Our most popular mid-temperature, smooth, plastic, semi-vitreous, buff burning, native functional stoneware.

Plainsman Polar Ice, P300, M370 and M340 (Whiteware transparent glaze on the porcelain, Stoneware transparent glaze on the M340).

The M340 is made from a combination of our A3 and B stoneware materials (as are H550, Buffstone, L212). It is our most popular body (widely used in schools, by hobbyists, and professional potters. Compared to bodies made from refined industrial minerals, M340's diversity of ultimate particle sizes produces a body of much higher dry strength. Its parent materials are also highly consistent, pure, and free of foreign particles. In fact, even without grinding, 95% of a raw sample of M340 will wash through a 200 mesh sieve.

Process Properties

M340 has medium to high plasticity and feels smooth (having a slight texture). There is some distribution of particle sizes in the plus 200 mesh range, these provide channels for faster drying than other bodies you may have used. You should have few problems drying smaller pieces, but care and attention are necessary when making larger pieces, especially flat plates, shallow bowls and sculptural ware. Make sure that the focus is on evenness of drying rather than speed; if sections of a piece dry faster, then either slow these sections down or slow down the drying of the entire piece to effect a more even process.


Fired test bars of M340 (left) and M325. Fired at cone 8, 7, 6, 5 and 4 (top to bottom).

M340 fires from a straw color at cone 4 to patchy stone-grey-buff at cone 6 to grey by cone 8 (the greying begins by cone 7). M340 is best used at cone 6. This temperature is a functional compromise between the maximum vitrification of cone 8 (where it will also tend to warp or bloat) and the higher porosities in the cone 4-5 range. We typically add 2-3% talc flux to maintain fine control over the body's fired maturity at cone 6 (and reduce the incidence of quartz inversion cracking problems).


M340 is quite fine particled in its natural state and takes glazes very well, producing fine homogeneous surfaces. It is high in silica and will craze fewer glazes porcelains. However crazing is possible if a glaze is high in sodium (i.e. from soda feldspar or nepheline syenite) or is very low in silica or alumina (little clay or silica). As a general rule, unbalanced glazes containing high feldspar and little kaolin or silica are usually a problem. For functional ware check glaze fit using a boiling water:ice water immersion test.

Since M340 does contain some iron oxide, brightly colored glazes will tend be muted compared to porcelain. This can be handled by using a well fitted slip between body and glaze or opacifying the glaze more.

Glaze Recipes

We specifically recommend our Stoneware transparent base glaze recipe (Whiteware transparent also works). We manufacture these but also provide the recipes if you want to mix your own.

Other Recipes

  • Ravenscrag base GR6-A glossy, you can find its recipe on the cone 6 recipes page at This transparent gloss is a slightly amber-colored because Ravenscrag does some iron content.
  • G1214W has been used for many years. It is among several starting recipes at the Digitalfire Reference Library. Information is also given on how to customize base glazes for colors, opacity, speck, variegation, etc. One of the recipes there, G1214M, works well with pink stains.

For slip decoration, be careful to match fired shrinkage of the slip with the body.

Casting Recipe

M340 has soluble salts that prevent the action of deflocculants so it cannot be slip cast. We have developed a casting body that is of similar color and maturity (made from refined materials). You can find information on it here.

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.

Thermal Expansion Chart. Average: 5.5.

Physical Properties

 Drying Shrinkage: 6.0-7.0%
 Dry Strength: 800 psi
 Water Content: 20.0-21.5%
 Drying Factor: C120
 Dry Density: 2.0

Sieve Analysis (Tyler mesh):

     +48: 0.0-0.1%
   48-65: 0.4-0.8
  65-100: 1.5-2.5
 100-150: 1.5-2.5
 150-200: 4.0-6.0

Fired Shrinkage:

 Cone 4: 4.0-5.0%
 Cone 5: 4.5-5.5
 Cone 6: 5.0-6.0
 Cone 7: 5.5-6.5

Fired Absorption:

 Cone 4: 4.0-5.5%
 Cone 5: 2.5-4.0
 Cone 6: 1.5-2.5
 Cone 7: 1.0-2.0

Chemical Analysis

 CaO       0.2
 K2O       2.1
 MgO       1.2
 Na2O      0.1
 TiO2      0.6
 Al2O3    17.7
 P2O5      0.0
 SiO2     69.2
 Fe2O3     1.4
 MnO       0.0
 LOI       7.5%


M340 at cone 6. Kathy Ransom.

This mug is made from M340 and glazed with GA6-C Alberta Slip rutile blue (outside) and GR6-A Ravenscrag white base (inside). By Tony Hansen.

M340 with GA6A base Alberta Slip glaze. However this one employs frit 3195 instead of 3134. A slow cool produced a flawless surface.

A very deep and rich blue (with no cobalt). This is M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt Umber instead of the normal 10% Zircopax) was applied inside and partway down the outside (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue.

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