I had the same blotchiness in this image:
http://www.deviantart.com/deviation/3518047/
I have an opaque bowl with a very thin transparent glaze on the surface and
the photon interactions in this, and from the glass marbles caused the
fabulous texture on the bowl.
In fact the color of the bowl is a uniform gray, but the blotchiness looked
so good I didn't do anything else to it.
