Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/308

300 exposed to the sulphuretted hydrogen or moistened with ammonium hydrosulphide it turns brown or black, whereby it is distinguished from zinc-white. When heated with the access of air it yields its carbonic acid, and at 572° Fahrenheit passes into lead oxide and finally into minium. When digested under pressure with carbonated water for some time, the water may contain 0.22 drachm of lead per quart. The difference in covering power is due to the form, size, density and composition of the smallest particles. The white lead obtained by the French or precipitation process is looser, of a coarser grain, and possesses less covering power than the product obtained by the Dutch or German process, which is denser, of a finer grain, and never crystalline, and, though of a greater specific gravity, requires less oil.

When exposed to the light and air, white lead is fairly permanent and will resist exposure to normal conditions for a great length of time; on the other hand, when exposed to the fumes of sulphuretted hydrogen and other sulphurous gases, white lead turns brown or black through the formation of the black sulphide of lead. The production of this body is more likely to occur in large towns, where great quantities of gas are used for lighting and other purposes, which usually contains some sulphuretted hydrogen or other sulphur compounds.

White lead can be mixed with all pigments except those which, like cadmium yellow, ultramarine or King's yellow, contain sulphur, such pigments sooner or later causing the formation of the black sulphide, and thus bringing about the discoloration of the pigment or paint.

In commerce white lead is found in two forms, as a heavy white powder, having a specific gravity of about 6.47 and weighing about 180 pounds to the cubic foot, or as a paste containing about 8 per cent of the linseed oil. To make the latter, the dry white lead is first mixed in a mixing mill with about 8 to 9 per cent of its weight of raw linseed oil; it is then run through a grinding mill several times to