Low to medium fire, smooth, medium plastic, buff burning, native material blend for earthenware or medium temperature stoneware. Buffstone is pure A3, one of our native stoneware materials, with a small amount of bentonite. Buffstone is pleasant to work with and is suitable for the production of decorative ware at low temperatures and functional ware at medium and high temperatures.
Buffstone has medium plasticity and feels smooth (very slight sandy feel on the wheel). It has good drying properties. However if ware is being modelled or handbuilt extra drying precautions are appropriate for pieces are of uneven thickness. To avoid cracking use slip of low water content to join, apply as much pressure movement as possible, and cover ware with a cloth and plastic to slow it down and even out the drying. You might consider preparing a batch of slip and storing in lidded plastic containers (add a little grog to it for even better joining).
Buffstone fired bars. Cone 04, 02, 2, 4 and 6 oxidation (bottom to top).
The natural soluble salts in Buffstone can come to the surface during drying and are left in a layer whose distribution across the surface is determined by the nature of the drying. After firing this surface film appears as a whitish scum in a manner similar to the efflorescing that occurs on red terra cotta brick walls. However, this behavior is not normally an issue if the ware is being glazed.
While it works well as a middle temperature stoneware (to cone 8), Buffstone has been sold primarily as a non-firing and low fire clay to the school markets. Like L211, Buffstone is not volatile and thus restricted to use at low temperatures. It gradually matures and vitrifies over a wide range from cone 2 to about cone 8. When fired at cone 04-02, it is a yellow buff color and is porous like any other earthenware body.
Buffstone does not contain talc as most other buff earthenwares and thus it does not exhibit the higher overall expansions associated with talc bodies. This makes it more suitable for ware which will be exposed to sudden temperature changes but also makes it harder to match glazes without crazing.
Alberta Slip base glaze. Fired at cone 6. Buffstone.
If you are glazing, we recommend that Buffstone be used for ware which is glazed entirely in order to avoid the appearance of the white solubles film on bare surfaces. If you are not using this body in a school setting, consider using L212 or L213.
If you wish to paint on commercial glazes consider using these in conjunction with under-glazes. To get the best defect-free surface, use a drop-and-soak firing schedule. Of course, we cannot guarantee that any glaze made by another manufacturer will frit.
If you wish to dip-glaze ware it then commercial glazes may not be practical (they often dry very slowly and drip badly). The answer is a recipe. Mixing your own glaze from a pre-mixed powder you buy from us (or weighing out your own ingredients) is more economical. However at low temperature it is very difficult provide one-glaze-that-fits-for-everyone. But if circumstances are narrowed, it is practical. For example, we have found that for making functional ware the Zero3 L Glaze variant fits this body at cone 03. We do not trust electronic controllers to be accurate, we verify using cones and manually program to compensate for error (we recommend you do the same). Technical info is available here).
We recommend stress testing by boiling-water-into-ice-water (and vice versa) to bring out any crazing or shivering in your functional ware. If that happens, make changes and retest.
The chart shown was produced from a specimen fired once to cone 04 in the Plainsman lab and tested in an Orton dilatometer. If you fire to a different temperature, employ different heatup or cooldown rates, or glaze-fire more than once the thermal expansion in your ware may be different than this chart indicates.
Thermal Expansion Chart. Average: 6.0.
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 clear glazes. But they are not the same. The G2931L glaze (left) has a lower thermal expansion (to prevent crazing on Buffstone). The G2931K glaze fits L215 because the latter contains some talc (which raises its expansion). Both of these mugs have 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, K and H. L is low expansion (for zero talc porous bodies). K is intermediate (for moderate talc bodies like L215 and Zero3 stoneware and porcelain). H is high expansion (for high talc bodies like L212, L213).
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508