Lightfastness Testing for Pastels

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What You’ll Need:

• Pastel paper (8×10 white, sanded pastel paper that will accommodate your samples)
• Wood board or very thick-ply matboard serving as a hard support. This needs to be an inch larger all around the perimeter than the sheet of paper that will hold the test samples.
• The pastels you typically use
• Opaque matboard cut to 1-inch-wide strips
• Blue wool test card*
• A mask* that contains a strip of colored material with blue-gray rectangle
• Pencil

Orienting the sheet with the longer side placed vertically, draw horizontal lines to form boundary divisions that will contain your pastel test samples. Your lines should be about 3/4 of an inch apart, stopping about 1 inch from the side edges of the paper. The sample sheet should appear ruled, similar to loose leaf notebook paper.

Draw three vertical lines with the first one at the right edge of the end of the horizontal lines. The next line should an inch to the left of the first vertical line, and the final line another inch over to the left. Your paper should have lots of horizontal lines ending on the right, with two vertical columns.

Use the horizontal line to write in the name of the pastel you’re testing. Place the pastel within the top and bottom boundaries of the horizontal line. Put the pastel over both of the columns on the right side of the paper laid out horizontally so that you have a strip of pastel 2 inches long and 3/4-inch high.

Rub pastel into the sample card, covering it completely with a dense layer of color. If the pastel is extremely soft and prone to lifting, the sample may be covered with a paper-like glassine and burnished lightly with the bottom of a spoon for good adhesion. (Do NOT spray the sample with fixative.)

Next, place a strip of inch-wide matboard (slightly longer than your paper) on the left vertical column so the sample won’t be exposed to light. The other vertical strip will be exposed to light, and will provide a comparison to the unexposed pastel samples. The matboard strip should be firmly taped to the board, completely covering the unexposed sample. (Avoid letting the tape come in contact with any of the samples.)

A blue wool test card provides a rough estimate of the amount of light the sample receives so that an adequate test of the samples can be done. Mount the card in a space reserved so half the card will be exposed to light and the other half will be covered by the opaque strip. The blue wool card should be oriented so that the test strips are horizontal.

The blue wool card has dyes that fade in a predictable way, thus it’s used to gauge the amount of light exposure. The strips will fade from the top down with each strip being a bit more resilient as one moves down. Periodically compare the color of the third blue wool strip with the printed sample on the mask—and stop the testing when the third blue wool strip matches the color of the blue-gray printed rectangle on the mask.

The sample card with the pastels, blue wool card and opaque mask should be placed in a window that gets bright light most of the day. Southern exposure is recommended. Make sure the window doesn’t have treatments that would block ultraviolet light. Many new windows have coatings, films or gas-filled cavities that block the transmission of harmful UV rays. In this case, you want those rays to rain down on your sample to provide a real-world test of your pastels’ lightfastness.

Examine the sample card every few days to see if any changes are taking place. Note on the sample card the first day that the unexposed side of the samples looks different than the exposed side, then note that on the test card. Watch the samples carefully and note any changes. Check again periodically and stop the experiment when the blue wool and the mask reference match.

Keep the pastels that pass the test and look for replacements for the ones that don’t. This is an effective way to assure that your pastels are maintaining long-term color fidelity.