Colorants
When a colorant is used in a formula and is observed in reflected light (light is incident on the surface and reflected to the eye), the masstone of the colorant is evaluated. When the same formulation is observed in transmitted light (i.e., the light is observed through the pigmented sheet or film), the transparency and undertone of the colorant is evaluated. When the colorant is evaluated in a combination with white pigment (generally at a ratio of 5 parts of colorant to 95 parts of white pigment), the tint tone of the colorant is evaluated. It must be understood that the undertone of a pigment (when observed in transparency) is not necessarily the same as the tint tone of the pigment when observed in combination with white [1].
Definitions of color and methods for numerical evaluation of the color properties of a pigment are discussed in Section XVI.A.
VIII. CLASSIFICATION OF PIGMENTS
Two general methods useful in classifying pigments involve chemical composition and color. Any general study of pigments should begin with their chemical composition, as this generally defines their performance properties. Individual studies can then be made, based on the color requirements of the particular problem.
IX. INORGANIC PIGMENTS: OXIDES (HYDRATED OXIDES)
Many of the most durable pigments are metallic oxides. Because this class of compounds is generally unreactive, the properties that are most often manifested are excellent heat stability, light stability, chemical inertness, lack of bleeding and migration, desirable electrical characteristics, and very low absorption. These properties are improved by calcining, and those oxides that have been so treated are especially suitable.