Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/179

Rh mills, a man and a boy are deemed sufficient; the attention of one is wholly given to the fire, while the other supplies coke and grey slag. The length of a shift is 14 or 16 hours, during which, the quantity of lead made varies from 10 to 21 cwt., according to the nature of the slags. Twenty to twenty four bushels of coke are required to produce one fodder of lead. The quantity of slag lead made in smelting, as may be conceived, is considerably greater in poor and refractory than in rich and free-running ores, but, it may be stated generally at one-thirteenth of the lead yielded at the smelting hearth, so that it is usual to reckon, in large transactions, 13 twelve-stone pigs of common lead, and 1 of slag lead, to the fodder.

HEARTH ENDS AND SMELTER'S FUME.

In the operation of smelting, as already described, it happens that particles of unreduced and semi-reduced ore are continually expelled from the hearth, partly by the force of the blast, but principally by the decriptation of the ore on the application of heat. This ore is mixed with a portion of the fuel and lime made use of in smelting, all of which are deposited upon the top of the smelting hearth, as mentioned in page 157, and are called hearth-ends. It is customary to remove the hearth-ends from time to time, and deposit them in a convenient place until the end of the year, or some shorter period, when they are washed to get rid of the earthy matter they may contain, and the metallic portion is roasted at a strong heat, until it begins to soften and cohere into lumps, and afterwards smelted in the ore hearth, exactly in the same way as ore undergoing that operation, for the first time, already described.

It is difficult to state what quantity of hearth-ends are produced by the smelting of a given quantity of ore, but, in one instance, the hearth-ends produced in smelting 9751 bings, on being roasted and reduced in the ore hearth, yielded, of common lead 315cwt., and the grey slags separated in this process gave, by treatment in the slag hearth, 47cwt. of slag lead; making the total quantity of lead 362cwt., which is at the rate of 3cwt. 2qrs. 23lbs. from the smelting of 100 bings of ore.