The red color of Fe exploited in some ceramic glazes and seen in many clays is not related to having Fe dissolved in the glassy part f the glaze. In silicate glasses iron will sort itself out into some balance of FeO and Fe2O3 depending on the oxidizing conditions during the firing and peak temperature. FeO gives blue colors and Fe2O3 yellow. Since the two are typically present together a green of some sort is usually had – see sheet glass or true Celadon glazes, for example.
The red comes from the situation of iron within a crystalline material that favors its being surrounded by 6 oxygens. This position is not the deal in glassy materials as used here. The reddish mineral garnet is a pure form of the red color given by Fe in this arrangement. This is why a fat Al2O3 component in the glaze is useful to developing Fe red – in effect one is making little garnet crystals within the glaze to color it. If Fe gets into SiO2 crystals it can also yield a pink, but the color is more faint – I can’t remember exactly which form of SiO2 favors this, but I think it’s crystobalite – and we all know the headaches of having much of this in a pot can cause.
Strongly reducing conditions act against the development of Fe reds. Fe will favor moving to FeO.
I’m not sure what specific questions have been posed here about Fe reds, but am hopeful that this brief note lends a little insight. Further questions should be directed privately.
KPP – doesn’t download e-mail w/ executable attachments.
