Some pigments are stable enough to withstand the ravages of light. The iron oxides and the heavy metals such as cobalt and cadmium are composed of solid particles of material unaffected by UV light.
That’s why barns are a brownish red: Iron oxide is one of the cheapest and most lightfast pigments. Farmers used to make the binder out of milk-based proteins.
Carb-Othello pastel pencils and Caran d'Ache colored pencils did very well when subjected to the same eight months of sunlight that faded the markers earlier. 
Even though watercolors can sometimes be more fugitive than oils or acrylics, these pan colors from a portable set by Schmincke hardly changed at all.
And the oils did OK, except for certain pigments. We'll look at that question tomorrow.
5 comments:
Thanks for elaborate tests!
So, surprizingly, the Sharpie Permanent Black is indeed fairly permanent.
I'm reading this post while waiting for a large, wet wash of Winsor Newton Cobalt Blue (Lightfastness I) watercolor to dry.
While still waiting for that wash to dry, I was mildly surprised by parts of what Wikipedia has to say about lightfastness:
The most stable painting medium is pastel, but modern lightfast watercolors are now more stable than oil or acrylic mediums. Unfortunately, paint manufacturer lightfastness ratings are not always trustworthy. However, because they have been demonstrated to be impermanent in watercolors, certain pigments (paints) should never be used under any circumstances. These include: aureolin (PY40), alizarin crimson (PR83), genuine rose madder (NR9), genuine carmine (NR4), genuine vermilion (PR106), most naphthol reds and oranges, all dyes (including most "liquid watercolors" and marker pens), and paints premixed with a white pigment, including paints marketed under the names naples yellow, emerald green or antwerp blue. Most of these are colorants invented in the 19th century or before that have been superseded by far more durable modern alternatives, and these are usually sold as "hue" paints (e.g., "alizarin crimson hue" is a modern pigment that resembles alizarin crimson). Industry labeling practice is to include a lightfastness rating on the paint packaging, and painters should only use paints that have a lightfastness rating of I or II under the testing standards published the American Society of Testing and Materials (now ASTM International).
This is great. Keep this coming. I'm watching for the inks and colors I use for sketching.
James - I've got a LOT of information about the lightfastness standard for coloured pencils and the lightfastness of different brands of coloured pencils on my information site Coloured Pencils - Resources for Artists. You (and others reading this post) may well find it interesting.
I started to take a very keen interest in lightfastness after seeing the CPSA blue wool test charts produced when they were trying to get the standard agreed - some of the colours had faded to nothing.
Ever since I've always advocated finding out about the lightfastness ratings because coloured pencils used to be used by illustrators who of course had no concerns about how long a drawing would last!
Now that coloured pencils are being used more and more for fine art, I think there's more effort going into to producing lightfast colours - but the effort and results are still very variable!
Thanks, Steve and Katherine for that helpful information. I'll try to do some links in tomorrow's post, but I'm sure I'll leave a lot out!
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