WHITE LEAD 1131 



height, and havo a series of strips of sheet lead, e, e, e, placed upon it, which are 

 kept apart by blocks or some other convenient moans, with a space open at one 

 end of the plates, for the passage of the vapours ; but above the upper plates, boards 

 are placed, and covered with tan, to confine them there. In the lower part of the 

 chamber, coils of stoam-pipes, /,/, are laid in different directions to distribute heat ; 

 g is a funnel-pipe, to conduct vinegar into the lower part of the vessel ; and h is a 

 cock to draw it off, when the operation is suspended. The acid vapours raised by the 

 heat pass up through the spent bark, and on coming into contact with the sheets of 

 lead, corrode them. The quantity of acid liquor should not be in excess ; a point to 

 be ascertained by means of the small tube i, at top, which is intended for testing it by 

 the tongue, k is a tube for inserting a thermometer, to WJitch the temperature, which 

 should not exceed 170 Fahr. Wo arc not aware what success attended this patented 

 arrangement. 



A factory was many years since erected at West Bromwich, near Birmingham, to 

 work a patent obtained by Messrs. Gossage and Benson, for making white lead 

 by mixing a small quantity of acetate of lead in solution with slightly-damped litharge, 

 contained in along stone trough, and passing over the surface of the trough currents 

 of hot carbonic acid, while its contents were powerfully stirred up by a travelling- wheel 

 mechanism. The product was afterwards ground and elutriated, as usual. The car- 

 bonic acid gas was produced from the combustion of coke. This factory has long 

 been abandoned. 



Messrs. Button and Dyer obtained a patent for making white lead by transmitting a 

 current of purified carbonic acid gas, from the combustion of coke, through a mixture 

 of litharge and nitrate of lead, diffused and dissolved in water, which was kept in con- 

 stant agitation and ebullition by steam introduced through a perforated coil of pipes 

 at the bottom of the tub. The carbonate of lead was formed here upon the principle 

 of Thenard's process upon the subacetate ; for the nitrate of lead formed with the 

 litharge a subnitrate, which was forthwith transformed into carbonate and neutral 

 nitrate, by the agency of the carbonic acid gas. It is known that all sorts of white lead 

 produced by precipitation from a liquid, are in a semi-crystalline condition ; appear, 

 therefore, semi-transparent when viewed in the microscope ; and do not cover so well 

 as white lead made by the process of vinegar and tan, in which the lead has remained 

 always solid during its transition from the blue to the white state ; and hence consists 

 of opaque particles. 



A patent was obtained in December 1833, by John Baptiste Constantino Torassa, 

 and others, for making white lead by agitating the granulated metal or shot, in trays 

 or barrels, along with water, and exposing the mixture of lead-dust and water to the 

 air, to be oxidised and carbonated. The whole of these projects for preparing white 

 lead are inferior in economy and quality of produce to the old Dutch process, which 

 may be so arranged as to convert sheets of blue lead thoroughly into the best white 

 lead, within the space of ten weeks, at less expense of labour than by any other 

 plan. 



The composition of the different varieties of white lead has been carefully examined 

 by J. Arthur Phillips. 1 The result of this investigation shows that those specimens, 

 which are obtained by precipitation from solutions of the nitrate by means of an 

 alkaline carbonate, contain very variable quantities of oxide of lead, whilst in white 

 lead prepared by the ordinary Dutch process, the relations existing between the 

 amounts of carbonate and oxide, although definite, are usually very simple. The 

 most usual composition of the white lead of commerce is represented by the formula 

 2(PbO.CO=) + PbO.HO (aPbCO'.PbH'O 2 ), although specimens represented by the 

 formula? 3(PbO.C0 2 ) + PbO.HO (3PbCO 3 .PbH 2 O 2 ), and 6(PbO.C0 2 ) + PbO.HO 

 (SPbCO'.PbH'-'O 2 ) arc also occasionally met with. 



On examining the ordinary corroded leads in a finely-divided state, by the aid of a 

 powerful microscope, no traces of a crystalline. structure will be perceived, but when 

 precipitated specimens are subjected to a power of 300 diameters, distinct hexagonal 

 plates become visible. These vary from s ^yth to ^6^i5o tn ^ an i QCU ^ n diameter, and 

 appear slightly yellow by transmitted light. 



Mr. Thomas Richardson, of Newcastle, obtained a patent in December 1839, for a 

 preparation of sulphate of lead, applicable to some of the purposes to which the car- 

 bonate is applied. His plan is to put 56 Ibs. of flake litharge into a tub, to mix it 

 with 1 Ib. of acetic acid (and water) of spec. grav. ] -046, and to agitate the mixture 

 till the oxide of lead becomes an acetate. But whenever this change is partially 

 effected, he pours into the tub, through a pipe, sulphuric acid of spec. grav. l - 597o, 

 at the rate of about 1 Ib. per minute, until a sufficient quantity of sulphuric acid has 

 been added to convert all the lead into a sulphate; being about 20 parts of acid to 



1 Joutnal of the Chemical Society, p. 145% 



