Color: An Introduction to Practice and Principles, 3rd Edition

8

Colorants and Their Mixture

Colorants are materials having light-absorbing and, in case of pigments, also scattering properties. If a colorant scatters light, it depends on the size of its molecules, its solubility in the substrate, and its tendency to form crystals or other aggregates. Colorants with small molecular size and good solubility are usually dissolved as single molecules in the substrate, that is, each colorant molecule is separated from others. This applies to most classes of dyes. Depending on solubility and attraction forces, some dyes form aggregates in the substrate. Colorants have a given solubility in a particular solvent, the maximum dissolved concentration depending on the temperature of the solvent. If the temperature is lowered, colorants precipitate, usually in crystalline form. In that case, there continues to be absorption but, depending on the particle size, there is also more or less scattering of light on the crystal surfaces. Light striking colorants is absorbed in a spectrally selective way due to the organization of certain electrons in molecules (see Chapter 1). Technically, the distinction between dyes and pigments is somewhat ambiguous. Vat dyes are called dyes even though in their marketed form they are pigments and in the final form on the textile substrate, they are in the form of crystalline particles that also scatter light. Disperse dyes, ...