MITRA Forum Question Details

​I asked a chemist recently if cobalt arsenate (the original cobalt violet light, prior to the rise of alternatives such as PV 14, PV 49, and PV 47) is a risk for producing arsine (a problem with the copper arsenates like emerald green and Scheele's), as I haven't heard of it blackening nor losing its color in some other way on the pigment level when used in artists' paints.

He is an organic chemist so perhaps he doesn't know but he was still concerned, off the cuff, about the possibility of arsine being formed. I have looked at the literature and haven't found any detailed information whatsoever about this pigment's stability. I couldn't find a single conservation article devoted to it via Google Scholar.

One hobby chemist who made some as a demonstration said he believes it is tightly bound enough to not react. Other things I have read that are vague and shallow have said roughly the same thing. My guess, based on the complete lack of writing about the pigment blackening, fading, changing color, et cetera — is that it's the only stable arsenic pigment used in art, as the sulfides (orpiment/realgar) have various stability issues including changing from one form to the other and losing chroma.

However, I would really like to know. Holbein was the last to produce it. Its catalogue from the 1980s still showed it. One Youtube chemist's product looked like PV 47 to me, so I am not sure if cobalt arsenate has any artistic value, given its higher toxicity. However, I am wondering if anyone has done measurements for its chroma and how they compare with PV 14, PV 49, and PV 47. Of course... how well that chroma stands up in terms of air pollution and pH issues is also the question. My understanding is that PV 14 has not only the typical cobalt phosphate heat sensitivity but is also sensitive to acidic and basic pH.

Additionally... for the chemists here... as PV 47 and PV 49 are alterations of the PV 14 molecule... is it possible to add lithium or ammonium to cobalt arsenate in like manner?