Inorganic Pigments-Other

‘Colourants’ (materials used to impart colour) may be either pigments or dyestuffs. A pigment is and remains insoluble when used in a surface coating, whereas a dyestuff at some stage is soluble either in the solvents or the binder or both. This solubility or insolubility is the reason a coating coloured with an insoluble pigment is to a greater or lesser degree opaque. A dyestuff, on the other hand, may impart a great depth of colour (their tinting strengths are usually very high) to the coating, but in a thin-enough section the coating will be coloured but transparent. Dyestuffs are used only to a limited extent in surface coatings because they are transparent, and generally their light-fastness is fairly poor. Typical examples of their uses are in flexographic printing inks, some metal decorating inks, foil coatings, and timber stains. Where maximum light-and weather-fastness is required with transparency (e.g. in metallic automotive finishes) a high light-fast pigment having poor opacity is used.