ASTM D 4303
Standard Test Methods for Lightfastness of Colorants Used in Artists' Coloring Materials

This technical method describes the ways in which colorants (pigments, dyes, lakes and so on) used in most single-colorant artists’ paints can be tested for their relative lightfastness. It describes two types of light exposures methods, how to prepare samples for testing, the conditions of the exposures, the instruments used for measuring the light intensity during exposures, the instruments that can be used to accelerate exposures, the specifics of the languages used to characterize color and the instruments that can measure color using specific color languages, how to measure the colors, and how to interpret the results.

Two theories should be simply explained here. 1) Many artists use at least some white paint in their mixtures—or paint thinly on white surfaces. This is an assumption based on observation of artists at work in their studios and the observation of works of art in display collections. ASTM D01.57 therefore specified that single-colorant paints would be made with the colorant mixed with a standard white for testing purposes. That is, ASTM D 4303 specifies the physical nature of the materials being tested. 2) The second theory is that accelerated testing is correlated with the passage of real time—the test method reflects a reasonably accurate picture of what would happen if 20 to 100 years of testing would occur.

The second theory was first tested in the 1970s, by Robert L. Feller, a well-known conservation scientist  at the National Gallery of Art, Washington DC, and reported in a paper published in 1978. To date, the theory has been verified in a number of unpublished studies. The principal investigators who did the research for ASTM D 4303, led by Joy Turner Luke with Henry Levison (Liquitex) and Robert Feller, tested hundreds of oil and acrylic dispersion paints under a large variety of lighting and environmental conditions—from making paints that were applied to car fenders and exposed to south Florida weather, to producing laboratory-controlled applications and accelerated exposures in various xenon and fluorescent instruments. As time passed and data accumulated, it became easier to have confidence in the results of controlled accelerated testing so that it was possible to write a standard that all the D01.57 participants (about 45 people), and ultimately the entire ASTM society (some 30,000 members) could agree upon. (Supporting data are available from ASTM. Request Research Report D01-1036.)

Since its first publication in 1983, D 4303 has undergone some minor revisions and two major overhauls. Its current version was published in July 2006. A summary of the methods is that art material samples are specially prepared for testing and exposed under at least two of four possible conditions: to daylight, outdoors under glass in Arizona (dry) or Florida (humid), and to simulated daylight in a filtered xenon lamp exposure instrument with or without humidity added. All conditions are monitored to ensure appropriate exposure to a standard amount of light that includes UV and IR, to simulate the exposure the art material would endure if displayed indoors under glass. Test specimens are measured before and after exposure, and the color differences are calculated.

Depending on the results, the colorants can be placed in a number of categories of lightfastness, of which only two are of interest to artists: Lightfastness I and Lightfastness II.

ASTM D 4303 does not predict how long an art material will last, or even how long the colorants used in the materials will last—no test can predict that. It only establishes categories of lightfastness that are recognizable to artists and useful to them, and to the manufacturers of products that use them. The standard does provide a basis for developing specifications for art materials, such as ASTM D 4302. In that regard, D 4303 has been used successfully to produce specifications for artists’ acrylic dispersion paints, transparent and opaque watercolors—major art materials used by artists—and colored pencils, also a well recognized art material. In addition, D01.57 is nearing a ballot on a specification for pastels.