MITRA Forum Question Details

Many pigments, when hydrated in paste form, emit a slight odor.  I currently have 20 open jars of pigment pastes (natural and synthesized colors, ultramarine among them) in my studio (I'm letting them dry out so they become "pigment cakes", or dry chunks of colors; this minimizes the issue of pigment dust) and the room smells faintly swampy.  There's always a bit of smell when I make pigment cakes and I've never had a problem with my colors.

The modern process of synthesizing ultramarine blue involves sulphur (it is, as a chemist told me, "a sodium-silicate zeolite trapping polysulfurs in a silicate cage").  Hence ultramarine blues can smell like sulphur (a rotten egg).  The strength of the odor depends on pH:  In a pH 6.5 or greater environment, ultramarine is stable.  If its environment falls below pH 6 (becomes acidic), the color can begin to breakdown and release sulphur fumes.  So if what you're smelling is a strong sulphur odor, it might be a good idea to toss out the color. 

Naturally occurring, genuine lapis lazuli has the exact same chemical formulation as synthesized ultramarine blue.  There is a history of genuine lapis fading or turning dark, called "ultramarine sickness", generally caused by acidic pollutants.  The same thing can happen to synthesized ultramarine (since it's chemically identical to lapis) - and yet, I've never personally heard of an artist experiencing problems with modern ultramarine blue, even among the many icon painters I know who preserve pigment pastes and egg yolk medium with vinegar (quite an acidic preservative).  I'm not saying this means modern, synthesized ultramarine isn't affected by acids; I mean to say only that the circumstances under which it happens must be unusual, or else I think we'd be hearing about more problems with modern ultramarine blue.  

If mold is the culprit and the pigment has been smelly for a while, you'd likely see mold (white, green, there are different kinds) on the surface of the pigment.  If it is mold, phenol is often suggested as a more effective anti-microbial than alcohol.  Phenol is acidic (carbolic acid) but I haven't heard of any issues with adding it to ultramarine blue.  Again, the degree to which ultramarine is reactive to acids isn't clear to me - it's generally considered a stable color, I believe it takes a strong acidic issue to cause it to break down.   So if there is a strong rotten egg smell, something in your studio environemnt my be affecting the color negatively.  Or, maybe it is just mold.  Good luck!

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Hi SRS,

This is a topic I remain confused about.  I understand that lapis lazuli and synthesized ultramarine are chemically reactive to acids and can darken in their presence, and that examples of Renaissance paintings that have this probelm are often cited.  Yet, despite being in communication with many professional & student painters, and reading/posting on various painting sites (including MITRA), I've never heard of a painter - even those adding vinegar to lapis or synthetic ultramarine - that's experienced a problem.  Am I just not hearing enough antecdotal evidence, or are the circumstances under which this happens very rare (and, consequently, perhaps a moot concern)?  I warn students about ultramarine blue and acids, but without more specific examples to cite, I'm not quite sure what I'm telling them!  Thanks for your input. ​​