The reaction of natural and synthetic mineral pigments with binders containing lipids, such as egg yolk, drying oils, etc., has been reported by various authors.
Generally, the formation of metallic soaps with the free fatty acids in the binders is assumed to be the reaction. These metallic soaps may act as driers, accelerating the oxidation and polymerization of the binder and providing a hard and resistant pigmented film.
Usually, and without much verification, pigments such as lead white, umber, zinc oxide, etc., are said to accelerate the drying of binders. The synthesis and use of metal soaps, such as those from lead, cobalt, and manganese, are typical in the paint industry and are used as driers in artists' colors.
A study was initiated a few years back that produced a definitive answer regarding the formation of metal soaps by natural and synthetic mineral pigments. The researchers studied the formation of metallic soaps of pigmented paint films when mixed with linseed oil.
These pigments did not form metallic soaps, so it is reasonable to deduce that they do not react with linseed oil. Interestingly, some of the pigments in the group above have long been designated reactive pigments, forming metallic soaps with linseed oil.
Minium (red lead)
Cobalt black oxide
These pigments are arranged according to their reactivity (from the most to the least) in forming metallic soaps.
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