Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/165

Mr. H.L. P ATTINSON on Smelting Lead Ore, &c. at other mills, the quantity roasted in eight hours by one man is three bings, at three charges, and thus three men working each 6 eight-hours' shifts, roast but fifty-four bings of ore per week.

The manner of conducting the process of roasting is the same in all cases. The proper charge of ore is spread evenly over the bed of the furnace to the depth of two or three inches, and the fire is at first pushed moderately, during which the ore is frequently turned and stirred, in order that the whole may be uniformly heated, but care is to be taken that no part is prematurely fused. If the fire is judiciously managed, the charge gradually attains a dull red heat — a greater heat is then given and the ore vigorously stirred, when, in a little time, it begins to feel soft and adhere slightly to the tool, in which state it is withdrawn from the furnace. The roasting process is conducted in the best manner, when great care is taken to apply the heat very gently at first, to keep, by constant stirring and change of place, the temperature of the whole charge as uniform as possible, and to withdraw it at the proper time from the furnace.

After the furnace is properly heated and working, two Winchester bushels, or about 1½ cwt. avoirdupois, of free coal, are required to roast one bing of ore; but some varieties of ore can be more easily reduced into the pasty state, mentioned above, than others; that is, they fuse at a lower degree of heat, and this in proportion to their purity. The least fusible ores are generally the most difficult to smelt, and undergo the greatest loss in that operation. It is well known that a considerably greater produce of Lead can be obtained from the same ore after being properly roasted, than before. This difference is of course variable, but in some instances, 20 bings of roasted ore have yielded 8 or 9 cwt. more Lead than 20 bings of the same ore smelted in its raw state.

At nearly all smelting mills long horizontal chimneys or flues are constructed (generally on the slope of an adjacent hill if practicable), which the smoke from the various processes of smelting is made to traverse before it escapes into the atmosphere. As the heat of the furnace in roasting, if incautiously applied, may volatilize a portion of the ore, and the draught has a tendency to draw along with it some of the smaller