Iron Oxide FeO, Fe₂O₃, Fe₃O₄, and Fe₂O₃·H₂O

Iron Oxide FeO, Fe₂O₃, Fe₃O₄, and Fe₂O₃·H₂O

• Strong fluxing colorant
• Melts at 2849°F
• Extremely sensitive to oxidation and reduction atmospheres
• Produces wide range of colors and effects in glazes
  • including browns, yellows, greens, olives, blues, oil spots, and metallic colors
• Very difficult to find material without iron
  • found in almost everything
    • from feldspars and kaolin to ball clays, earthenware clays, and many other colorants
  • Many materials require processing to reduce iron levels that are acceptable for their intended purposes
• 4 major forms of iron oxide
  • Red iron oxide (RIO)
    • RIO, Fe2O3, is refractory similar to Al2O3. Will remain in this state during oxidized firing until about cone 7-8, when starts to thermally decompose to BIO (FeO), strong flux
    • If RIO is fired in early reduction cycle, will reduce to BIO from atmosphere and then act as flux in glaze
  • Magnetic iron
    • Magnetic iron (Fe3O4), forms iron spangles in glazes during cooling.
  • Black iron oxide (BIO)
    • BIO has larger particle size than RIO
    • In glazes it’s prone to speckling
    • easily strongest form of Fe used in glazes
    • Very fine and doesn’t cause speckling
  • Yellow iron oxide (YIO)
    • YIO (Fe2O3·H2O), pigment, is purest and has ultra-fine particles.
• Many sources of iron oxide form which potters may choose
  • Each contains varying degrees of impurities that add to unique expression of iron glazes
• Most common form is RIO, finely ground material that disperses well in glaze curries
• (Note: Spanish iron oxide is produced by bacterial digestion.)
• Other sources are crocus martis, yellow ochre, burnt umber, burnt Sienna, earthenware clays (Redar, Albany, Alberta, Barnard, Michigan Slip, Ranger Red Clay, and the like), Iron Chromate, iron sulphate, rutile, ilmenite, and rust.