Pigments - Definitions

Dyes can be used to make pigments. This was kown long before the invention of coal-tar dyes made production possible on a lage scale. There are a considerable number of organic dyes in nature that were valued as pigments by the old masters. If a piece of white chalk is dipped in a solution of eosin dye and left in water, a substantial part of the eosin will be extracted, dyeing the water red, and only a small proporiton wil remain in the chalk as a pale pink tint. There are, however, some chemical methods for fixing dyes to a white substrate in such a way that the combination will be insoluble. As this chemical process happens too rarely in nature, it is performed artifically in chemical plants and yields a multitude of pigments of may different hues. There are many white minerals suitable as bases or substrata, e.g., barite, gypsum, and clay, as well as pale varieties of green earth, the color of which will not interfere wih strong dyes. In many cases the bases used are not natural minerals, but are artificialy precipitated during the dyeing process. This process is known as color striking, and pigments thus obtained are sometimes called lakes.

Ocasionally certain dyes will be difficult to fix onto a base, because unsuitable substrates were chosen or, owing to faulty manufacture, traces of soluble dye remained in the igment. These can be dissolvrd later in the vehicle and that means not only in water, but in oils, alchohol, or other liquids usd in painting. In the trade, one speaks of "oil-fastness,' or "fastness to oil, water, spirit," etc. If a pigment dissovles in the medium, it is said to "bleed." This is most unpleasant, [p. 51] as such a pigment imparts its color to any paint applied over it. Some fo the brilliant shades of poster color are particularly prone to bleed in water. Commercial artists will be embararrassed to find that their white lettering on a bright red ground suddenly turns pink. In paintings, such shortcomings can have dire consequences. Most brands of permanent rrd, for instance, are notorious for their bleeding. Many artists may have noticed when using a students' gouache or watercolor box that an orange, green, or violet stain often remains on the interior surface of the lid, quite resistant to all cleaning attempts. Paint from an artists' watercolor box sold in Austria could not be washed out of the paper properly because soluble components had dyed the paper fibers.

Another related case concerns an artist who had been asked to varnish some valuable color reproductions. He spoiled several prints because the solvent of his varnish partly dissolved the red printing color. These examples should suffice. Special care must be taken when using cellulose lacquers as coatings, for otherwise quite reliable pigments will react unfavorably with the strong solvents necessary for them. This is further proof that in case of failures all three basic elements of painting terhnique must be considered. The artist should keep to a small range of reliable pigments, and even within this range he should choose carefully. The followng will aid him in his choice. [pp. 51-52]

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