WHITE LEAD 1129 



sheets ; because a rough surface presents more points of contact, and is more readily 

 attacked by acid vapours than a polished one. 



These plates are now placed so as to expose an extensive surface to tho acid 

 fumes, by "folding each other over a square slip of wood. Being suspended by their 

 middle, like a sheet of paper, they are arranged in wooden boxes, from 4 to 5 feet 

 long, 12 to 14 inches broad, and from 9 to 11 inches deep. The boxes are very 

 substantially constructed ; their joints being mortised, and whatever nails aroused, 

 being carefully covered. Their bottom is made tight with a coat of pitch about an 

 inch thick. The mouths of the boxes are luted over with paper in the works where 

 fermenting horse-dung is employed as the means of procuring heat, to prevent the 

 sulphuretted and phosphuretted hydrogen from injuring the purity of the white lead. 

 In Carinthia it was formerly the practice, as also in Holland, to form the lead sheets 

 into spiral rolls, and to place them so coiled up in the chests ; but this plan is not to 

 be recommended, because these rolls present obviously less surface to the action of 

 the vapours, are apt to fall down into the liquid at the bottom, and thus to impair the 

 whiteness of the lead. The lower edges of the sheets are suspended about two inches 

 and a half from the bottom of the box ; and they must not touch either one another 

 or its sides, for fear of obstructing the vapours in the first case, or of injuring the 

 colour in the second. Before introducing the lead, a peculiar acid liquor is put into 

 the box, which differs in different works. In some, the proportions are four quarts 

 of vinegar, with four quarts of wine-lees ; and in others a mixture is made of 20 

 pounds of wine-lees, with 8 pounds of vinegar, and a pound of carbonate of potash. 

 It is evident that in the manufactories where no carbonate of potash is employed in 

 the mixture, and no dung for heating the boxes, it is not necessary to lute them. 



The mixture being poured into the boxes, and the sheets of lead suspended 

 within them, they are carried into a stove-room, to receive the requisite heat for 

 raising round the lead the corrosive vapours, and thus converting it into carbonate. 

 This apartment is heated generally by stoves, is about 9 feet high, 30 feet long, 

 and 24 feet wide, or of such a size as to receive about 90 boxes. It has only one door. 



The heat should never be raised above 86 Fahr. ; and it is usually kept up for 15 

 days, in which time the operation is, for the most part, completed. If the heat be too 

 high, and the vapours too copious, the carbonic acid in a great measure escapes, and 

 the metallic lead, less acted upon, affords a much smaller product. 



When the process is well managed, as much carbonate of lead is obtained as there 

 was employed of metal ; or, for 300 pounds of lead, 300 of ceruse are procured, besides 

 a certain quantity of metal after the crusts are removed, which is returned to the 

 melting-pot. The mixture introduced into the boxes serves only once ; and if carbon- 

 ate of potash has been used, the residuary matter is sold to the hatters. 



When the preceding operation is supposed to be complete, the sheets, being 

 removed from the boxes, are found to have grown a quarter of an inch thick, though 

 previously not above a twelfth of that thickness. A few crystals of acetate of lead 

 are sometimes observed on their edges. The plates are now shaken smartly, to cause 

 the crust of carbonate of lead formed on their surfaces to fall off. This carbonate is 

 put into large cisterns, and washed very clean. The cistern is of wood, most commonly 

 of a square shape, and divided into from seven to nine compartments. These are of 

 equal capacity, but tmequal height, so that the liquid may be made to overflow from 

 one to the other. Thereby, if the first chest is too full, it decants its excess into the 

 second, and so on in succession. 



The water poured into the first chest passes successively into the others, a slight 

 agitation being meanwhile kept up, and there deposits the white lead diffused in it 

 proportionally, so that the deposit of the last compartment is the lightest and finest. 

 After this washing, the white lead receives another in large vats, where it is always 

 kept under water. It is lastly lifted out, in the state of a liquid paste, with wooden 

 spoons, and laid on drying-tables to prepare it for the market. 



The white lead of the last compartment is of the first quality, and is called on the 

 Continent ' silver white.' It is employed in fine painting. 



When white lead is mixed in equal quantities with ground sulphate of baryta, it is 

 known in France and Germany by the name of 'Venice white.' Another quality, 

 adulterated with double its weight of sulphate of baryta, is styled 'Hamburgh white ;' 

 and a fourth, having three parts of sulphate to one of white lead, gets the name of 

 ' Dutch white.' When the sulphate of baryta is very white, like that of the Tyrol, 

 these mixtures are reckoned preferable for certain kinds of painting, as the barytes 

 communicates opacity to the colour, and protects the lead from being speedily dark- 

 ened by sulphurous smoke or vapours. 



The high reputation of the white lead of Krems was by no means due to the barytes, 

 for the first and whitest quality was mere carbonate of lead. The freedom from silver 

 of the lead of Villach, a very rare circumstance, is one cause of the superiority of its 



