Native Raw Clay Materials - Plainsman
These are the quarry materials from which Plainsman native bodies are made. These materials have a number of advantages:
Working and Drying Characteristics
These materials provide a wide range of plasticity and particle sizes. Many are notable for a particular property (e.g. color, texture, plasticity, maturity) while still being balanced enough to produce ware with no further additions. Exceptions are Kaosand (very non-plastic), A2 ball clay (very plastic) and Helmer Kaolin (too refractory and silica deficient).
To make a body of typical pottery plasticity these materials will produce a drying shrinkage of 6.0-7.5% (about 1% more than for bodies made from refined materials). While this suggests more problems with drying cracks, it is balanced by high dry strength (often double that of most refined alternatives), this helps the clays resist drying cracks.
Bodies made from these materials tend to dry fairly fast compared to those made from refined materials (because the larger particle sizes tend to vent ware better).
Since these materials are not ground to 200 mesh (as are refined materials), they do contain some particles that will 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 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 amounts of pure quartz in their recipes (while zero quartz needs to be added to our materials to make bodies from them).
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 that have no feldspar (although you might like to use a little to bring into solution any cristobalite that might develop during firing).
Since these material are not fine-ground the coarser particle matrix creates channels for gas passage concentration (which certain other unground particles generate as they decompose during firing). 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. For example, fire at 100F/hour to slowly approach cone 6, then drop 100F (to 2100F) and hold there (30 minutes for example). The glaze will still be fluid enough to heal defects, bubbles will break (as the increased viscosity overcomes surface tension) and gas generation from the body should slow or stop. Of course, firing your bisque as high and long as possible will help to reduce gas generation. There is no limit to how high a bisque can be if you can find a way to apply, adhere and dry the glaze.
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 Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508