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How to Test for Lightfastness

Artists, photographers, printers and other users of colored materials need to be familiar with how lightfast the elements of their work are. Some materials can be exposed to strong light for decades without showing a visible color change. Others may start to change within a year. For paints, particularly oils and acrylics, this information is often readily available on the product label, which indicates ASTM (American Society for Testing and Materials) lightfastness ratings of I (Excellent), or II (Very Good).1 Other rating systems, of varying reliability, may also be found on art materials, and many products carry no rating.

Fortunately, it is possible for the user to establish lightfastness ratings for materials or to confirm manufacturers' information. This may be done by using an experimental adaptation of the mechanism which causes fading; i.e., predicting effects of long term exposure to artificial and natural light sources by exposing samples for a relatively short time in strong direct light. At its simplest, this test can be accomplished by applying the material to be tested to a suitable support, cutting the sample in half, and exposing part of the test specimen behind a window that provides as much sunlight as possible, while keeping the other half shielded from all light sources. Observing color change of the test against the retained sample over a period of several months will provide a good indication of how the material will fare after years of hanging on a wall, out of direct sunlight.2

A limitation to this approach is that it is difficult to get repeatable test results, due to daily, seasonal, and yearly variations in the intensity and duration of sunlight coming through a particular window. Geographical location can also cause tremendous variations. For example, a test run in the northeast in the summer months may show fading, but if exposed for the same number of days during the winter, there may be no visible color change. However, if the samples were exposed for the same number of days in the winter in Florida, they could fade drastically.

This problem of delivering consistent levels of total exposure was solved with the idea of simultaneously exposing materials of known lightfastness and terminating the test at a point when a predetermined amount of fading occurs. With art materials, this task is accomplished with the use of a series of eight strips of dyed blue wool fabric, with each piece being sequentially more lightfast. Exposed samples are then rated in comparison to the behavior of the fabrics. A sample fading at the same rate as "Blue Wool #1" gets a rating of "1". On the other end of the scale, if it shows no fading as "Blue Wool #1-7" fade, the material would receive a rating of "8". If "Blue Wool #6" fades before the material being tested, it would be analogous to an ASTM rating of II (Very Good) and the material would be considered durable enough for archival intentions. If the specimen continues to show no change by the time "Blue Wool #7" fades, it would be similar to an ASTM rating of I (Excellent).

A refinement of this test is made when the sample is left in one piece, with one half of it covered to shield it from sunlight. In this way, the adjacent areas of exposed versus not exposed sample provide easier determination of color change. There are many other variables, particularly in how the samples are viewed and rated, that make the test even more reproducible. Great efforts were made to refine the method enough that it could be published as an ASTM Practice.3

After this work was done, it was apparent that it would be cumbersome for most art material users to obtain the "Blue Wools", a suitable support, directions for running the test and then assemble an apparatus to hold it. Now, with the development of the Lightfastness Test System, everything needed to test your own materials is provided in one package, with no assembly required. It also makes a great teaching and demonstration tool.

1 Under conditions of museum exposure, ASTM Lightfastness II ( "Very Good"), materials should remain unchanged for approximately 100 years. Materials rated as worse than II (III and above) are not recommended for art that is intended to last.

2 For further discussion on assembling your own lightfastness test, consult Gottsgen's The Painter's Handbook (Watson-Guptill, 1993, pp. 127-131)

3 Practice for the Visual Determination of the Lightfastness of Art Materials by the User (ASTM D 5398) and Practice for the Visual Determination of the Lightfastness of Art Materials by Art Technologists (ASTM D 5383), available from: ASTM, 1916 Race St., Philadelphia, PA 19103

ACKNOWLEDGEMENT - The ASTM Test Methods upon which the GOLDEN Lightfastness Test System is based are largely the result of work done by Joy Turner Luke, Artist and Educator.

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