Notebook, 1993-



Even when chemically compatible colors are mixed together one can encounter unpleasant surprises, for compatibility also depends on the pigments' physical characteristics. If red lead is mixed with lamp black, i .e., soot, a pigment twenty times lighter than the former, the heavy red pigment particles will obviously settle out, leaving the light soot floating on top. This may be experienced in practice with grades of umber that have had their color modified with a black pigment of lower specific weight. Chrome yellow and Prussian blue can also separate in this manner, as can mixtures of cobalt blue and burnt sienna in watercolor, as well as a number of other pigments. Recent research has shown that this is a colloidal phenomenon, and is caused by different electric charges carried by particles of different pigments. Sometimes this effect can be prevented with wetting agents. [p. 62]

[Wehlte, Kurt. The Materials and Techniques of Painting. Translated by Ursus Dix. New York: Van Nostrand Reinhold Company. 1975.]



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