If you think of paint as a two-part material, with the oil being the glue that binds the pigment particles into place, it is the pigment that is responsible for delivering the colour. The pigment must therefore have the following qualities:
- it must be light-fast to a reasonable level (i.e. far more light-fast than would be acceptable for house-paint)
- it must not be soluble in oil or in thinners like turpentine
- it must have the right pH balance (i.e. acid to alkaline) with the linseed oil or other oil into which it is ground, otherwise the pigment might have little resistance to the bleaching effect of an acidic oil (which would cause a marked colour shift)
- it must not, in normal circumstances, react chemically with other pigments in the range
So paint manufacturers choose pigments which have these properties, or rather the ability to resist these forms of attack. Broadly speaking we divide pigments into two groups: inorganic and organic.