THE BLUE WOOL SCALE

The Blue Wool Scale measures and calibrates the permanence of colouring pigments. Normally two identical pigment samples, often of blue wool, are created. One is placed in the dark as the stable original and the other is placed in the equivalent of sunlight for a 3 month period. The amount of fading is then measured by comparison to the original colour and a rating between 0 and 8 is awarded. Zero denotes extremely poor colour fastness whilst an Eight rating is deemed not to have altered from the original and thus credited as being lightfast and permanent.