Then as to its effect on color: it is so delete rious that for grounds the great Venetian paint ers never used it. The Venetian painters pre pared a ground of plaster of Paris. We may be certain that if we start with white lead—a min eral—and put any vegetable color with it, we shall find the mineral kill the vegetable. The safest way to use colors so that they are not de stroyed is to use only mineral with mineral col ors, and only vegetable with vegetable colors.
White lead is not suitable for water color, as it turns black when used without oil. Age gives white lead a yellow tinge, and damp will turn it black. If coated over dark colors, it will sink into them, and the darker color will show through. There are instances of this in old pic tures. Raphael, after painting a red cap on a figure in one of his pictures, desired to do with out the cap, and so painted it out with a color whose principal part consisted of lead. After a time the cap came through, and now shows—of course, not so bright as it originally was, but like the ghost of the cap.
The ordinary process of obtaining white lead is by the slow corrosion of small castings of met allic lead, caused by its exposure over acid in small earthenware vessels. This is known as the Dutch method, and it requires at least ninety days for the corrosion, which, of course, adds to the price to no small extent, especially as consid erable time is required, in addition, for the lead to mature. During the past few years, however, there have been patented from time to time a number of new processes, most of them having for their immediate object the saving of time in manufacture.
One of the results of using white lead that is too fresh is chalking—that is, powdering of the lead under the action of the atmosphere. Of the new processes of corrosion that have been suc cessful, the most interesting is probably that in which the metallic lead is reduced to particles as fine as powder, and then subjected to the action of acid, which reduces it in a short time to hydro carbonate of lead.
Men have become confirmed invalids and cripples through the effects of lead in their sys tems; strong, healthy young men have been at tacked, and incapacitated from work for as long as fourteen and fifteen weeks at a time, through the effects of what is known in the trade as lead colic.
The question of an efficient substitute for white lead is, therefore, one of literally vital im portance. It has been frequently asserted that there is no substitute for it; that a pigment can not be made that will answer the purpose, hav ing all the advantages claimed for the carbonate of lead. Certainly, sulphate of lead has not the body or covering properties that carbonate of lead has; but there are other white pigments. For instance, oxide of zinc can claim to be one of the best white paints in the market, and it has cer tainly a decided advantage over white lead, inas much as it is not the least injurious to health, and retains its color under all atmospheric condi tions. But there, again, body is wanting. There is also another pigment—sulphide of zinc— which has a greater advantage still; while being cheaper than oxide of zinc, it has a much better body and as great a covering power as carbonate of lead.
Zinc oxide is in very general use in Japan and in France. Its merit is greatest in point of long-retained whiteness; and its defect, in cov ering power. When zinc oxide is ground up with lead sulphate under heavy edge runners, the covering power of the compound is considerably greater than that of either component if applied separately. Laboratory experiments show that the quantity wanted for three successive coats is about equal when either the mixture named, or Freeman's white, or white lead is used: sub-: limed oxide of zinc requires rather more. The relative quantities of oil required are about the same. The color value is distinctly greater in the case of the substitutes.
Probably few persons outside the trade are aware that by use of white lead paint they have in their houses a sensitive detector of poisonous sewer gas—a sanitary quality of which most of the "sanitary enamels" that are now the rage are, by reason of the entire absence of lead in their composition, almost or entirely destitute.
In practical house painting, zinc-white is often used in obtaining the finest white surfaces for enameling and hand polishing of woodwork. Since this class of work is of a very laborious and expensive nature, it naturally follows that a pigment should be used which maintains its color for a great number of years.