FeO (iron oxide) is a very powerful flux in reduction

This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux, converting from Fe₂O₃ to FeO, 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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A cone 10R high iron glaze has run off the vase during firing. And crystallized.

An high iron glaze has melted too much run

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Iron, in the FeO form, is among the most powerful of fluxes in reduction firing. That fluxing action, dependent on the percentage of iron oxide in the recipe, produces two obvious consequences: Running (depending on the degree of reduction) and crystallization (depending on the speed of cooling and the chemistry of the glaze hosting the iron). This piece was slow-cooled during firing, resulting in total crystallization of the surface. The crystals are larger and densely packed at the neck. Their presence, as a thin surface layer, has completely matted it. And, because of the fluxing power of the FeO (present because of the reduction atmosphere in the kiln) enough glaze ran downward off the piece that it was left sitting in a pool of molten glass.

Additions of iron oxide are coloring, fluxing and crystallizing this base transparent

Glazes range from celadon to heavy iron crystal surface

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Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% produces an amber glass (and the iron reveals its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold or Teadust), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln using the C10RPL firing schedule, the crystallization mechanism would be heavier if it were cooled more slowly (or less if cooled faster). The 7% one in this lineup is quite interesting, a minimal percentage of cobalt-free black stain could likely be added to create an inexpensive and potentially non-leaching jet-black glossy.

A cone 10R iron crystal glaze using only Ravenscrag Slip and Iron

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Ravenscrag Slip, by itself, produces a silky transparent glaze at cone 10R. It is an excellent base to which to add colorants and modifiers. This is a simple addition of 10% iron oxide (Ravenscrag Slip already contains 2% iron, making about 12% total Fe₂O₃). This GR10-L recipe produces a stunning crystalline fired surface on these two porcelains. This "beyond-tenmoku" effect happens because of the extra iron and a slow cooling rate. The 12% iron dissolves in the glaze melt during heat up in the firing, but during cooling, the extra 2% precipitates out to produce these surfaces. The iron also acts as a flux in reduction firing, greatly increasing melt fluidity. Take that last statement seriously: The iron is a flux and the glaze will melt much more (it can wreck your kiln shelves if it runs). That being said, Ravenscrag Slip is more melt-stable than other bases, making it a more stable host for the iron addition.

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Links

Materials	Iron Oxide Red Red iron oxide is the most common colorant used in ceramic bodies and glazes. As a powder, it is available in red, yellow, black and other colors.
Oxides	FeO - Ferrous Oxide
Oxides	Fe2O3 - Iron Oxide, Ferric Oxide
Glossary	Reduction Firing A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.
Glossary	Tenmoku Tenmoku is a kind of high temperature reduction firing ceramic glaze. Glossy, very dark brown or maroon, fluxed by iron oxide to have high melt fluidity.

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