1130 



WHITE LEAD 



carbonate ; as well as the skilful and laborious manner in which it is washed, and 

 separated from any adhering particle of metal or sulphide. 



In England, lead is converted into carbonate in the following way : The metal is 

 cast into the form of a network grating, in moulds about 20 inches long, and 8 or 9 

 broad. Several rows of these are placed over cylindrical glazed earthen pots, about 

 6 or 7 inches in diameter, containing some wood-vinegar, which are then covered 

 with planks and spent tan ; above these pots another range is piled, and so in suc- 

 cession, to a convenient height. The whole are imbedded in spent bark from tho 

 tin-pit, brought into a fermenting state by being mixed with some bark used in a 

 previous process. The pots are left undisturbed under the influence of a fermenting 

 temperature for 8 or 9 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 horsedung and litter, has now ceased to 

 be used ; because the coil is not uniformly acted on by the acid vapours, and the sul- 

 phuretted hydrogen evolved from tho dung is apt to darken the white lead. 



In the above processes, the conversion of lead into carbonate seems to be effected by 

 keeping the metal immersed in a warm humid atmosphere, loaded with carbonic and 

 acetic acids. 



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 pre- 

 ceding operations. M. Thenard first established the principle, and MM. Brechoz and 

 Lesseur contrived the arrangements of this method, which was subsequently executed 

 on a great scale by MM. Eoard and Brechoz. 



A subacetate of lead is formed by digesting a cold solution of uncrystallised acetate, 

 over litharge, with frequent agitation. It is said that 65 pounds of purified pyrolig- 

 neous acid, of specific gravity 1*056, require, for making a neutral acetate, 68 pounds 

 of litharge ; and hence, to form the subacetate, three times that quantity of base, or 

 174 pounds, must be 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, 

 precipitates them in the form of a white carbonate, while the liquid becomes a faintly 

 acidulous acetate. The carbonic acid maybe extricated from chalk, or other com- 

 pounds, or generated by combustion of charcoal, as at Clichy ; but in the latter case 

 it must be transmitted through a solution of acetate of lead before being admitted into 

 the subacetate, to deprive it of any particles of sulphuretted hydrogen. When tho 

 precipitation of the carbonate of lead is completed and well settled down, tho superna- 

 tant acetate is decanted 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 acetate : after which 

 tho white lead is thoroughly elutriated. This repetition of the process may be inde- 

 finitely made ; but there is always a small loss of acetate, which must be repaired, 

 either directly or by adding some vinegar. 



It is customary on the Continent to mould the white lead into conical loaves before 

 sending it into the market. This is done by stuffing well-drained white lead into 

 unglazed earthen pots, of the requisite size and shape, and drying it to a solid mass 

 by exposing these pots in stove-rooms. The moulds being now inverted on tables, 

 discharge their contents, which then receive a final desiccation; and are afterwards 

 put up in pale-blue paper, to set off the white colour by contrast. 



It has been supposed that the differences observed between tho ceruse of Clichy and 

 the common kinds, depend on the greater compactness of the particles of the latter, 



produced by their slower aggregation ; 

 as also, according to M. Robiquet, on 

 the former containing considerably less 

 carbonic acid. 



Mr. Ham proposed, in a patent dated 

 June 1826, to produce white lead with 

 the aid of the following apparatus, a, a, 

 Jiff. 2116, are the side-walls of a stove- 

 room constructed of bricks ; b is the 

 floor of bricks laid in Roman cement ; 

 c, c, are the side-plates, between which 

 and the walls a quantity of refuse tan- 

 ners' bark, or other suitable vegetable- 

 matter, is to bo introduced. The same 

 material is to be put also into the lower 

 part at d (upon a false bottom or grating ?). The tan should rise to a considerable 



