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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").
This iron crystal glaze is Ravenscrag slip plus 10% iron oxide fired to cone 10R on Plainsman H550. Since Ravenscrag slip is a glaze-by-itself at cone 10, it is an ideal base from which to make a wide range of glazes.
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.
Oxides | FeO - Ferrous Oxide |
---|---|
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. |
Glossary |
Iron Red Glaze
A type of ceramic glaze, typically fired around 2200F, where iron oxide in the cooling glass precipitates out to form a striking red crystalline mesh on the surface. |
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