Hydrated iron oxide. Yellow ochre. This is a high purity, refined oxide – the same oxide giving natural yellow ochre earths their distinctive colour. When fired above 500º C, the hydrated portion is driven off leaving red iron oxide as the functional colorant. Expect the same range of earthy colours, typically, yellow, brown, rust, black and celadon. As a replacement for red iron oxide in a glaze recipe, use 17% more.
For more information on yellow iron oxide, see the Digitalfire.com Reference Library of ceramic materials