Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/163

Mr. H.L. P ATTINSON on Smelting Lead Ore, &c. ore itself may be cubical, steel-grained, or antimoniated, or a mixture of two or the whole of these in various proportions; and, besides heterogeneous impurities, it frequently contains a greater or less proportion of iron, as an ingredient in its composition; these circumstances render its quality so various, that, when laid down at the smelting-house, it is very difficult to pronounce two parcels from different veins, or from the same vein in different strata, exactly alike. It consequently happens that one variety of ore is sometimes found, without any very obvious cause, to be much more refractory in the fire and less easily reducible than another, and among a number of parcels this difference is occasionally very considerable.

At nearly all the smelting-houses in this district, the practice prevails of smelting the ore from each vein, where the quantity is considerable, by itself, although most smelters admit that a mixture of different kinds of ore has frequently a very beneficial effect in promoting the reduction of each. In smelting ore from the same vein, it is the practice to treat the different portions, into which it is divided in the washing process, separate from each other; that is, the portions called, technically, seive ore and smiddam, are smelted separate from the slime ore, lest the latter, which is in very small particles, should be driven away by the draught and blast, by which the former is roasted and reduced into Lead.

The process of smelting may be most conveniently described under four heads, viz. :—

ROASTING OF THE ORE.
The process of roasting is nothing more than heating the ore to a proper temperature in a reverberatory furnace, during which it undergoes a change, by the partial expulsion or acidification of the sulphur it contains, which renders it afterwards more easily reducible.