The Blue Wool Scale measures and calibrates the permanence of colouring.
Traditionally this test was developed for the textile industry, but it was later adopted by the printing industry as a measure of lightfastness for ink colourants.
Light fastness tests, using the Blue Wool scale as a reference, are carried out using a Xenon arc lamp as a light source. Light from the Xenon lamp has the nearest artificial wavelength distribution to that of the sun.
The Blue Wool scale, of 1 to 8, uses samples of wool dyed with 8 different blue pigments each of which fade after different exposure times: 1 being the least resistant and 8 being the highest. The test simply compares which blue pigment fades at the same time as the ink sample on test. The ink is then allocated that number on the Blue Wool Scale.
Generally the time the ink takes to fade depends where in the world you are. A dull climate will be very different to a desert exposure in Africa. In addition the film weight of the ink applied can make a difference, as does the degree of white pigment within a given formulation.
Average values for time as measured in the U.K for full strength ink is as follows :-
- Blue Wool 1 –A few days up to a week in dull conditions. A few days down to a few hours in full sun. Very Low resistance.
- Blue Wool 2 –About twice as good as 1. Very Low resistance.
- Blue Wool 3 –4/6 weeks. Low resistance.
- Blue Wool 4 –Up to 3 months. Moderate resistance.
- Blue Wool 5 –Up to 6 months. Good resistance.
- Blue Wool 6 –Over 6 months. High resistance.
- Blue Wool 7 –Excellent performance, only a limited number of pigments give this level of light fastness even at full strength.
- Blue Wool 8 –Exceptional – virtually no fade.
Direct comparisons of the Blue Wool Scale to the light fastness required to meet BS 5609 do not exist. Labels passing BS 5609 have light fastness values over Blue Wool 6.