Notebook

Notebook, 1993-

MATERIALS & METHODS - Pigments

Compatibility with
Other Pigments
Although fully light-fast, a pigment can, by chemical reaction with other pigments, quickly discolor even without the influence of certain media. For instance, emerald green, a copper arsenate, is not compatible with sulfur-containing colors like cadmium yellow or ultramarine. Theoretically, mixtures of ultramarine and flake white should form brown lead sulfide, as should mixtures of flake white and vermilion [mercury sulfide]. Strangely enough, this never happens in practice. The expected chemical reaction does not take place with oil colors. Because the particles are isolated by a film of oil. In aqueous media flake white is avoided anyway, because it blackens on reacting with sulfur compounds in city air. In fresco techniques lead white is replaced by lime. Until recently, painters were cautioned against the use of titanium white with coal-tar pigments, because it can bleach these under the influence of light. Now there is a new grade of Kronos titanium white, which does not have this defect. Tests carried out by Dr. Robert Haug at the author's institute have shown that Prussian blue and Milori blue can bleach somewhat when mixed with white. However, this would only occur in aqueous techniques under the prolonged influence of moisture, and it has only rarely been observed in practice.

If experienced artists today avoid all copper pigments as a matter of course, they will have eliminated the greatest danger. After all, they cannot be expected to continually consult the commercially issued mixing tables that list all dangerous mixtures. [p. 61]

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









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