The Blue Wool Scale
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 a measure of "lightfastness" of ink colourants and also within the polymer industry for measurement of pigment & colour stability (lightfastness). Lightfastness is the chemical stability of the pigment or dye under long exposure to light.
Note: this should not be confused with Permanence or fastness which refers to the chemical stability of the pigment in relation to any chemical or environmental factor, including light, heat, water, acids, alkalis, or mold. For example, the pigment ultramarine blue is extremely lightfast, but it will fade if brushed with a dilute acid.
The Ultraviolet (UV) radiation in light is responsible for the ink fading and hence the change in the blue wool. Hence the blue wool scale has been widely adopted as a standard during UV exposure or UV weathering tests.
The normal procedure is to take two identical dye or pigment samples. 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. Most apparel will have a light fastness of 4 whereas most furnishings will have a light fastness of 6 and most polymer materials for outdoor use would require a light fastness of 7 or above.
Blue wool textile fading cards or kits typically consist of 8 swatches of blue wool dyed to various levels. They consist of eight strips of wool mounted side by side on a small card; each strip or reference is colored with a blue dye that fades after exposure to a known amount of light. The dyes have been chosen so that each reference takes about two to three times longer to begin fading as the next lower reference in the scale. (Under normal solar testing conditions, reference 1, the least permanent, will begin to fade in 3 hours to 3 days, depending on geographic location, season, cloud cover and humidity; reference 3 will fade in 5 days to 2 weeks; reference 6 in 6 to 16 weeks; and reference 8, the most permanent, in 6 to 15 months.) These scales are used for paint lightfastness testing under international standard ISO 105-B, and are also used by gallery curators to measure the accumulated amount of light received by museum displays of paintings, textiles or photographic prints. The blue wool scale cards willl normally be used in conjunction with grey scale cards in order to assess the degree of change.