For my “Physical Analysis of Artworks” class this quarter I was preparing a little demo of cooking indigo and palygorskite clay to make Maya blue pigment, and was surprised to see in a recipe I found online the claim that indigo sublimates at 180C. Certainly, cooking a mix of indigo powder and palygorskite on a hotplate at 200C was sufficient to drive the Maya blue reaction as seen in the before/after images below:

Why does this work? I have seen scientific papers in which people create indigo thin films using thermal evaporation with a source temperature of 220C, but this is done with an effusion cell in an ultrahigh vacuum environment. There doesn’t seem to be any buildup of indigo “film” — at all — on any of the exposed metal surfaces in the above setup. I’m going to get back to the nebulizer approach when I next have some time in the lab, but I’ll have to keep thinking about this…