Aureolin is a common primary yellow choice for watercolorists who have been in the game for a long time. It’s prized for being transparent and a polite mixer, but it is known to turn brown when exposed to light. Thanks to a friend with a dot, I was able to try Aureolin without buying it. I already knew I wouldn’t like to use it much because of its lightfast issues and because I prefer stronger colors, but it is such a well-known color that I was curious to try it.
Pigment Stats for PY40
Pigment Category: Inorganic Synthetic
Chemical Name: Potassium Cobaltinitrite
Also Known As: Cobalt Yellow
Lightfastness: ASTM rates it at II, but anecdotally appears to be lower. I have not personally tested the lightfastness, but lightfast tests by Bruce MacEvoy and Kim Crick show it to fade and turn gray-brown.
Toxicity: Toxic; made from cobalt. Comes with Prop 5 and ASTM ratings. ArtisCreation.com gives it a B/C rating (between “possible hazard” and “hazardous”).
Observations of Daniel Smith Aureolin
Hue: A middle yellow with a slight brownish undertone. Not as brownish as Nickel Azo Yellow (PY150), though it also doesn’t get as dark due to being a lower-tinting color. Mixes like a “cool” lemon yellow with the oranges having a cool, muted quality and the greens being more vibrant.
Transparency: It is indeed fully transparent and has that pleasant liquidy quality that some transparent colors have. It mixes like a chameleon.
Dispersion: Low! It stays put. This surprised me as the only other truly transparent yellows I know of, the heavy metal yellows PY150 and PY129, are wildly dispersive. As a yellow that is both transparent and low-dipsersion, PY40 is quite unique.
Lifting: Lifts very cleanly. This also makes it unique; PY150 and PY129 are very staining.
Tinting Strength: Low to moderate.
What Others Say
AVOID. Aureolin (like alizarin crimson) is a pigment that modern chemistry has made superfluous. There are many excellent yellows available with better lightfastness.
Bruce MacEvoy, handprint.com (2010)
Conclusion
While the poor lightfastness concerns give a solid reason for anyone to avoid this pigment, it’s not too hard for for me because even its strengths are not ones that I particularly enjoy. “It’s not too strong” and “it’s lifting” are benefits that to me are weaknesses: I prefer overly strong, staining pigments that I can glaze as I tend to lift only by accident.