In order to ascertain the exact amount of the precious metal—that is, the silver—contained in the buttons of lead obtained as the results of the foregoing operations, they are subjected to a purifying process by the metallurgist, called cupellation. By this means the lead and other impurities are driven off by heat in contact with a current of air, and the silver is left behind in a pure state. To perform this operation it is necessary to expose the buttons on some absorbing medium or porous support, and this support is commonly known as a cupel. No doubt many porous substances could be made available for the formation of cupels, but bone-ash is the best for all practical purposes, such as are required by the assayer. The bone-ash, in the condition of a very fine powder, is mixed with a little water in which has been dissolved a small quantity of potash, and moulded into the desired shape. The cupels are tightly consolidated by pressure in an iron mould of the form shown in [Fig. 7], which is the best in use, being well adapted for the manufacture of cupels. It consists of a slightly conical steel ring, 2 ins. in depth, and about 1½ in. in diameter at the top internally; a steel die with a wooden handle ([Fig. 8]) is made to fit the mould. To make a cupel the space in the ring is nearly filled with the moistened bone-ash, and pressed down by the hand, and afterwards by the die, the latter being driven into the ring by the application of a wooden mallet ([Fig. 9]) to the handle of the die. It will be seen from the illustration that the die forms a cavity in the cupel capable of receiving the charge of metal for assay. When the bone-ash has been sufficiently compressed, the die is withdrawn, and the cupel removed from the ring. This is a delicate operation, as sometimes the edges of the cupel are liable to be injured; to prevent which and facilitate the removal a loose plate of iron, exactly fitting the bottom of the mould, should be introduced previous to putting in the bone-ash. The iron plate of course being removed with the cupel, it must be replaced before another can be made. By introducing a cylindrical piece of wood to the lower aperture of the steel ring, the cupel can be removed without difficulty.
| Fig. 7. Cupel Mould. | Fig. 8. Die for Cupel. | Fig. 9. Wooden Mallet. |
The size of the cupel should always be regulated according to the quantity of foreign matter to be absorbed, it being generally understood that the material of which it is formed takes up double its weight of lead. The process of cupelling is conducted in the furnace of the assayer, an apparatus of peculiar construction, the most important part of which, however, is the muffle ([Fig. 10]), consisting of a small arched oven of fire-clay closed at one end, and furnished with perpendicular slits in the sides, in order to allow of a free access of air to the cupels inside.
