The Blue Wool Scale measures and calibrates the permanence of colouring pigments. Traditionally this test was developed for the textiles industry but it has now been adopted by the printing industry as measure of lightfastness of ink pigments. Normally two identical pigment samples are created. One is placed in the dark as the control 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 a rating of eight is deemed not to have altered from the original and thus credited as being lightfast and permanent.
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