FeO (Ferrous Oxide)
Notes-Fe2O3 is easy to reduce to the FeO state with a light reduction firing as follows:
Fe2O3 + CO -> 2FeO + CO2
-Some suppliers quote iron in its reduced form as part of a materials formula.
-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.
Reduction high temperature iron crystal glaze
This is what about 10% iron and some titanium and rutile can do in a transparent base glaze with slow cooling at cone 10R on a refined porcelain.
1970s cone 10 reduction stoneware bowl by Tony Hansen
This bowl was made by Tony Hansen in the middle to late 1970s. The body was H41G (now H441G), it had large 20 mesh iron stone concretions that produced very large iron blotches in reduction firing. Luke Lindoe loved to use these clays to show off the power of the cone 10 reduction firing process that he was promoting in the 1960s and 70s.
The same glaze in reduction (left) and oxidation at cone 10
It is not just iron oxide that changes character from oxidation to reduction. Of course, cobalt can fire to a bright blue in oxidation also, but this will only happen if its host glaze is glossy and transparent. In this case the shift to reduction has altered the character of the glaze enough so that its matte character subdues the blue.
Alberta slip fired in reduction (left) is much darker than in oxidation at cone 10.
This is 100% of the pure material. Notice how the iron is fluxing it more on the left, it is beginning to run. And how the reduction atmosphere amplifies the color of the iron (by changing it to the metallic state).
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
FeO (iron oxide) is a very powerful flux
This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux in reduction firing and produces a glaze melt that is much more fluid. In oxidation, iron is refractory and does not melt well (this glaze would be completely stable on the ware in an oxidation firing at the same temperature, and much lighter in color).
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