WHITE-LEAD MAFUFACTURE. This branch of industry exhibits many interesting features, both on account of the chemical relations which govern it, and the mechanical arrangements adopted.
White lead is made from common metallic sheet lead ; it is a carbonate, and is produced in the following way. The lead is first cast into the form of flat pieces, about 20 inches long, by 6 broad, .1-th of an inch thick, and liteighing 5 lbs. In a large lofty room a pile is built up, formed in the following singular way ; a layer of fine ashes is strewed over the floor: then comes a .layer of tanner's spent bark, two or three feet thick ; then a layer of earthen pots, about five inches in diameter, each containing a pint of strong -vinegar ; then a layer of the leaden plates, five or six in depth, over the open mouths of the pots ; then a layer of boards ; then a second layer of tan, vinegar pots, lead plates, and boards— then a third layer of tan, vinegar pots, lead plates, and boards, and so on, until the room is filled to the ceiling, a height of perhaps'20 feet. The whole is called a stack, which consists of as many beds as there are recurrences of the above series, perhaps seven or eight. A stack usually comprises about 30 tons of lead plates besides the weight of the pots, vinegar, &c. The room is then closed up, and left un touched for many weeks.
During this period a somewhat complicated chemical process goes on. The spent bark,
by fermenting, gives off heat, which raises the general temperature to'180° Fahr. The vinegar, at this temperature, evaporates slowly, and gives off an acid vapour which acts upon the lead ; the lead first becomes an oxide, then it becomes an acetate by combining with the acetic acid vapour, and this is transformed to a carbonate by carbonic acid arising from the tan. This action takes place in about one half of the thickness of the lead, at the two surfaces ; the central part remaining un acted upon. The door is then opened, the stack taken down, and all the lead plates removed. Each piece is passed between two brass rollers, which crush and remove the earthy carbonate from the surface ; the remaining metallic lead is remelted, to be used again ; and the carbonate goes through further processes. The carbonate is mixed with water, brought to the state of a fine cream, evaporated, and dried: it then constitutes white lead, which is brought to market either in the earthy state, or ground up with oil as a paint.
There are other and more expeditious methods which have been patented within the last few years , but the above is the general method, as practised at Newcastle.
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