All Minerals

Iron Pyrite

Formula: FeS2

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.

Pyrite (FeS2) is a hard mineral (Mohs hardness 6–6.5), making it more difficult to grind than most clay minerals. In ceramic raw materials, coarse pyrite particles can survive processing and produce dark iron-rich speckles after firing, with the size of the fired speck depending largely on the original particle size. This effect is often valued in buff-, brown-, and red-burning pottery bodies, particularly those fired in reduction.

Finely ground iron-rich concretions can also be used as a specking agent in glazes. Compared with refined iron oxide powders, they tend to produce softer, more natural-looking speckles with less blooming or diffusion into the surrounding glaze. Their coarser particle size also helps them remain suspended in glaze slurries.

Related Information

Pyrite - Fools Gold

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An ironstone concretion found in a quarry in southern Saskatchewan

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These iron-rich concretions occur in the Battle Formation across Saskatchewan. Plainsman Clays mines the lower part of the Battle Formation, just above the Whitemud Formation, where a 1–2 foot layer is stockpiled as A1 clay. This clay is added to reduction-fired bodies to provide speckle, color and plasticity. The A1 contains thousands of these concretions, ranging in size from lemons to toasters. When freshly mined they are extremely hard and difficult to break with a hammer. After weathering (repeated wetting and drying, oxidation, shrinkage, and freeze-thaw), they gradually break down into small fragments that can be crushed and dispersed during clay processing. These are the same Battle Formation beds exposed in the Eastend, Sask. area that yielded the Tyrannosaurus rex fossil “Scotty,” displayed at the T. Rex Discovery Centre there.

Reduction speckle: a product of iron particles in the body

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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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