Doing a lightfastness test on a paint color or pigment isn't complicated, but takes patience as the paint will need to be exposed to the sunlight for some time. It is a test that can be done for any medium, whether watercolors, oils, or pastels.
Difficulty: Easy
Time Required: Several weeks, even months (waiting while the paint colors are exposed to sunlight)
Here's How:
- Paint a swatch or area of the color(s) you want to test for lightfastness on a sheet of heavy, smooth watercolor paper. If you're testing multiple colors, leave a section of white paper between them. Either apply the paint as you would usually use it in a painting or, be more thorough and paint a range, from thin to totally opaque.
- Paint the samples large enough and in such a way that you'll easily be able to cut it in half. Write the name of each color next to the sample.
- When you've painted all the colors you want to test for lightfastness, cut the sheet in half. Place one half in a dark drawer; this will be used as the "before exposure to sunlight" comparison.
- Place the other half of the sheet in bright, direct sunlight. Stick it to the inside of a window that gets a lot of sun or if your car is parked in the sun, on the dashboard (provided the glass isn't UV-protective!).
- Now you have to be patient and let the sun do its thing. After a month, but ideally several more, compare the half of the sheet kept in the shade to the one in the sunlight. Any differences indicate that a color is not lightfast.
Tips:
- Check that the sheet of watercolor paper doesn't have a yellowish bias to it, especially if you're doing a lightfastness test with watercolor paints.
- If this simple lightfastness test grabs your interest, you might want to move on to do a comprehensive lightfastness test.
