Notes
Pyrite is quite hard (6.0-6.5 Moh) and is even used in resin bonded abrasives (i.e. brake linings). It is difficult to grind and clay materials containing pyrite can fire with an iron-speckle (in accordance with the size of the iron pyrite particles). This effect is much sought after in buff and brown/red firing pottery clay bodies burned in reduction firing.
Ground iron concretions can be used as a specking agent in glazes as they do not blossom as much as pure metallic compounds and they tend to stay in suspension better.
Related Information
1970s cone 10 reduction stoneware bowl by Tony Hansen
This bowl was made by Tony Hansen in the middle 1970s. The body was H41G (now H441G), it had large 20 mesh ironstone 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.
An ironstone concretion found in a quarry in southern Saskatchewan
These are very hard and high in iron oxide. They are found in the Battle Formation across Saskatchewan. Plainsman Clays extracts the bottom part of the Battle layers, just above the Whitemud Formation layers, about 1-2' thick, and stockpiles it as the material A1 (that clay is added to reduction fired bodies to impart speckle, color and plasticity). The A1 contains thousands of these concretions, ranging in size from lemons to toasters. When first mined they are hard and very difficult to break with a hammer. But upon aging in the sun they dehydrate slowly and crumble into small lumps. These layers are the same as those in which the worlds largest T-rex was found (learn more at the T.rex Discovery Centre).
Reduction speckle: a product of iron particles in the body
In reduction firing, where insufficient oxygen is present to oxidize the iron, natural iron pyrite particles in the clay convert to their metallic form and melt. The nature of the decorative speckled effect depends on the size of the particles, the distribution of sizes, their abundance, the color of the clay and the degree to which they melt. The characteristics of the glaze on the ware (e.g. degree of matteness, color, thickness of application, the way it interacts with the iron) also have a big effect on the appearance.
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| Typecodes |
Colorant
Metallic based materials that impart fired color to glazes and bodies.
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| Minerals |
Iron Pyrite
Iron pyrite, or Fool's Gold, is a common metallic mineral containing about 50% iron and 33% sulfur.
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| Materials |
Magnetite Granular
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| Glossary |
Reduction Firing
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.
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Mechanisms
