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The Blue Wool Scale measures and calibrates the permanence of colouring dyes. 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 colourants. The American Association of Textile Chemists and Colorists pioneered the work on these test methods and together with the ASTM D13 committee.
Normally two identical dye samples are created. One is placed in the dark as the control and the other is placed in the equivalent of sunlight for a three-month period. A standard bluewool textile fading test card is also placed in the same light conditions as the sample under test.The amount of fading of the sample is then assessed by comparison to the original colour.
A rating between 0 and 8 is awarded by identifying which one of the eight strips on the bluewool standard card has faded to the same extent as the sample under test.
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.