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M340

Our most popular mid-temperature, smooth, plastic, semi-vitreous, buff burning, native functional stoneware. 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 plasticity and feels slick. 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. However, as with any other fine-grained material, care and attention in drying 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.

Firing

M340 fires to a dense clean stone-grey color at cone 6-7, it is very hard and strong. At cone 4 it is yellow buff and at cone 5 the color is an attractive variegated grey buff. M340 is designed to be semi-vitreous and grey at cone 6, We typically add 2-3% talc flux to maintain fine control over the body's fired maturity and reduce the incidence quartz inversion cracking problems. Most users work at cone 5 since this provides a good compromise between stability against warping in the kiln and the vitreous character of cone 6. We do not recommend firing the body higher than cone 7.

If you need a lighter burning body, try P380 or M370. They will also 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.

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.

Glazing

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

You can develop a compatible glossy or matte base for this body from our suggested starting point base recipes available on our Internet web site at http://digitalfire.com/education/glaze/cone6.htm. Information is given on how to fit the glaze to your body and how to customize it it for colors, opacity, speck, variegation, etc. 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: 4.5-5.5%
 Cone 5: 5.0-6.0
 Cone 6: 5.5-6.5
 Cone 7: 6.0-7.0

Fired Absorption:

 Cone 4: 3.0-4.5%
 Cone 5: 2.0-3.5
 Cone 6: 1.5-2.5
 Cone 7: 0.8-1.5

Chemical Analysis

 CaO       0.2
 K2O       2.1
 MgO       1.0
 Na2O      0.1
 TiO2      0.6
 Al2O3    17.1
 P2O5      0.0
 SiO2     70.5
 Fe2O3     1.3
 MnO       0.0
 LOI       7.1%

News

M340 is one of our most stable bodies and we regularly make 3000 or more boxes at a time. We watch its fired maturity very closely and adjust the amount of talc in the body to control the degree to which it vitrifies.

We have still not solved a rather odd problem regarding the tendency of certain runs to stick to plaster batts making it more difficult to get leather hard ware off. We were never able to relate this to body particle size, solubles, plasticity, or any other property, it is still a mystery. If you encounter this we recommand making a slip from another body that does release (or just mix kaolin:ball clay 50:50), apply a thin layer of this to the batt and let it dry for a few seconds before attaching the M340 slug.

Logo Plainsman Clays Ltd.
Box 1266, 702 Wood Street, Medicine Hat, Alberta T1A 7E5
Phone: 403-527-8535 FAX: 403-527-7508 		Top

Revised: 02/20/08, Author: Tony Hansen