Coloring With Iron Oxide Pigments
Synthetic (manufactured) iron oxide pigments are in the form of particles ranging approximately from 0.1 to 1.0 micron. The difference in color between one pigment and another is due to the shape and surface structure of the particle. The characteristics of the surface of each particle determines which of the color components in the light that is striking it will be reflected and thus recognized by the eye. This is the reason why pigments appear different in color when subjected to different light conditions, i.e., daylight vs. artificial or northern vs. southern skylight.
Light from these various sources have different components (wavelengths) which when selectively reflected by the same pigment will not appear to the eye as the same color.
Iron oxides, when used as a colorant, work by masking the surface they are covering so that the eye will see the particles of the pigment or more specifically recognize the light component being reflected by it, thus establishing a particular shade.
It is important to recognize that the particles of dry iron oxide pigments do not exist individually. Electrostatic forces and packing compaction cause the individual particles to clump together to form agglomerates ranging in size from 20 to 150 microns with the majority falling between 60 and 120 microns.
This article comes from archimg edit released