Lightfastness is the retention of a pigment’s chromatic properties over many years. A lightfast pigment is able to resist change when exposed to light and the atmosphere. This is important in paintings. Information about lightfastness helps the manufacturers and users to determine which pigment is suitable for use in artists’ materials.
Lightfastness can be determined in one of two ways: first, by examining the pigments used in past paintings that have not shown any visible change for seventy-five years; second, by an accelerated ageing test. A pigment that has shown. no change visible for seventy-five years is likely to remain lightfast for one hundred years or more.
The accelerated ageing test - which induces colour changes that may occur following long term indoor illumination - is theoretical and speculative. The manufacturers of pigments and paints have not adopted a standardised test for all because there are so many variables in the assessment of lightfastness.
The two common testing procedures are the 'ASTM Standard Test Methods for Lightfastness of Colorants Used in Artists’ Materials' and the 'Blue Wool Standard Lightfastness Test'. The ASTM abbreviation stands for the American Society for Testing and Materials. The Blue Wool test was originally developed for the textile industry and was later adopted by other industries that use colourant in their products.
The ASTM test requires tinted pigment samples to be exposed to either sunlight, fluorescent light, or xenon arc light filtered through a glass in a specified way for a specified time. Light from the Xenon arc lamp is the nearest artificial light equivalent to sunlight. The lightfastness of the pigment is determined based on the colour difference between samples before and after exposure to light as measured with a spectrophotometer. The pigment sample is then assigned one of the following ratings.
I—Excellent Lightfastness (very slight to no colour change after the equivalent of one hundred years of indoor museum exposure)
II—Very Good Lightfastness (Less permanent than Category I, but satisfactory for most indoor painting)
III—Not Sufficiently Lightfast to be used in artists’ paints (Borderline. Use with caution)
V—Very Poor Lightfastness
Manufacturers who adopt the ASTM guidelines for their testing procedures will specify the rating on the individual paint tube labels. An artist who wishes for their paintings to withstand the test of time should only use materials containing pigments with ASTM ratings I and II.
The Blue Wool test exposes pigment samples alongside eight samples of wool dyed with different blue pigments to a Xenon arc lamp. The light from the lamp causes accelerated changes in the blue pigment samples which can be matched with a number of years of indoor illumination under normal conditions. The test simply compares which dyed blue pigment (numbered 1 to 8) fades at the same time as the pigment sample and assigned one of the following ratings.
1—Very poor lightfastness (fugitive, less than 2 years)
2,3—Poor lightfastness (fugitive, 2–15 years)
4,5—Fair lightfastness (impermanent, 15–50 years)
6—Very good lightfastness (50–100 years)
7,8 —Excellent lightfastness (over 100 years)
Some manufacturers have their own testing procedures or will adopt the ASTM or Blue Wool procedures with modification to determine the lightfastness of pigments. The results of the testing are translated into proprietary information about “permanence” on the label. That information may sometimes be coupled with the ASTM rating. The manufacturer's permanence information usually uses letters and symbols such as AA, A, B, C or asterisk ****, ***, **, * with nomenclature such as 'Extremely Permanent', 'Permanent/Durable', 'Moderately Durable' or 'Fugitive' to indicate the durability of the pigment. There is usually no information about the length of time attached to each category because the manufacturers are concerned that it would be construed as a warranty. Under these circumstances, it's best that artists do not purchase materials where the pigments are rated to be less than 'Permanent/Durable'.