|Dry M.O.R. (50% Silica)||771C|
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
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.
Since iron oxide is a strong flux in reduction, iron-based pigments can run badly if applied too thickly.
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.
Fired on a porcelain in a gas kiln.
The recipe contains 6% red iron oxide. The chemistry is high alumina (from 45 feldspar and 20 kaolin), zero silica silica (4:1 Al2O3:SiO2 ratio) and 20% calcium carbonate. The remainder is a little talc and calcium phosphate. The reduced iron is fluxing what would otherwise be a very matte surface. Reducing the iron percentage to 4% produces a yellow mustard color.
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).
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).
|Oxides||Fe2O3 - Iron Oxide, Ferric Oxide|
|Materials||Iron Oxide Black|
|Glaze Color||When 1-5% iron is used in a transparent reduction glaze which has some calcia and potash (barium also helps) celadon glazes are produced. 'Celadon' glazes are glossy shades of green which exhibit depth of color due to suspended micro-bubbles in the glass.|
|Glaze Color||A typical high-temperature fluid reduction glaze with 15% iron will freeze to a sparkling rust colored mesh of crystals. Alkaline glazes work best. Barium can impede this effect.|
|Glaze Color||Saturated reduction iron glazes normally firing to black in reduction will move toward brown if alumina is high, toward blue if alumina is low.|
|Glaze Color||The presence of phosphorous pentoxide, lithia and soda also encourage blue in both normal and saturation conditions in reduction firing. Iron glazes will move toward blue if alumina is low.|
|Glaze Color||Classic reduction black-breaking-to-brown tenmoku glazes are made with 8-12% iron.|