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.[1][2]
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 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. [3][4]. 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. The Ultraviolet (UV) radiation in light is responsible for ink fading. As the intensity of UV radiation differs from place to place, the ink fading also depends on place. It will be more in areas with more UV radiation and vice versa.
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