Lead Salts

Lead and several of its salts have been long and widely used as dryers. Metallic lead in the form of foil, litharge or lead protoxide, minium or red lead, lead peroxide, sugar of lead or lead acetate, the basic lead acetate, and white lead itself, have all been used in this way, chiefly for the purpose of making linseed or other painting oil dry more quickly. Some of these compounds, particularly sugar of lead, have been introduced into the very picture itself. It was a common practice to employ powdered sugar of lead or a solution of this salt in water to hasten the drying of vehicles and of slow-drying pigments which have been ground in oil. I have seen one of the results of this commingling of sugar of lead with the medium or the paint in the production of an immense number of small spots in the picture, sometimes appearing through the surface-varnish in the form of a white efflorescence. This efflorescence consists at first of lead acetate in crystals, but these soon attract carbonic acid from the air and become lead carbonate, which, in its turn, is changed into lead sulphide by the action of sulphuretted hydrogen. This tendency of the lead compounds to yield brown or black lead sulphide is, indeed, the great drawback to any use of these substances as dryers.

When oil is left in contact with them, and especially when heat is applied to the mixture, some of the lead dissolves, forming, with the fatty acids of the oil, lead-soaps. These soaps are distributed uniformly throughout the oil, and help to make it dry and harden quickly. The same action occurs when white lead is ground as a paint with oil, and has been urged as an objection to the use of those white leads which contain hydrate of lead, a compound which acts upon oil more quickly and thoroughly than the carbonate of lead.

It will be seen, however, that while there may be reasons for permitting the use of a single lead pigment which possesses this peculiar property, there can be none for introducing into every part of a picture oils or other materials which contain a metal, like lead, so liable to cause discoloration and darkening, when other and perfectly innocuous substances are available for producing the same siccative effects. On this account we omit further reference to the lead compounds, which have been and are still employed in the preparation of strongly-drying oils, etc., but pass on to the Manganese compounds, of which the dioxide, the hydrated protoxide and sesquioxide, the borate, the oxalate, and the linoleate are the most important.