1132 WHITE LEAD 



112 of the litharge. The sulphate is afterwards washed and dried in stores for the 

 market, but is very inferior to ordinary white lead. 



Mr. Leigh, surgeon in Manchester, prepared his patent white lead by precipitating 

 a carbonate from a solution of the chloride of the metal by means of carbonate of 

 ammonia. On this process, in a commercial point of view, no remarks need be made. 



A patent was granted to Mr. Hugh Lee Pattinson, in September 1841, for improve- 

 ments in the manufacture of white lead, &c. This invention consists in dissolving 

 carbonate of magnesia in water impregnated with carbonic acid gas, by acting upon 

 magnesian limestone, or other earthy substances containing magnesia in a soluble 

 form, or upon rough hydrate of magnesia in the mode hereafter described, and in 

 applying this solution to the manufacture of magnesia and its salts, and the precipi- 

 tation of carbonate of lead from any of the soluble salts of lead, but particularly the 

 chloride of lead ; in which latter case the carbonate of lead so precipitated is tri- 

 turated with a solution of caustic potash or soda, by which a small quantity of chloride 

 of lead contained in it is converted into hydrated oxide of lead, and the whole rendered 

 similar in composition to the best white lead of commerce. The manner in which 

 these improvements are carried into effect is thus described by the patentee : 



' I take magnesian limestone, which is well known to be a mixture of carbonate of 

 lime and carbonate of magnesia in proportions varying at different localities ; and on 

 this account I am careful to procure it from places where the stone is rich in magnesia. 

 This I reduce to powder, and sift it through a sieve of forty or fifty apertures to the 

 linear inch. I then heat it red-hot, in an iron retort or reverberatory furnace, for two 

 or three hours, when the carbonic acid being expelled from the carbonate of magnesia, 

 but not from the carbonate of lime, I withdraw the whole from the retort or furnace, 

 and suffer it to cool. The magnesia contained in the limestone is now soluble in water 

 impregnated with carbonic acid gas, and to dissolve it I proceed as follows : I am 

 provided with an iron cylinder lined with lead, which may be of any convenient size, 

 say 4 feet long by 2 feet in diameter ; it is furnished with a safety-valve and an 

 agitator, which latter may be an axis in the centre of the cylinder, with arms reaching 

 nearly to the circumference, all made of iron and covered with lead. The cylinder is 

 placed horizontally, and one extremity of this axis is supported within it by a proper 

 carriage, the other extremity being prolonged and passing through a stuffing-box at 

 the other end of the cylinder, so that the agitator may be turned round by applying 

 manual or other power to its projecting end. A pipe, leading from a force-pump, is 

 connected with the under side of the cylinder, through which carbonic acid gas may 

 be forced from a gasometer in communication with the pump, and a mercurial gauge 

 is attached, to show at all times the amount of pressure within the cylinder, in- 

 dependently of the safety-valve. Into a cylinder of the size given I introduce from 

 100 to 120 Ibs. of the calcined limestone with a quantity of pure water, nearly filling 

 the cylinder ; I then pump in carbonic acid gas, constantly turning the agitator, and 

 forcing in more and more gas, till absorption ceases under a pressure of five atmospheres. 

 I suffer it to stand in this condition three or four hours, and then run off the contents 

 of the cylinder into a cistern, and allow it to settle. The clear liquor is now a solution 

 of carbonate of magnesia in water impregnated with carbonic acid gas, or, as I shall 

 hereafter call it, a solution of bicarbonate of magnesia, having a spec. grav. of about 

 1 - 028, and containing about 1,600 grains of carbonate of magnesia to the imperial 

 gallon. 



' I consider it the best mode of obtaining a solution of bicarbonate of magnesia from 

 magnesian limestone, to operate upon the limestone after being calcined at a red heat 

 in the way described ; but the process may be varied by using in the cylinder the 

 mixed hydrates of lime and magnesia, obtained by completely burning magnesian 

 limestone in a kiln, as commonly practised, and slaking it with water in the usual 

 manner: or, to lessen the expenditure of carbonic gas, the mixed hydrates may 

 be exposed to the air a few weeks till .the lime has become less caustic by the absorp- 

 tion of carbonic acid from the atmosphere. Or the mixed hydrates may be treated 

 with water, as practised by some manufacturers of Epsom salts, till the lime is wholly 

 or principally removed ; after which the residual rough hydrate of magnesia may be 

 acted upon in the cylinder, as described ; or hydrate of magnesia may be prepared 

 for solution in the cylinder, by dissolving magnesian limestone in hydrochloric acid, 

 and treating the solution, or a solution of chloride of magnesium, obtained from sea- 

 water by salt-makers in the form of bittern, with its equivalent quantity of hydrate 

 of lime, or of the mixed hydrates of lime and magnesia, obtained by completely 

 burning magnesian limestone, slaking it as above. "When I use this solution of 

 bicarbonate of magnesia for the purpose of preparing magnesia and its salts, I 

 evaporate it to dryness, by which a pure carbonate of magnesia is at once obtained, 

 without the necessity of using a carbonated alkali, as in the whole process ; and 

 from this I prepare pure magnesia by calcination in the usual manner ; or, instead 





