Colourants: dyes VS pigments
The ‘Blue Wool Scale’ was created to measure these levels of lightfastness. Strips of colourant are placed in laboratory ultraviolet conditions designed to mimic sunlight over many years. Part of the strip is protected from the light and after a period of exposure time the strip is revealed and the fading can be measured on the Blue Wool Scale from 1-8; 8 being the most fade resistant. This test was designed for the textiles industry, and has now been adapted to test pigments as well.