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.
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 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.
An iron stone concretion found in a quarry in southern Saskatchewan
These are very hard, high in iron and can be as large as volkswagens. Tiny iron concretion particles cause specking in fired ware, especially in reduction.
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.
Metallic based materials that impart fired color to glazes and bodies.
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.
|Glaze Variegation||Granular iron pyrite can added to glazes to produce speckle at any temperature. At higher temperatures the speckles will bleed, in reduction they will melt and blossom.|
By Tony Hansen
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