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Native Raw Clay Materials


Raw Quarry Materials List: Click here

Working and Drying Characteristics

These materials have a wide range of plasticity and particle sizes. Many are notable for a particular property (e.g. color, texture, plasticity, maturity) and yet still being balanced enough to produce ware with no further additions. Exceptions are Kaosand (very non-plastic), St. Rose Red (non-plastic), A2 ball clay (too refractory) and Helmer Kaolin (too refractory and silica deficient).

To make a body of typical pottery plasticity aim for a drying shrinkage of 6.0-7.0%. Bodies made from these materials tend to dry fairly fast compared to those made from refined materials (because the larger particle size vents water better).

Firing

Since these materials are not air-floated (e.g. to 200 mesh), they do contain some particles that can cause specking, especially in reduction and at higher levels of vitrification. At the same time, the raw lump forms of some of our materials are very clean, nature has cooperated in removing mechanical impurities during their sedimentation (some are so clean; e.g. 3D, 3B; that the lumps can simply be slaked and quality pottery made directly form the dewatered material).

Many of these materials are quite high in free silica. This prevents crazing by helping to put compressive force on glazes. While it can mean that bodies are a little more susceptible to thermal shock failure, almost all bodies made from refined materials have significant pure quartz (silica) in their recipes.

Many of our clays contain natural fluxes that make them mature at a relatively low temperature. It is entirely feasible to produce bodies at cone 8-10 with no added feldspar (for others a little feldspar is needed, for maturity for also to bring into solution any cristobalite that might develop during firing).

Glazing

Since these material are not fine-ground, the coarser particle matrix creates channels for gas passage concentration. This can pinhole or blister some glazes. These problems can almost always be solved by firing to temperature and then doing a drop-and-hold firing schedule (e.g. C6DHSC). It is a good idea to fire your bisque as high as possible (while still having sufficient porosity to absorb water during glazing).

The dry strength of our materials makes single-fire glazing quite viable if you can find a way to apply the glaze evenly and dry it quickly (e.g. heating the ware before application, applying when ware is leather hard).

If you would like the recipe of any of our native bodies as a starting point in your formulation, we can supply it.

Plainsman Clays, 702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535, FAX: 403-527-7508, Email: plainsman@telus.net