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.


Commercial brush-on glazes offer many colors and surfaces. For functional ware check for glaze fit (vital for quality functional ware). Do not assume food safety of brightly colored glazes in your kiln and with layering without a leach test (e.g. GLLE test). Consider using transparent or white liner glazes for food surfaces.

Mixing your own glazes is practical (with our clear guidelines even beginners can make dipping glazes that go on silky smooth and evenly and dry in seconds). If you already do this using recipes from the web, be careful. High-feldspar glazes (having more than about 35%) often craze. Ones that rely on high melt fluidity to encourage crystallization and variegation (often because of excessive Gerstley Borate, lithium carbonate, zinc or Frit), view these with suspicion for leaching and cutlery marking; test them well (also test the additionless versions). Be suspicious of any glaze not having good documentation.

The best approach is to begin with a good transparent base you understand and that fits. We supply (as products and recipes) G2926B glossy whiteware and G2934 matte frit-fluxed bases. Their documentation describes how to mix, use, fire and adjust them and showcases stain, color and variegator additions to create an infinite number of effects. The former, G2926B, may not have a enough melt fluidity to create non-food-surface reactive visual effects with certain colors and variegators. G3806C fluid-melt recipe is an alternative (but check for crazing). These pages also reference other base glazes that might be of interest.

Consider also making glazes based on Alberta Slip (especially the GA6-A amber base) and Ravenscrag Slip. These materials have their own websites with lots of helpful information.

Crazing: Functional ware must remain craze-free (crazing is unsanitary and drastically reduces ware strength). Because ware is not crazed out of the kiln does not mean it will not do so with time. Do cycles of a boiling water:ice water immersions (BWIW test) on a piece to test glaze fit (by stressing it to bring out any crazing or shivering tendencies).

Glaze slurry consistency and quality: A secret to achieving even glaze coverage is controlling the thixotropy and specific gravity of the slurry, both in freshly mixed and stored batches. A glaze of the right specific gravity and having a slightly gelled condition goes on to bisque ware evenly, does not drip and dries in seconds. 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 and engobes be careful to match the fired shrinkage of the slip with the body. Where we do not recommend a specific engobe recipe use a formula based on the porcelain 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 (the mismatch with body can cause flaking) and can compromise opacity.

If you want to develop and mix your own glazes and engobes consider getting an account at You can organize a methodical development program and adopt better methods of testing (e.g. melt fluidity, thermal stress, slip-fit tests).

M370 with GA6-C 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 (using an IWCT test, 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%

Compared to Others

The decline of the ceramic industry in North America has impacted the price, availability and quality-for-ceramics of raw materials from which porcelains are made (especially ball clays and kaolins, which have seen increases in soluble salts, foreign particles and iron specks). In addition, we do not have filter-pressing, pre-mixing and stainless-steel pugging equipment (these would drastically increase prices). While you cannot make Wedgewood-quality ware it will be far whiter and cleaner than our stonewares. Notwithstanding that, be aware that transparent glazes carry a chance of isolated specks using our standard porcelains, it is better to use white and colored glazes on these. If you absolutely need a clean, white, translucent porcelain consider using our premium products, Polar Ice and P700 (they cost more but you get much more). We are also developing engobes for low, middle and high temperature ranges, using these you can apply a porcelain-like surface of almost any color at leather hard stage and completely hide the underlying stoneware. These engobes are so opaque that a white one can completely mask a black body underneath using only one coat. So, if you can master their use (there is lots of documentation here) many new design opportunities will offer themselves.


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 GA6-A 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.

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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702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508