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Buffstone

Low to medium fire, smooth, medium plastic, buff burning, native material blend for earthenware or medium temperature stoneware. Buffstone is a simple mix of two of our smooth fine stoneware clays with 10% added fine sand. Like Terrastone, this body contains no barium carbonate. Buffstone is pleasant to work with and is suitable for the production of functional and decorative ware in low and medium temperature electric oxidation firing. If you are using this material in an educational setting, please check our web site for an article on making the children's first experience with clay a memorable one. We also have a number of videos available on clay basics.

Process Properties

Buffstone has medium plasticity and feels slick yet has good drying properties. However if ware is being modelled or handbuilt extra drying precautions are appropriate if pieces are of uneven thickness. To avoid cracking use slip of low water content to join, apply as much pressure and lateral 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 (you can add a little grog to it for even better joining).

Firing


Cone 04

Because it does not contain barium carbonate, Buffstone has one property that you must be aware of. The natural soluble salts in the clay 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

Most people use commercial glazes. You may have to try different brands to find one that fires craze-free in your circumstances. However in many educational settings, crazing will not matter.

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, you might consider using L212 or L213.

Glaze Recipes

Try a starting glaze recipe of Frit 3124 42.5%, Frit 3134 42.5%, EP Kaolin 15% and test to get the degree of melting needed. If the glaze melts too much, add 5% silica or increase the kaolin. If it is not melting enough reduce the kaolin. Stress test in boiling water and then ice water to bring out any crazing or shivering. Adjust the fit by increasing the amount of Frit 3134 if the glaze shivers or 3124 if it crazes. Once your transparent is working you can use it as a base to create other glazes by adding colorants, opacifiers, variegators and specking agents. You can find more detail at http://digitalfire.com/4sight/education/g1916 m_cone_06-04_base_glaze_226.html.

For slip decoration, be careful to match drying and fired shrinkage of the slip with the body and tune the amount of frit to get good adherence with the body. You can find more information at http://digitalfire.com/4sight/e ducation/creating_a_non-glaze_ceramic_slip_or_engobe_97.html.

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

Chemical Analysis

 CaO       0.2
 K2O       2.0
 MgO       1.0
 Na2O      0.1
 TiO2      0.6
 Al2O3    19.3
 P2O5      0.0
 SiO2     66.8
 Fe2O3     1.5
 MnO       0.0
 LOI       8.3%

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