Fusion.—The ore generally subjected to the process of fusion is lead glance, which almost always contains a small quantity of silver. When lead mines are wrought on account not only of the lead, but also of the silver, the earthy matter mixed with the ore is considerable, and the quantity of silver is very varims in different mines. In some, as those formerly wrought in York• shire, the silver amounted to about 230 oz. in the ton, while the Durham and Westmoreland mines :Here? only 17 oz. per ton.
The first part of the process for procuring silver from these ores consists in stamping and washing, as has been already described, after which the substance contains silver, lead, iron pyrites, blende, and earthy matter. The ore is then roasted in a reverberatory to drive off the sulphur, taking care that the heat he not so high as to fuse it, and that when it begins to adhere together on the surface, it be well stirred. In about five or six hours, the flame from the ore changes from blue to white, charcoal in powder is then thrown in, by which part of the lead ore is reduced, and collects at the bottom ; when a sufficient quantity is formed, quicksilver is mixed with the scoria to thicken it, and the fused metal is drawn off. After this, the heat is continued, and the metal, as it is formed, is repeatedly taken out, the temperature being increased towards the end of the process, to cause the scoria to become more liquid, so as to allow the metal to fall through it. The metal, as it is collected, is covered with char coal ; and the scoria which is formed is removed. It is then covered with saw-dust, pieces of wood, and a lit tle resin, and constantly stirred ; and when the flame ceases, it is poured into moulds. The scoria procured in the different operations is afterwards heated in a blast furnace, to procure the lead from it.
The lead containing the silver obtained by the above operations, is then subjected to the process of refining, which is the same as cupellation, only performed on a large scale. For this purpose, a reverberatory furnace is employed, but the process differs in different places, owing to a difference in the nature of the ore. In England, the cupel, or vessel in which the refining is performed, is composed of 6 parts of bone ashes, and 1 of fern ashes, made into a paste with water. In the centre of this there is a shallow cavity, for the recep tion of the metal, and at one end there is a hole for the escape of the litharge formed during the operation; the litharge flowing into this, along a groove made in the cupel. When the cupel is heated, the metal is put
in through an aperture in the furnace, and a stream of air is made to play on it, by which the litharge is formed, and escapes along the groove. As this groove is de stroyed, the litharge is made to run along another, and in this way the whole of it is withdrawn. The metal in the cupel is again subjected to a similar operation, after which it is poured into moulds, and formed into in gots.
In other places, the lower part of the reverberatory is covered with wood ashes and clay, so as to form a cupel. On one side of the furnace there is a hole for the exit of the litharge ; and on the opposite side is another for the admission of air to the surface of the metal, which is introduced through an aperture above, to which a cover is adapted. After the lead is melted and brought to a red heat, the blast of air is admitted, and the scoria as it collects is removed. When the litharge is formed, the heat is increased, and the quan tity becomes greater, and is withdrawn through the opening in the furnace. At the same lime some lead is volatilized. Towards the end of the pro cess, the litcarge which comes off contains a small quantity of silver. and is therefore kept separate from the rest. After the whole of the litharge is removed, and the surface of the metal in the furnace becomes bright, a quantity of water is poured en it„to provent the metal from spirting, which it is apt to do when con gealing. The metal thus obtained, is subjected to a si milar operation for about five hours, in a smaller furnace, and at a higher temperature, by which it is freed from the lead.
Silver is also occasionally procured front the silver ores by the process of fusion. The process, as carried on in France, consists in mixing the ore (which con tains native silver, sulphuret of silver, iron pyrites, ar senic and cobalt,) with lime, the scoria of a former operation, and slag from an iron furnace, to make it more fusible. Galena and litharge of a previous pro cess arc also added. These, when mixed, arc exposed to heat in a blast furnace, along with charcoal, by which a metallic matter, composed of silver and lead, is obtained. This is again used with litharge, and the metal procured is afterwards subjected to the pro cess of cupellation, at a temperature higher than that in the other instances, owing perhaps to the presence of iron, which occasions a considerable loss of silver.