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

Three porcelains and a buff stoneware with transparent base glaze at cone 6.

The M340 family of clay bodies (H550, H460, L212 are a similar base) are our most popular and are widely used in schools, by hobbyists, and professional potters. M340 is made from balanced native materials mined principally from the extensive 'whitemud' formation of southern Saskatchewan. 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 very smooth. There is some distribution of particle sizes in the plus 325 mesh range and these open the matrix up and are a factor in its faster drying. You will have not 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.

If you need a body that is a little more open, a grogged version of M340 is available. For an all-around coarser material, try M325. However, these bodies can produce pinholing in some glazes and can yield a rough surface when particles of grog expose themselves through the fired glaze.


Fired test bars of M340 (left) and M325 from cone 8 (top) to cone 4.

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 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. Cone 6 is thus the most 'functional' temperature for this clay blend given by mother nature.

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). While many work at cone 5, recommend a full cone 6 get the designed compromise. As noted, we do not recommend firing the body higher than cone 7.

If you need a lighter burning body, try M370. Bodies like M370 will produce brighter colors in some glazes. However, they do not have the robust working properties of M340, are not as consistent, and are more expensive.

Fired test bars of M340GS from cone 8 (top) to cone 4.

M340 Speck: We make a version of M340 with a 0.2% manganese granular added. This produces a fine black fired speck that will bleed up through glazes producing a pleasing visual character for some applications. However this body must not be fired higher than cone 6 or it will blister. Since slight overfiring is unavoidable in typical hobby kilns it is essential that you fire lower (i.e. cone 5) to avoid loss of ware.


M340 is very fine in its natural state and takes glazes very well, producing fine homogeneous surfaces.

M340 is high in silica and will craze fewer glaze than our porcelain bodies. However its overall thermal expansion is still low and crazing is possible on M340 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 flint). As a general rule, unbalanced glazes containing high feldspar and little kaolin or flint are usually a problem. For functional ware we recommend you check glaze fit using a boiling water:ice water immersion test. Please contact Plainsman if you need help to adjust your glaze.

Since M340 does contain some iron oxide, brightly colored glazes will tend be muted compared to their use on 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 liner and base glazes at our Liner and Base Glazes page. (e.g. M340 Transparent, Matte Base). 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 Ravenscrag.com. This transparent gloss is a slightly amber-colored because Ravenscrag does have some iron content.
  • G1214W has been used for many years. It is amoung 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 drying and fired shrinkage of the slip with the body since low temperatures generate little glass to adhere the slip.

Thermal Expansion

The chart shown was produced from a specimen fired once to cone 6 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: 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: 0.0-0.2%
   48-65: 0.2-0.6
  65-100: 0.5-2.0
 100-150: 1.0-2.0
 150-200: 1.5-4.0
 200-325: 7.0-10.0

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.

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