Notebook, 1993-


Fastness to Media

The behavior of pigments in binding media is in practice far more important than the problem of compatibility between pigments. For example, Prussian blue in oil is a perfectly satisfactory artists' color; in casein media it becomes unreliable and in fresco it will turn dirty brown within a few hours. Chrome yellow turns slightly green in oil, the dark shades turn an ugly brown, and the middle chromes change to an earthy ocher color. Artists can therefore only use them in aqueous [p. 61] techniques. Chrome yellow is also unsuitable for fresco painting, since the alkaline lime decomposes the light varieties to form chrome orange. By analogy, chrome greens are quite good in tempera techniques and adequate for oil studies, but they are quickly destroyed in fresco because both its components [chrome yellow and Prussian blue] are not fast to lime. Several otherwise quite satisfactory, light-fast pigments react chemically with lime, even when applied on lime mortars or cement, or in alkaline casein. For practical purposes, there is an adequate range of alkali-proof pigments, sometimes called fresco colors. Some of them are not only alkali-resistant but are also stable in cement.

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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