Print

Woodfire Clay

Description

A high temperature, plastic, light buff burning lightly grogged stoneware containing Helmer Kaolin. Most wood firing is done to achieve an effect called "flashing", gengerally an orange/red/brown coloration on the unglazed body surface. While the mechanisms of flashing are not well understood, engobe recipes that flash well are widely known and potters often simply apply these to stoneware bodies to get the effect. However it is also known that certain ball clays and kaolins flash better than others (specifically Helmer Kaolin, also #6 Tile). The recipe of this body is:

  • 30% Helmer kaolin
  • 20% #6 Tile kaolin
  • 20% Spinks Blend ball clay
  • 10% silica
  • 10% Nepheline Syenite
  • 10% Minus 35mesh grog.

Philosophies and body preferences vary greatly in the woodfire community, please test this material in your circumstances.

Process Properties

While 10% fine grog has been added, the feel of this body is quite smooth. The high kaolin content coupled with the plasticity of additional ball clay impart a throwing character similar to a Lincoln Fireclay body, very pleasant to work with. Since this body is quite plastic we recommend care in drying (even though it contains grog).

Casting: Helmer kaolin responds well to deflocculants so this body should work well (although it might cast slower than others because of the high clay content). It would be best to remove the grog from the recipe, this will lower the fired porosity somewhat. Consider using 50% Helmer (instead of 30 Helmer and 20 #6 Tile) for the best possible flashing.

Firing

Plainsman Woodfire clay is a stoneware, suitable for functional pottery. It vitrifies a little less than our other cone 10R buff burning stoneware bodies (e.g. H550, H450) to give margin for over-firing. If your wood kiln goes higher than cone 10 the typical 3% porosity will drop producing a fairly vitreous result (by cone 12 this could drop to 1% producing quite a vitreous product). Some isolated iron speckles will occur in reduction firing, these are from the grog and Helmer.

Woodfire Clay fired bars. Cone 10R (top), cone 10, 9, 8 oxidation below. These are all fired in an electric kiln, thus there is no flashing effect.

Here is an example of a flashing effect achieved in a the kiln at the Medalta Artists in Residence program.

Thermal Expansion

We do not supply a thermal expansion value. The reason is that such numbers often mislead users. First, a body has different thermal expansion characteristics when fired at different temperatures, schedules and atmospheres. Dilatometers are only useful when manufacturers can measure bodies and glazes over time and in the same firing conditions. If a chart is supplied here, please view only as a way to compare one body with another.

Another significant issue is that many customers compare measured thermal expansion numbers with calculated values of glazes in efforts to fits those glazes to a body. This does not work. Calculated values are relative only and have limitations that must be understood. The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (using your account at insight-live), fire a glazed piece and thermal stress it (300F into ice-water). If it still crazes, repeat the process.

If we recommend a base clear or glossy glaze, try calculating the expansion of that as a rough guide to know whether your glazes will fit.

Physical Properties

 Drying Shrinkage: 6.0-6.5%

Sieve Analysis (Tyler mesh):

 +35mesh: 2.0-3.0%
   35-48: 2.0-3.0
   48-65: 3.0-5.0
  65-100: 3.0-5.0
 100-150: 2.0-4.0

Fired Shrinkage:

  Cone 8: 6.5-7.5%
 Cone 10: 7.0-8.0
Cone 10R: 7.5-8.5

Fired Absorption:

  Cone 8: 6.0-7.0%
 Cone 10: 3.5-4.5
Cone 10R: 2.5-3.5

Safety Data Sheet

Click here for web view.

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