Iron Chromate
In its coarse form, it is used as a blue brick stain. The fine version (2 – 5%) exhibits a grey/brown to black colour in glazes. In reduction conditions, the colour will be blue to grey.

These iron oxides generally give a wide range of colours ranging from honey yellow, brownish reds, brown, black, yellow and purple depending on firing
conditions. Under reducing conditions, a blue colour can be obtained.