Julie Beck’s demonstration of voodoo darkening with titanium white and carbon black has really bugged since I saw it. “Darker when wet” is explained well with some fairly easy to understand physics. “Darker when dry” is sort of the exact opposite and doesn’t admit an easy explanation.
I was pretty sure it had something to do with the spatial distribution of particles, but hard to do really do anything other than jazz hands.
I caught a line in an article about Kubleka-Munk theory on how it was well known that a limitation of the approach is how it can be inaccurate for paint films where a finer particle distribution ends up floating on top of a coarser one. Titanium white is finer (~0.3 microns) vs. lead white ~3 microns or carbon black ~10 microns.
That got me to Hess’s Paint Film Defects: Their Causes and Cure:
Naturally paint films have had such industrial importance, it is no surprise that there are books like this that catalogue the understanding of different sorts of defects in great detail. “Flooding” seems to be a good first order explanation, the book mentions how it is known that titanium white/carbonblack mixtures are known for this. The colors dry to a “deeper shade than the bulk paint”, which I believe it what Julie saw. I believe I see “dark when dry” when using titanium white/carbon black in gouache (it should be very easy to replicate) and I think it would largely be for the same reasons.
This explanation makes a lot of sense to me. It would be difficult to really “prove” that is what is happening in Julie’s swatches, but there seems to be a really good match as to what is happening. Perhaps Julie or someone else already figured this out?