Manufacture of White Lead.

Scientific American 19, 4.11.1868

White lead, or ca[r]bonate of lead, is extensively used in the arts. As a pigment, when are and mixed with linseed oil, it produces a beautiful white. It is also the base and vehicle for colors used in painting. Cements for metals are composed mainly of it, and in the preparation of vulcanized rubber and liquid gutta percha it enters largely. In medicine it is employed mixed with linseed oil as an ointment for burns, scalds, ulcers, and excoriations. Of all the different preparations of lead the carbonate is the most poisonous to the human system, inducing what is know as the painter's colic in those engaged in its manufacture and in painters. This terrible disease, even if not fatal, frequently produces local paralysis, and the victim becomes a permanent cripple.

The method of manufacture in simple. The material, usually in pigs, of the purest quality, is melted in a fixed kettle and then run into very thin sheets. When made by hand, the process of casting these sheets requires considerable skill. The operator holds in his left band, by a suitable handle, a sort of shovel of sheet brass, the sides turned up, and dipping up a small quantity of the melted metal, he dexterously thrown it over the surface of shovel, when it almost instantly cools in a thin sheet, the superfluous portion of the metal running back into the kettle. A number of there sheets are loosely coiled, forming a sort of cylinder to be submitted to the after action of the acid.

In large concerns, however, this band casting has been aupereeded by a method very much superior, the invention of Mr. Augustus Graham, of Brooklyn, N. Y. A series of molds, corresponding to the shovel just mentioned, and connected to an endless chain, are successively presented to a current of melted lead, forming sheets in the shape of grates, called "buckles" from their resemblsoce to the large shoe and knee buckles worn in former times. There buckles ate dischargrd at the further end of the apron and placed in earthen pots, their edges renting on inward projecting ledges about three inches from the bottoms of the pots. Each pot contains a small quantity of acetic acid, not however reaching the lead buckles. The pots have holes near the top and they are set on a floor covered with tan, the holes of the pots opposite each other to insure a free passage, from one to the other, of the acidulated gases. The first layer of pots is covered with bards over which is spread another layer of tan and on this another layer of pote, and so on to the hight of perhaps twenty feet. The whole is covered with a thick layer of tan.

Then the process of decomposition begins. The tan ferments, generating heat, which causes the vinegar to evaporate and its vapors to circulate among the lead. This goes on for several weeks, and the white carbonate falls down in snowy heaps. When the process is supposed to be completed, or the action of the acid ceases, the pile is taken down, the carbonate removed, and those portions of the lead which have not been reduced, called "blue lead," are cleansed of their white coating and returned to the melting pot.

The carbonate or white lead in the form of powder is then washed in tanks with water. These tanks are placed high enough to draw off the lead paste from their bottoms to immense pans called drying kilns, which have false bottoms between which and the true bottoms steam is admitted to hasten the evaporation of the water. When dry the powdered lead may be packed ready for market, but usually it is ground in oil in which form it is generally sold.

It is seldom, however, that it is offered pure; sulphate of barytes being extensively used to adulterate it. This substance is nearly as heavy as white lead, and is perfectly white but not so brilliant. It has not the body of white lead, but is not so easily affected in color by noxious gases. white lead being soon discolored by sulphureted hydrogen gas.