Red iron oxide, hematite, Fe2O3, produces a wide range of earthy glaze colours – yellow, brown, rust and near-black in oxidation firings, and blue/green celadons to black in reduction firings. Actual effects depend on oxide concentration, purity, particle size, degree of solution, glaze chemistry, temperature and firing atmosphere. Variability is due, in part, to the different oxidation states that develop during firing.
Spanish red iron oxide (~81% Fe2O3) is an economical, micaceous iron oxide mineral ore which produces similar effects to refined grades. As a replacement for a high purity grade of red iron oxide (>97% Fe2O3) in a glaze recipe, use 20% more. Results from substitution may not be exact.
For more information on red iron oxide, see the Digitalfire.com Reference Library of ceramic materials.