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Buffstone

Description

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.

Process Properties

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).

Firing

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.

Glazing

Many find that commercial brushing and dipping glazes dry very slowly and drip badly. And they often do not fit well (craze or shiver). And they are expensive. With a little effort you can make your own of both types. And add stain and opacifier powders to make almost any color. There is no need to search for a recipe, we have done it for you. You can mix your own glaze from a pre-mixed powder you buy from us or weigh out your own ingredients. However, at low temperature, there is no one-glaze-that-fits-all bodies. To solve this we have created a three-recipe solution, these have thermal expansions that span the entire range of low fire bodies. And they fire crystal clear (in a drop-and-hold firing). Our Zero3 K transparent glaze fits low talc bodies (like L215) and low fire stoneware (like Zero3), Zero3 H fits high talc bodies (like L213, L212) and Zero3 L fits zero talc bodies (like Terrastone and Buffstone). We recommend cone 04-03 using a drop-and-hold firing schedule (for defect free surface and good transparency). Do not trust electronic controllers to be accurate, we verify using cones and manually program to compensate for error. If you need to fire lower or higher than 03, be sure and do testing first.

Blending the K, H and L glazes is encouraged to get the best fit. We recommend stress testing by boiling-water-into-ice-water (and vice versa) to bring out any crazing or shivering. If crazing occurs, blend in some of the L. If shivering occurs, blend in some of the H. In this way these glazes can be optimally fitted to almost any clay body.

It is also practical to make your own brushing glazes, ones that will work identically to the commercial ones mentioned above. Like the dipping versions, you can blend to tune the thermal expansion to fit your clay body. Click here for more information on how to do this. You will be surprised at how easy it is and how much money you can save.

Alberta Slip base glaze. Fired at cone 6. Buffstone.

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.

[RecommendG2931B]

Thermal Expansion

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.

Physical Properties

 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

Fired Shrinkage:

 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

Fired Absorption:

 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

Gallery

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).

Safety Data Sheet

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