Freeman’s process (see [p. 254]) consists of grinding together, in a dry state, under great pressure, and consequently with great friction, sulphate of lead and sulphate of zinc. While neither of these two substances alone possesses good body or opacity, when treated by this process they are so changed in character that the new substance is stated to be superior in these respects to the best form of ordinary white lead.
The white lead thus obtained mixes well with oil, and has also the advantage of not becoming blackened when exposed to the fumes of sulphuretted hydrogen, and of not peeling off in a saline atmosphere. As a basis for coloured paints, it is recommended on the ground that, being decomposed with greater difficulty, it can be mixed with almost any colouring substance; and, being free from acid, it does not change the tints of other substances.
The details of Lewis’s process, and the plant employed, are more fully set forth below.
Heretofore the manufacture of dry white lead from galena or the native sulphurets has been effected by roasting or desulphurising the galena, and then mixing the residue, after roasting, with carbon, and subjecting the mixture to the action of heat in a compound reducing or subliming and oxidising furnace, and collecting the resulting fumes in textile bags.
Lewis’s process, however, is based upon the discovery that by subliming unroasted galena, or the native or raw sulphuret of lead, and then oxidising the volatile products, cooling the fumes, and collecting them by means of a textile fabric, a superior basis of a pigment can be obtained. The admixture of carbon with the raw ore will facilitate the subliming process, or it may be carried on in a muffle or reverberatory furnace without the previous admixture of carbon.
The furnace, which has been found to answer well for the purposes above mentioned, is commonly known as the Wetherill furnace, and is represented in plan, in [Fig. 21]; in front and back elevations, in Figs. [22], [23]; and in central longitudinal section, in [Fig. 24].
a represents the main chamber, the bottom b of which is composed of iron bars perforated with small holes of about ¼ inch in diameter and about 1 inch apart, and preferably made slightly conical, with the larger diameter downward, that is to say, the said holes are of such a size as to prevent