Currently, pastel manufacturers are working to get their ASTM ratings. Soon the pastel public will have the same information that oil, watercolor and acrylic painters have been enjoying. Since certain pigments are less permanent than others, it will prove helpful to have this rating available when choosing individual pastels. General pigment ratings have been around for a long time; the interaction they have with certain binders and fillers, used in the manufacture of the pastel sticks, has not been tested. Some unstable pigments when mixed with other very stable pigments and fillers perform very well, while other very stable pigments will perform poorly when mixed with certain pigments and fillers. Red is one of the most vulnerable color families in pastel; the best hues are often made with heavy metal pigments. Since pastel is a dry medium, easily made airborne, heavy metal-based pigments can be dangerous. This has led some manufacturers to use more fugitive pigments that are prone to fading. Knowing the lightfast rating of these individual sticks will allow us to choose the most permanent hues possible, adding to the longevity of our finished pastel paintings, and increasing the medium’s reputation.
It is not just our pigments that are susceptible to the effects of light. Pastel surfaces can be as well. Colored papers and pre-toned surfaces need to be scrutinized for their permanence and ability to stand up to years of light exposure. Many colored surfaces are similar to colored mat board used in framing. These are colored with dyes, instead of more expensive pigment-based colors. If you work in a fashion that allows a degree of the surface to show through, it is imperative that you understand its lightfastness rating.
If the manufacture of your favorite pastel brand or surface does not provide an ASTM lightfastness rating, you can easily make tests for yourself. In fact, these are wise to do even when the ratings are available. Start by taking the surface you wish to test and place strong pastel marks from the individual sticks you want to test (making note of what they are). Place an opaque strip of cardboard across a portion of the test sheet. Place the test sheet in a sunlit window for a couple of months (see my example above). At the end of the time frame, remove the opaque strip and analyze the effect light has had on the exposed area. If there is virtually no difference, you are using the most lightfast materials. If there is considerable change, you need to rethink some of your choices. We owe it to the purchasing public and ourselves.