WHITE LEAD, a white pigment very largely used in painting. Many processes have been devised for its manufacture. The old Dutch method was to expose sheet lead in coils, placed in earthenware jars partly filled with vinegar, to the combined action of air, moist ure and carbonic acid gas. This was done by immersing the jars with their contents in de composing horse, manure which furnished the heat and carbonic acid gas necessary for the process. The English process, which is the one by which most of the white lead is made, differs from the Dutch process by the use of ferment ing tan hark instead of manure. The pots con taining the lead and dilute acetic acid or vinegar are piled in rows in the so-called stacks and surrounded with the fermenting tan bark. After about three months the lead is removed and purified. The process is slow and the methods of purification dangerous to the workmen, but the resulting white lead is of the best quality. In the French method a solution of basic lead acetate is first prepared by the action of an im pure acetic acid on litharge or lead oxide. Car
bonic acid gas is then passed through this solu tion and the white lead precipitated. White lead is also produced electrolytically, by employing a solution of sodium-nitrate, preferably in a cell of wood and a porous diaphragm. The anode Li of lead and the cathode of copper. When the current is turned on, nitric acid appears at the anode as lead nitrate and from the cathode ap pears caustic soda. The lead hydroxide ob tained is later treated with sodium bicarbonate to secure the carbonate of lead. White lead is subject to frequent adulteration. It may be tested for purity by making a solution of the powder in acetic acid, which should take up all the lead and leave the adulterant. White lead is a basic carbonate of lead of somewhat vari able composition. That formed by the Dutch and English processes may be represented by the formula 2PbCO,,Pb(OH),.