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This is what about 10% iron and some titanium and rutile can do in a transparent base glossy glaze (e.g. G1947U) with slow cooling at cone 10R on a refined porcelain.
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.
Glossary |
Crystallization
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation. |
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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 |
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