Alternate Names: Iron stone concretions
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.
Glaze Variegation - Speckle
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.
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.
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.
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.
Out Bound Links
Iron pyrite, or Fool's Gold, is a common metallic mineral containing about 50% iron and 33% sulfur. It is found in many types of rocks and is common in clay deposits in small chunks and even large bou...
A kiln atmosphere which is deficient in free oxygen. In traditional ceramics, reduction firing requires a specially designed fuel fired kiln that restricts the flow of incoming air so there is enough to burn the fuel and no more (in some cases it is restricted so that is actually less than enough to...
In Bound Links
FERROSO-FERRIC OXIDE, FEROSIC OXIDE