Assay.
The assay of the ores of bismuth, with the view of ascertaining the quantity of that metal, is perlormed by means of nitric acid. The ore is dissolved in this acid, diluted with one-half its weight of water, by the aid of heat. To the solution, a large quantity of water is added, by which oxide of bismuth is precipitated. The fluid after filtration is then evaporated as far as pos sible, without causing the deposition of any of its ingredients. Muriatic acid is then added, and the pre cipitate is digested in nitric acid. \Vith the solution thus obtained, a large quantity of water is mixed, and the whole of the oxide of bismuth is thrown down. This, with the former precipitate, is then fused with black flux, by which the bismuth is procured in the metallic state.
If the ore contain silver, the precipitate thrown down by muriatic acid will become black on exposure to light.
To ascertain the quantity of this metal present in the ore, the residue of the precipitate, after the digestion in nitric acid, must be washed, and then exposed to a red heat, 100 gr. of it = 75.3 of silver.
Or, the metallic button, obtained by the fusion of the ore with black flux, may he subjected to cupellation, by which Ihe base metals will be oxidated, and absorbed by the dish, and the silver will be left.
Antimony.
The sulphuret of antimony is the only ore of this metal that is used in commerce. This is purified merely by fusion, in which state it is usually employed in the arts. For purifying the sulphuret, two methods are employed.
1st, The ore, after being broken, and freed as much as possible from stony matter, is put into a large cru cible, in the bottom of which there is a small hole. From this there proceeds a tube, which passes through the furnace in which the crucible is placed, and termi nates in a reservoir. As the heat is applied, the sul phuret fuses, and runs through the tube into the reser voir, while the stony matter is left in the crucible.
2d, The fragments of the sulphuret, after being freed from the earthy ingredients, are put on the hearth of a reverberatory furnace, and covered with charcoal.
They are then brought to the fluid state, while the fo reign ingredients float on the surface, and are removed by ladles. When fluid, the sulphuret is poured into moulds, in which state it forms common, or crude an timony.
From the crude antimony the metal is procured by different processes. The first of these is, by what is called scarification. For this purpose 8 parts of finely powdered sulphuret are mixed with 6 of crude tartar and 3 of nitre, and the mixture is thrown in successive portions into a red-hot pot. When the vessel is nearly filled, it is covered, and a strong heat is applied to it for about half an hour; the fused matter is then either al lowed to cool, or it is poured into a conical iron vessel, greased in the inside, in which it separates into different layers ; the upper consisting of scoria; of alkaline mat ter and oxide of antimony ; the lower one of antimony in its metallic state.
It has been recommended by some to deflagrate the nitre and tartar, previous to the sulphuret being mixed with them; as, however, the use of the nitre is to aci dify the sulphur, it. is best to mix the whole of the sub stances together, and then to deflagrate them. In this process it is necessary to avoid using an excess of tartar and nitre, otherwise not only the expense is increased, but less metal is obtained. The proportions above stat ed are what Lemery found to answer best.
The second method of reducing the sulphuret is by roasting The ore, reduced to small pieces, is placed on the hearth of a reverberatory furnace, and heat is applied to it, by which the sulphur is consumed, and the antimony is oxidated. In this part of the process it is necessary that the heat be at first slight, otherwise the sulphuret runs into cakes. As the sulphur is con sumed, and the fusibility diminishes, the residue is brought to a red heat. When it ceases to emit the odour of sulphurous acid, the roasting is stopped, and the oxide is removed. In this state it still contains a small quantity of sulphuret.