Classification Of Pigments

The painter naturally classes pigments according to their colour, or more exactly according to what are called the constants of colour - namely, hue, brightness and purity. He also takes into account transparency and opacity, although these terms are conventional and comparative only, since no pigment is perfectly transparent, none perfectly opaque. Another basis of classification, and a very important one, that of stability, will be considered in the next chapter; at present we are concerned with none of these methods of grouping pigments, but with others founded either upon their origin, or their physical characters, or their chemical composition.

Pigments are often classified into two groups - the mineral, and the organic. It is necessary to divide these groups further, in some such way as this:

Such a classification brings into prominence one marked distinction between the two groups, for, in accordance with one's expectation, the mineral pigments are, as a rule, characterized by a much higher degree of permanence than those of organic origin. [The chemist looks upon all compounds containing carbon, save the carbonates, as organic; but at the same time the distinction between organic and inorganic, or organic and mineral, is nothing more than a convenient convention.]

Other bases of classification are afforded by physical or mechanical characters. Pigments may be fixed or volatile, soluble or insoluble, crystalline or amorphous, substantive or adjective. There are difficulties in carrying out these schemes of classification, and it will be found that distinctions of physical character are utilized to the best purpose when connected with such a chemical classification as is offered below.

The simplest chemical classification is this:

Elements ; as graphite, lamp-black, gold. Compounds ; as aureolin, viridian, vermilion. Mixtures ; as yellow ochre, brown pink, rose madder.