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Iron, Oxide Red

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Red Iron Oxide
FeO (Ferrous Oxide)
-In clays and glazes, firing in reducing conditions, or with clays containing significant organic matter, the Fe2O3 converts to FeO as early as 900C. FeO is a very powerful flux. Once iron has been reduced and becomes active in glass forming, it is difficult to reoxidize it again. For this reason, reduction firings for iron effects should be light throughout to reduce the iron early before glaze melts. They should then be fired slowly through the 250-500C range to provide adequate time for organics to burn away. A period of clearing in oxidation at the end of a firing does not affect the color of iron in the molten glass.

-FeO is so active as a flux that it can often be introduced by substituting for other fluxes like lead and calcium oxide.

-Most glazes will dissolve more iron in the melt than they can incorporate in the cooled glass. Thus extra iron precipitates out during cooling to form crystals. This behavior is true of oxidation but doubly so of reduction. For example, a typical high-temperature fluid glaze with 15% iron will freeze to a sparkling rust colored mesh of crystals.

-Many popular iron glazes and slips for pottery are based on clays highly stained with iron. For example, Albany slip was used for many years to produce a wide variety of glazes which exploited its unique blend of high iron, low melting point, moderate plasticity, low thermal expansion, low cost and unique character. For example, using Alberta Slip (an Albany substitute) one can make a tenmoku glaze with 90% Alberta slip and a little added iron and feldspar.

-If clay is not allowed to oxidize thoroughly through the 700-900C range during firing, carbon present within will rob the Fe2O3 of its oxygen and escape as CO2 leaving the FeO as an active flux within the body to break it down from within. This is called black coring.

-Iron bearing clays fire much darker in reduction than oxidation. In addition, reduction fired iron bodies experience sudden color changes from red or tan to dark brown across a narrow temperature range characteristic to each formulation. Classic iron reduction mottled effects are created by firing to the transition point where color is just changing producing light and dark patching of color as the darker color invades the surface.

-In reduction firings it can produce greens and blues (i.e. celadons), and yellows and maroons (i.e. mustard, oatmeal glazes). In higher amounts it saturates to produce crystalline deep brown and black effects (i.e. tenmoku 10-13% and kaki 13%+).

-Iron pyrite and similar minerals often contaminate stonewares and fireclays; and they are responsible for the popular speckling effects in reduction fired stonewares.
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