A general purpose medium plasticity clay. Because of its lower price it is most commonly used in schools at cone 06-04 (often with bottled underglazes and glazes). However, do not use this if you can afford the better choices: L213 or L212 (they fit commercial glazes better, dry slower giving students more time, they crack less and are smoother and more plastic).
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 other clays we recommend for use by children (already noted above). Since Buffstone dries faster, children have less time to complete projects (and less opportunity to start over on something else using the same piece of clay). Some plasticity is also lost 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 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).
To get the best defect-free surface please consider using a hold-rise-drop-hold firing schedule. For 04-03 consider using the C03DRH schedule.
Talc is added to buff and white burning low fire bodies to increase their thermal expansion so commercial glazes do not craze. Buffstone does not contain talc. Crazing is not considered an issue for decorative ware but functional ware is not possible using Buffstone at cone 04-06 with these glazes (because liquids will pass through the crack network in the glaze and absorb into the porous clay below).
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 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).
Glaze slurry consistency and quality: A secret to achieving even glaze coverage is controlling the thixotropy and specific gravity of the slurry, both in freshly mixed and stored batches. A glaze of the right specific gravity and having a slightly gelled condition goes on to bisque ware evenly, does not drip and dries in seconds. Always screen glazes when first making them (80 mesh). Be alert to any particulate that may appear after storage (e.g. precipitates) and screen again if needed.
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: 0.5-1.5% Cone 02: 2.0-3.0 Cone 2: 3.0-4.0 Cone 4: 4.0-5.0 Cone 6: 5.0-6.0
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. L is low expansion (for zero-talc porous bodies like Buffstone). K is intermediate (for moderate-talc bodies like L215, Zero3 stoneware and porcelain). H is high-expansion (for higher-talc bodies like L212, L213).
Buffstone mug at cone 10R. Outside glaze is AlbertaSlip/Ravenscrag Celadon. Inside 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.
Alberta Slip base glaze. Fired at cone 6. Buffstone.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508