|Monthly Tech-Tip |
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).
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). It produces a stunning crystalline fired surface on these two porcelains. We can call this a "Beyond-Tenmoku" (crystals happen because of more iron or a slow cooling rate than a Tenmoku). The 12% iron dissolves in the glaze melt during firing, but during cooling in the kiln, 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, the glaze will melt much more. So just adding iron oxide to a glossy transparent will wreck your kiln shelves when it runs down off the ware. Ravenscrag Slip does not melt-to-glossy, it has just enough feldspar to fire to a durable surface, making it a more stable host for the iron addition.
Ceramic glazes melt and flow according to their chemistry and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
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.
Tenmoku is a kind of ceramic glaze. Glossy, very dark brown or maroon, edges crystalizing, firing at high temperature in reduction atmospheres.
|Materials||Iron Oxide Red|
|Oxides||FeO - Ferrous Oxide|