Friday, April 22, 2016

Pigment

Pigment is basically a colored particulate material of various origins most commonly employed to add to or change the color of a substrate. Pigment colors occur because the particles reflect certain wavelengths of light while absorbing others. Unlike dyes, which can be dissolved in a liquid, pigment particles are insoluble, or nearly so. Pigment is suspended in a liquid to make paint by a process called "mulling." Dyes may be soluble in water and attach to underlying substrates, such as cloth, with variable strength. Many dyes are obtained from plant material but may also come from animals, minerals, or be chemically synthesized. On the other hand, pigment can come from the ground--earth pigments like umbers--from plant, vegetable or animal sources, or from chemical synthesis or extraction.

These particles reflect blue and absorb red & green
Pigments have variable physical properties. Some pigments comprise relatively large sized  particles or even widely variable particle sizes while others are divided almost infinitesimally so that the pigment is like very fine dust. On the other hand, ochres are often "gritty" because of their relatively large and hard particle size. Some pigments may have such tiny particles they act almost like a stain. Some pigments are very opaque while others are semi- or completely transparent and allow underlying colors to show through. Titanium white is extremely opaque, for example, and will cover and hide almost any other color while lead white allows colors underneath to glow through when thinly painted over them.  Transparent colors can be painted over other colors, allowing optical mixing. Or transparent color can be mixed with opaque color with less reduction in color intensity (also called chroma).

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
Of course one of the most important properties of pigment is it's color. Painters of long ago, like Rembrandt, Velazquez, and their peers, used a severely limited selection of colors in their paintings, owing to a limited number of pigments available in those times. For many of the "old masters" of the 17th into 19th centuries a full palette of colors might have consisted of lead white, a bone black (ivory black today), with an earth red plus an earth yellow--generally ochres. Blues were available, but were either weak in color or could only be obtained in small quantities at enormous cost. Lapis lazuli was a well-known natural pigment was more expensive than gold in the 17th and 18th centuries. Lapis was so expensive that the amount to be used was often specified separately in contracts for paintings. It was and is also called ultramarine blue, meaning beyond the sea, because it came from such enormous distance. Today ultramarine blue is chemically manufactured for considerably less money and is one of the best known colors in use. Smalt was another blue but weaker than lapis that was made from cobalt-containing glass, coarsely ground. Smalt tended to be muddy and faded, often only in a few years.

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.


Pigment Library

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