|These particles reflect blue and absorb red & green|
Speed of drying is another property of pigments that an artist ought to understand. Some pigments dry rapidly and some dry so slowly they need help. Umbers dry quite quickly--a few hours to a day or so--owing to the presence of manganese in the pigment. On the other hand, colors like the cadmiums are slower to dry. The speed with which oil paint dries also depends significantly on the vehicle (the oil) used to compound it, which can slow or speed drying, as can specific mediums and dryers. Linseed oil dries by oxidation, by taking up oxygen from the atmosphere in a slowly-proceeding chemical reaction that can take months to complete and turn the oil into a solid polymer. Most artist-grade linseed oil will dry to touch in a day or two at most. Walnut oil, another commonly used vehicle for paint, may take more than a week to dry. The nut oils take up oxygen more slowly.
Another very important property of pigment (and dyes) is light-fastness, or permanence. If one is to make a picture that will last, the pigments ought to resist fading as much as possible. Some pigments are prone to rapid and severe fading and are termed fugitive. Today, artist-grade paint is made with light-fast pigment, for the most part, and resists being faded by sunshine. Manufacturers label their paint tubes with grades of light-fastness. The only pigment I know that's in common use and rated as less than permanent is original alizarin crimson, which can fade over a long period of light exposure. There are synthetic substitutes for that color today that are impervious to light exposure, though.
|Ground lapis lazuli|
There is considerably more to think about when it comes to artist-quality pigments, and today's range of synthetic and natural colors is truly astounding. As I've been working through these paragraphs, it occurred to me that synthetics are a whole topic in themselves and I need to look at more of that information before I write much about them. Suffice it to say that many of the most valued pigments these days are synthetic--Prussian blue, alizarin crimson, the cadmium colors, and a host of others have come to dominate the palettes of most artists.