[Clayart] iron red turns plum

[sumi]

I'm wondering if anyone can tell me, just because I'm curious, what 
makes an iron red go plum or burgundy colored. I mean, I know how it 
happened, just wondering what form the oxide is in. One of our other 
instructors had some pots with Ketchup glaze on them that came out of a 
cone 6 firing blah and brown. I know the answer is to slow cool an iron 
red but I suggested putting them in a bisque firing just to see what 
would happen. I did that once with a temmoku pot and it turned red. When 
the ketchup pots came out of the cone 04 firing they were a beautiful 
burgundy color. You'd think it was a chrome/tin glaze. Is that color a 
particular form of iron oxide? Anyway, now I know how to get it!

Sumi