In the firing of ceramic glazes you can make an effect called oil spot. This is done by making two layers of glaze. A bottom layer rich in iron oxide (+15%) and a top white layer that's fairly permeable. Once your reach 1225° C, the iron loses it's bond with the oxygen and goes into reduction, despite being in an oxygen rich electric kiln. So the oxygen bubbles up to the surface, taking along a bit of the iron. It then turns into FeO, and it also has something to do with magnetite which I don't understand.
Anyway, the result is that you get a white glaze with dark spots surrounded with grey/green parts, or some yellow.This effect is very nice but it only works with iron oxide as far as I know.
So my question is: are there other oxides that can have the same thermal reduction effect, and if so at what temperature. I know somebody found a way to make it in blue (it took him 4 years) so maybe he used cobalt oxide, though that looks to me to be to obvious to take 4 years. Anyway, he's not a chemist, he's a potter.
For a potter this is hard to find out but maybe for a chemist this is not that difficult to resolve, so I thought why not go to the people who know this stuff much better than us potters.