The Blue Wool Scale measures and calibrates the permanence of colouring dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s. Traditionally this test was developed for the textile

Textile

A textile or cloth is a flexible woven material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by spinning raw fibres of wool, flax, cotton, or other material to produce long strands...

s industry but it has now been adopted by the printing

Printing

Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....

 industry as measure of lightfastness of ink colourants.
Normally two identical dye samples are created. One is placed in the dark as the control

Scientific control

Scientific control allows for comparisons of concepts. It is a part of the scientific method. Scientific control is often used in discussion of natural experiments. For instance, during drug testing, scientists will try to control two groups to keep them as identical and normal as possible, then...

 and the other is placed in the equivalent of sunlight for a 3 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.

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. This difficulty is overcome by the bluewool testing method. Absolute values of fading will depend on light intesity. Relative values of fading, comparing the sample with the standard blue test strip, will depend less on intensity. For example if a pigment is rated as "BW5" it can be expected to fade to the same degree as strip number 5 on a bluewool test card, for any specific light exposure.

The method of comparison between the sample and a test strip enables accelerated testing to be carried out under intense artificial illumination.