The fading [or darkening] of the color of a pigment on exposure to light is a definite chemical reaction, set in motion by the wave lengths to which the pigment is exposed. It is also dependent upon the intensity and the duration of such exposure, and in many cases it will not take place unless other chemical agents are present [for example, water or moist environment].
All the pigments listed in this book as approved for use in the various techniques of permanent painting are inert to such actions. Only when the paints contain inferior pigments will the colors fade or darken. Some of the permanent artists' pigments can be made to undergo chemical changes [and consequently be altered in color [p. 26] effect] by use of drastic methods, but inasmuch as these paints will resist fading or other chemical changes indefinitely under the conditions to which works of art are normally exposed, they are accepted as permanent, lightproof colors. For example, alizarin crimson is an acceptably permanent artists' pigment showing no color change on normal exposure over long periods of time when used in easel paintings. However, if the same pigment is exposed to a powerful carbon arc machine for some days, a definite change can be induced; also, long and continued exposure to the direct rays of the sun, as on a roof, may have the same effect.
Likewise, other chemical treatments can alter certain pigments. Oil paints made with cadmium yellow mixed with flake white will not darken on exposure to light, but if cadmium yellow and flake white are taken into the laboratory and boiled together with water in a test tube, the mixture will turn quite dark. For this reason some chemists have stated that these two pigments should not be used together. Paintings, however, are seldom boiled, and there have been no examples of an oil painting having darkened because of this mixture. Cadmium yellow has been on the artist's palette for more than a century, and flake white for more than twenty centuries. Flake white is on the approved list for oil painting only.
Because all the failures of nonpermanent colors are caused by chemical reactions, it follows that the pigments of the highest degree of permanence are those that have the most suitable and inert chemical properties. [pp. 26-27] [ The dry powdered pigments, from which artists' paints are made, must conform to several requirements in order to be acceptable for use in painting]
[Mayer, Ralph. The Painter's Craft. An Introduction to Artist's Methods and Materials. Revised and updated by Steven Sheehan, Director of the Ralph Mayer Center, Yale University School of Art. New York: Penquin Group. 1948. 1991. ]
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