Q: I recently painted a watercolor that contained a larger,
dark-brownish area where I combined Van brown, burnt umber, and
green earth. In that spot, the paint took nearly two weeks to dry. What
do you think occurred?
A: In general, the ingredients in watercolor (particularly
the amount of glycerin in the paint) rather than pigments themselves
that most affect drying time. Specifically, the colors you mentioned
can be problematic. Green earth tends to behave like a clay in that it
absorbs a good deal of water, particularly when applied heavily. And
Van brown is not considered a permanent pigment: it tends to be
unstable, whether used alone or in mixtures.
Another possibility that comes to mind is the opacity of your paint.
If you applied the paint too thickly, the binding action of the paper
on the pigment particles would lessen, causing the paint to dry more
slowly and inviting the possibility of cracking.