Middle temperature, smooth, very plastic, white-firing, semi-vitreous, refined material body for functional stoneware. A casting version is also available.

Base Glaze and Slip Recipes: See the glazing section below for more information

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

M370 is our must popular white burning clay. M370 is semi-vitreous (it is somewhat porous at cone 5-7). Compared to similar competitor bodies, M370 is whiter (because it uses more kaolines and less ball clays). It is white enough that the powder is a good base for slips and engobes.

Process Properties

Since M370 is less vitreous than a zero-porosity porcelain it does not require as much feldspar in the recipe (enabling us to maximize the clay content for greater plasticity). Of course, since M370 is still a fine grained plastic body, good drying practice is necessary (make sure pieces dry evenly across their cross section). If you need to attach elements (i.e. handles), use slip (not water) and apply as much pressure as possible during the joining process.

M370 generates less slip on throwing than competitor bodies that utilize more ball clay, you will find this makes throwing of large numbers of pieces less messy.

M370 thrown texture.


M370 is semi-vitreous at cone 6 (having about 1% porosity). Because we target this degree of density, M370 fires whiter and is tolerant of over-firing (having less tendency to bloat, stick to shelves, and glue lids to lips of ware). However, on second fix-up firings, it can bloat if fired above cone 6 (do not trust electronic controllers, verify with cones).

M370 is quite high in free quartz. This is an asset to achieving glaze fit but also implies caution in cooling the kiln through quartz inversion temperatures (e.g. 950-1100F) too quickly or dunting cracks can occur (especially large bowls and plates). If you glaze-fire a piece more than once be careful during heat-up through this temperature. Also, be aware that cooling too quickly through any stage may set up temperature gradients within pieces that can linger through inversion temperatures.

These fired bars show the progression of color from cone 4 (bottom) to cone 8. This body has plenty of margin for overfiring.


Since M370 fires so much whiter than stonewares glazes and colors are quite bright on it in comparison.

We stock commercial cone 6 bottled glazes in many colors and surfaces. However you should check for glaze fit to assure the highest quality ware. For food surfaces (and lower cost), it may be better to mix your own glazes. Develop or adopt a good transparent base and add colors and opacifiers to it. For best quality, consistency, slurry workability we recommend avoiding recipe fluxed with Gerstley Borate, use frits. We supply (as products and recipes) transparent glossy whiteware and matte frit base glazes. Their documentation describes how to mix, use and fire them and showcases stain and other additions to create an infinite number of effects.

Crazing: Function ware you make must remain craze-free for years (crazing is unsanitary and drastically reduces ware strength). Do a boiling water:ice water test to check for it. M370 and Polar Ice fit our recommended base glossy and matte glazes (mentioned above). P300 is the most difficult. More glazes will fit it than in the past (because of the higher silica content of its new recipe). The G2934 matte base works well on P300. However, while our glossy whiteware recipe fires without crazing on P300 it does not survive a 300F:IceWater test (thus crazing would appear over time). We are working on lower expansion recipes. Two promising examples are G3838A and G3813B are among them.

Our standard whiteware glossy may not have a enough melt fluidity to create reactive visual effects for additions of certain colors and variegators. The G3806C fluid melt recipe is an alternative (however crazing can be an issue, especially on P300).

Glaze slurry consistency and quality: A secret to achieving even glaze coverage is controlling the thixotropy of the slurry (prepare a more-fluid-than-normal glaze slurry and flocculate it to gel it slightly). In the gelled condition the glaze goes on much more evenly, does not drip and can be applied in a thinner layer (for more information google "glaze thixotropy"). Always screen glazes when first making them (80 mesh). Be alert to any particulate that may appear after storage (e.g. precipitates) and screen again if needed.

For slip decoration be careful to match the fired shrinkage of the slip with the body. We recommend using a slip (or engobe) recipe based on M370 itself. Add 2% VeeGum or Bentonite (the extra stickiness helps it adhere well to leather hard ware). Be careful about adding fluxes (e.g. frit), this increases fired shrinkage and can make the slip translucent. Stains can do the opposite (lowering fired shrinkage).

M370 with Alberta Slip rutile blue glaze. Fired at cone 6 oxidation.

Casting Recipe

Do not try to cast regular M370, the bentonite it contains (for throwing plasticity) will make it cast very slowly. Plainsman produces a powder version called M370C (it has no bentonite), it has excellent casting properties.

Suggested procedure to make a 5 kg. batch of dry material (yields approx 4 litres and has a water content of less than 30%):

  • M370 casting dry: 5 Kg.
  • Water: 2.1 kg.
  • Darvan #7: 38 grams

Do not add all the Darvan at once, hold a little back for fine tuning. Add it in small amounts to thin to the consistency where it can be mixed effectively and you have measure a specific gravity (by weighing, not a hydrometer). This recipe produces a slurry of 1.77 specific gravity after 1 hour of propellor mixing in our lab. Tune the amount of deflocculant such that the slurry gels after 30 minutes or so. If you are a potter you may be accustomed to making casting slips of much higher water content, but if you follow this procedure you will find the performance of a properly deflocculated slurry is superior.

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: 5.5-6.5%
 Water Content:21.7-22.7 Drying Factor: C120
 LOI: 5.0-6.0%

Sieve Analysis (Tyler mesh):

   48-65: 0.0-0.2%
  65-100: .01-0.3
 100-150: 0.1-0.4
 150-200: 1.0-4.0
 200-325: 8.0-11.0

Fired Shrinkage:

 Cone 4: 5.5-6.5%
 Cone 5: 6.0-7.0
 Cone 6: 6.5-7.5
 Cone 7: 6.5-7.5

Fired Absorption:

 Cone 4: 3.0-4.0%
 Cone 5: 1.5-2.5
 Cone 6: 0.5-1.5
 Cone 7: 0.2-0.8

Chemical Analysis

 CaO       0.2
 K2O       1.2
 KNaO      0.1
 MgO       0.2
 Na2O      2.6
 TiO2      0.7
 Al2O3    22.4
 P2O5      0.0
 SiO2     65.6
 Fe2O3     0.5
 FeO       0.0
 MnO       0.0
 LOI       6.4%


M370 bowls by Dawn Candy.

Mocca M370 mugs by Victor Duffhues

P300 and M370 Cone 6 mugs. By Tony Hansen. Outside glaze is G3806C plus copper oxide.

P300 and M370 mugs with GA6A Alberta Slip at cone 6 (glaze uses Frit 3249 instead of Frit 3134). Firing soaked 30 minutes at 2100F on the way down and slow cooled 100F/hr down to 1400F. By Tony Hansen.

Left: M370 mug fired to cone 10R. This is not recommended, this one has survived without bloating (but it could have bloated). Right: M370 mug fired to cone 8. Single fire. Quick drying (by heat gun). Obviously this type of treatment is not recommended, but it does demonstrate the margin for over-firing and drying properties.

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