It has already ben mentioned how readily ultramarine blue is affected by acids. There are also pigments that were insufficiently washed during manufacture and still contain traces of acids. These can affect other pigments or cause flocculation in casein media. This property appears most frequently in caput mortuum [violet iron oxide].
Some pigments like burnt sienna tend to set in oils while others contain oil-soluble components, which accounts for the already-mentioned bleeding and permeation of subsequent paint layers. If a watercolor cannot be washed out properly once applied on paper, or when it stains the lid of the paint box , it usually contains soluble coal-tar dyes. Prussian blue can also contain water-soluble constituents as a result of faulty manufacture.
Certain colors partly dissolve in alcohol and therefore cannot be used to tone isolating varnishes made with methylated spirit. The same applies to pigments unsuitable for cellulose lacquers. Some colors, normally regarded as reliable in a certain medium, unexpectedly fail to perform well. The reason for this may be the methods employed to manufacture certain grades. It will be sufficiently obvious by now that some pigments cannot be used in all media and hence the efforts to introduce a universally applicable standard range of colors were bound to fail. The painter must therefore study his own material and assess its limitations. Artists are inclined to demand the impossible from the manufacturers. [pp. 61-62]
[Wehlte, Kurt. The Materials and Techniques of Painting. Translated by Ursus Dix. New York: Van Nostrand Reinhold Company. 1975.]
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