The Blue Wool Scale measures and calibrates the permanence of colouring

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

s. Traditionally this test was developed for the

textile Textile is an Hyponymy and hypernymy, umbrella term that includes various Fiber, fiber-based materials, including fibers, yarns, Staple (textiles)#Filament fiber, filaments, Thread (yarn), threads, different #Fabric, fabric types, etc. At f ...

s industry but it has now been adopted by the

printing Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...

industry as measure of

lightfastness Lightfastness is a property of a colourant such as dye or pigment that describes its resistance to fading when exposed to light. Dyes and pigments are used for example for dyeing of fabrics, plastics or other materials and manufacturing paints o ...

of ink colourants.Textile Fading Card example 1
Gaylord. Retrieved 2010-10-18.Textile Fading Card example 2
, Pel. Retrieved 2010-10-18. The

American Association of Textile Chemists and Colorists AATCC—the American Association of Textile Chemists and Colorists— is a 501(c)(6) not-for-profit professional association that provides test method development, quality control materials, educational development, and networking for textile and a ...

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 Control may refer to: Basic meanings Economics and business * Control (management), an element of management * Control, an element of management accounting * Comptroller (or controller), a senior financial officer in an organization * Controlli ...

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.Faded Card example
, Talas. Retrieved 2010-10-18.Faded Card example
PDF. Retrieved 2010-10-18. 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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

(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 intensity. 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.

References

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