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A quick & dirty lightfastness test
2019/08/09
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The good news is that a lightfastness test is really easy to do. For the simplest, "quick and dirty" paint test (shown at right), here is all you do:
(1) buy (a) one premade art store frame, in size about 10"x12" or so, with glass (not acrylic) glazing and a sturdy backing board, and (b) a sheet of good quality, cold pressed (CP) watercolor paper, preferrably 300GSM weight or heavier to reduce cockling or warping over time
(2) cut the sheet of the watercolor paper to fit into the frame
(3) brush out equal sized samples of watercolor paints, diluted 1 part paint to 8 parts water, in evenly spaced rows on the paper
(4) check the fit of the watercolor paper in the frame, then securely mount the sample sheet to the frame backing.
(5) remove the glazing sheet and apply strips of aluminum metallic tape to the inside surface of the glass glazing, spaced so that one half of each paint sample is protected from light
(6) mount the samples in the frame under the glass glazing, making sure that the watercolor paper presses firmly against the strips of metal tape, with no gaps that can allow light to leak underneath
(7) put the framed samples in an unshaded, south facing window of your home or studio, a window that is not made with UV protective glass, and
(8) let sit undisturbed for four months between May and October.
When four months have passed, remove the samples from the frame and look for shadows of the aluminum tape mask across the paint samples. Any paint showing a tape shadow is unsuitable for artistic use.
This quick and dirty test is sufficient to reveal any problems in your customary materials, and is easy and informative enough to do at least once to ensure the quality of your chosen watercolor brands.
There is no "exposure meter" in the form of a blue wool scale, but unless you live in northern Canada or Europe, or your weather inflicts continuous overcast on your sky, four months is more than sufficient to provide a diagnostic dose of light. (One month, usually, is enough to identify bad paints.)
You can go about the tests with more rigor, and the procedures are explained below for reference and, dare I hope, actual use. However, the quick and dirty test is sufficient for the basic purpose — identifying paints that fade. Do not pretend that testing your paints is something too complicated for you to do.
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