Thermal reduction of other metal oxides beside Fe2O3
One candidate is manganese dioxide, $\ce{MnO2}$.  This decomposes at 535°C, releasing oxygen.  As the phase diagram from Ref. [1] shows below, the reduced manganese fuses with excess silica between 1250 and 1300°C, forming rhodonite ($\ce{MnSiO3}$).  You would see this as a pink color.  But, of course, I'm a chemist rather than a potter, so I make no guarantees on whether or how it works in pottery!



Reference:

1.
Y.-B. Kang, I.-H. Jung, H.-G. Lee, "Critical thermodynamic evaluation and optimization of the MnO–SiO2–TiO2–Ti2O3" system, Calphad
Volume 30, Issue 3, September 2006, Pages 226-234.  https://doi.org/10.1016/j.calphad.2006.05.002