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

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.

Related Pictures

A cone 10R high iron glaze has run off the vase during firing. And crystallized.

An high iron glaze has melted too much run

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.

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).

Iron-Red high temperature reduction fired glaze

This recipe, our code 77E14A, contains 6% red iron oxide and 4% tricalcium phosphate. But the color is a product of the chemistry. The glaze is high Al₂O₃ (from 45 feldspar and 20 kaolin) and low in SiO₂ (the recipe has zero silica). This calculates to a 4:1 Al₂O₃:SiO₂ ratio, very low and normally indicative of a matte surface. The iron oxide content of this is half of what is typical in a beyond-tenmoku iron crystal glaze (those having enough iron to saturate the melt and precipitate as crystals during cooling). The color of this is also a product of some sort of iron crystallization, but it is occuring in a low-silica, high-alumina melt with phosphate and alkalis present. Reducing the iron percentage to 4% produces a yellow mustard color (we thus named this "Red Mustard").

Links

Recipes	GR10-L - Ravenscrag Iron Crystal Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
URLs	http://ravenscrag.com Ravenscrag web site
Materials	Ravenscrag Slip A light-colored silty clay that melts to a clear glaze at cone 10R, with a frit addition it creates a good base for a wide range of cone 6 glazes.

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