WHITE LEAD. Carbonate of Lead ; Painters' White. Lead is converted into carbonate in the following way :—The metal is oast into the form of a network grating, in moulds about fifteen inches long, and four or five broad. Several rows of these are placed over cylindrical glazed earthen pots, about four or five inches in diameter, containing some trea cle-vinegar, Which are then covered with straw ; above these pots another range is piled, and so in succession, to a conve nient height. The whole are imbedded in spent bark from the tan-pit, brought into a fermenting state by being mixed with some bark used in a previous pro cess. The pots are left undisturbed under the influence of a fermenting tem perature for eight or nine weeks. In the course of this time the lead gratings become, generally speaking, converted throughout into a solid carbonate, which when removed is levigated in a proper mill, and elutriated with abundance of pure water. The plan of inserting coils of sheet lead into earthenware pipkins containing vinegar, and imbedding the pile of pipkins in fermenting horse-dung and litter, is now little used ; because the coil is not uniformly acted on by the acid vapors, and the sulphuretcd hydro gen evolved from the dung is apt to darken the white lead.
In the above processes, the conversion of lead into carbonate seems to be ef fected by keeping the metal immersed in a warm, humid atmosphere, loaded with carbonic and acetic acids ; and hence a pure vinegar does not answer well, but one which is susceptible, by its sponta neous decomposition in these maim atances, of yielding carbonic acid. Such are tartar, wine lees, molasses, &c.
Another process has been practised to a considerable extent in France, though it does not afford a white lead equal in body and opacity to the products of the preceding operations. M. Thenard first
established the principle, and MM. Bre choz and Lesenr contrived the arrange ments of this new method, which was subsequently executed on a great seals by MM. Board and Brechoz.
A snbacetate of lead is formed by di gesting a cold solution of uncrystallized acetate, over litharge, with frequent agi tation. It is said that 65 pounds of puri fied pyroligneous acid, of specific gravity 1.056, require, for making a neutral ace tate, 58 pounds of litharge; and hence, to form the subacetate, three times that quantity of base, or 174 pounds, must be i used. The compound is diluted with water as soon as it is formed, and being decanted off quite limpid, is exposed to a current of carbonic acid gas, which, uniting with the two extra proportions of oxide of lead in the subacetate, pre cipitates them in the form of a white car bonate, while the liquid becomes a faintly acidulous acetate. The carbonic acid may be extricated from chalk, or other compounds, or generated by combustion of charcoal, as at Clichy ; but in the lat ter case, it must be transmitted through a solution of acetate of lead before being admitted into the subacetate, to deprive it of anv particles of sulphureted hydro gen. When the precipitation of the car bonate of lend is completed, and well set tled down, the supernatant acetate is de canted off, and made to act on another dose of litharge. The deposit being first rinsed with a. little water, this washing is added to the acetateafter which the white laud is thoroughly elutriated. This repetition of the process may be indefi nitely made • but there is always a small loss of acetate, which must be repaired, either directly or by adding some vine gar.