Buffstone is pure A3, one of our native buff-color-burning stoneware materials (thus the name "Buffstone"). It contains a small amount of bentonite to improve plasticity and slow down drying. This body is less expensive than some of our other stoneware clays. Some plasticity is lost to the clay body when scrap clay is reconstituted. Extra drying precautions are appropriate for pieces of uneven thickness.
At cone 06 all buff and white burning clays fire to a porous matrix and have low strength (Buffstone is a little more porous than most). Notwithstanding this, cone 06 is convenient for schools and a wide array of commercial low-fire glazes are available for the making of decorative objects. Buffstone requires no special firing procedures if ware is completely dry. Buffstone will withstand kiln firings up to cone 10. The higher you can bisque the stronger ware will be (we recommend cone 03 or 02).
Buffstone fired bars. Cone 04, 02, 2, 4 and 6 oxidation (bottom to top). This spans the range of up to 14% porosity on the low end to near storeware density around cone 8 (representing a range of about 300 degrees F).
Fitting glazes at low temperature is more difficult than for stonewares. We have found that Spectrum 700 and our G2931L recipe (a variation of G2931K) both fit well if ware is bisque fired at cone 03 (lower bisques craze for both). An extra benefit is that the higher bisque also produces stronger ware.
Colored "underglaze" products will bond best if applied at the leather hard state. Second best is applying underglazes at the dry or bisque state (but not too thick, and avoid covering the entire surface, just decorate). Then bisque-fire ware (cone 03-04) and paint-on the top-coat of transparent glaze, dry and glaze-fire the ware. Children can also decorate larger areas of the surface with colored glazes (no final coats of transparent are needed over these). Of course if you are just glazing all the ware with one cover glaze (without decorating) then bisque fire, apply the glaze and refire according to its instructions.
If you are dip-glazing bisque ware it is possible that the glaze will partially or fully shed off areas of underglaze decoration (these glazes dry fast but do not have gum to hold them in place over the harder underglaze layer). If an underglaze does cover well it means that it is not melting enough and, implicitly, it is not forming a good bond with the bisque. For dipping it is best to bisque fire lower to get better porosity to hold the glaze (e.g. cone 06-05) and glaze fire hotter (e.g. cone 03).
Thixotropy: Many people mix their glazes the traditional way, just adding water until the slurry appears to be the right viscosity for dipping. However, if you want better application properties for one-coat dipping, consider creating a thixotropic slurry. Thixotropic glazes are creamy because they have been thinned and then gelled by the addition of a flocculant. They go on evenly, hang on without dripping and dry quickly. Achieving (and maintaining) this state involves targeting a specific gravity (usually around 1.43) and adding epsom salts (1-2g/1000g of powdered glaze).
Drying Shrinkage: 5.5-6.5% Dry Strength: n/a Water Content: 19.5-20.5% Drying Factor: C120 Dry Density: n/a
Sieve Analysis (Tyler mesh):
+65: 0.1-0.5% 100-150: 1.5-3.5 150-200: 3.5-5.5 200-325: 8.0-11.0
Cone 04: 11.0-14.0% Cone 02: 9.0-11.0 Cone 2: 8.0-10.0 Cone 4: 5.5-7.5 Cone 6: 2.0-3.0
Buffstone and L215 with underglaze decoration and G2931 weigh-and-mix-yourself clear dipping glazes. But they are not the same. The G2931L glaze (left) has a lower thermal expansion (to slow crazing on Buffstone). The G2931K glaze fits L215 because the latter contains some talc (which raises its expansion). The L215 mug has survived a 300F-to-ice-water thermal shock without crazing or shivering. If you switch the glazes the Buffstone mug would craze it and the L215 would shiver it off in flakes.
Buffstone, L212 and L213. Each has its own clear glaze. G2931L, G2931K and G2931H.
Buffstone mug at cone 10R. Outside glaze is AlbertaSlip/Ravenscrag Celadon. The liner glaze is Ravenscrag plus 10% talc. Notice the foot ring has soluble salts that can stick ware to shelves, if you use this clay at cone 10R be sure to use kiln wash and grind the foot rings well after firing.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508