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 with G2926B Whiteware transparent glaze on the porcelain (G2916F Stoneware transparent 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.

Substitutes For

B-Mix cone 6. M370 is much higher in kaolin and lower in ball clay than B-Mix, that is why it fires whiter. There is a plasticity-loss associated with this, M370 makes up for that with an addition of bentonite (given it a drying shrinkage of about 0.5% higher than B-Mix). Both bodies are highly plastic and fine-grained and require care in drying to avoid cracking. The best practice is to keep the water content of all parts of pieces as similar as possible throughout the drying process.

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). 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.

Drying: Porcelains do not dry as well as stonewares or bodies with particulates. You will get the best results if the clay is not too soft, ware is not too thick, contours are smooth, wall thicknesses are even, joins are few and done with thick slip, the degree-of-wetness in all parts of a piece is kept equal throughout all stages of drying. Large pieces are best made on plaster bats so the bottom can stiffen with the walls. The worst drying performance will occur with thick ware made from very soft clay, the use of non-absorbent bats, where vessel walls are thick at the bottom and thin at the lips or edges, walls are of uneven thickness with lots of joins or abrupt angles (giving cracks a place to start) and where drying is uneven (e.g. lips and edges are permitted to stiffen early on while lower sections remain soft). Large, flat plates are the most difficult shapes to dry, it may be necessary to stretch the time out to a month or more to achieve the even drying needed for success.

M370 thrown texture.


M370 is semi-vitreous at cone 6 (having about 1% porosity). Because we target this degree of density, it 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 requires 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.

As with any body, bisque fire it to the highest practical temperature (high enough to burn out undesirables, but low enough to have the needed porosity for glazing). Cone 06 is a good starting point.

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

To get the best defect-free surface please consider using a drop-and-hold firing schedule, for example the PLC6DS schedule. If crystallization during cooling is not an issue, glazes will give optimum results if slow-cooled also (e.g. the C6DHSC schedule).


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 a transparent or white liner glaze 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-B 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). Even though ware may not be crazed out-of-the kiln it may 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).

Thixotropy: Many people mix their glazes the traditional way, just adding water until the slurry appears to be the right viscosity for dipping. However, if you want better application properties for one-coat dipping, consider creating a thixotropic slurry. Thixotropic glazes are creamy because they have been thinned and then gelled by the addition of a flocculant. They go on evenly, hang on without dripping and dry quickly. Achieving (and maintaining) this state involves targeting a specific gravity (usually around 1.43) and adding epsom salts (1-2g/1000g of powdered glaze).

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 one 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).

G2934 matte can crawl if applied too thick. These are M370 (normal thickness left, too thick right).

Casting Recipe

Use M370C (the regular M370 adds 3% bentonite, making it deflocculate poorly and cast very slowly). M370C has the plastic strength and shrinkage to pull itself away from the mold (when properly deflocculated), however casting time is still longer than a body optimized for industrial fast-casting). It is important to understand the principles of deflocculation, be able to measure specific gravity efficiently and have a good propeller mixer to make good casting slip.

Suggested recipe (yields approx 4 litres):

M370C powder5000
Darvan #738

Mix the Darvan with the water first, then add the clay to that. Hold back some of the Darvan for fine-tuning. Measure 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.

It is possible to make whiter, stronger, faster casting and translucent casting porcelains if you are willing to mix your own recipe. Please check this page for some examples.

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. The liner glaze is G2926B.

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.

M370 with G2934Y Red and G2926B Black

M370 with GA6-C Alberta Slip rutile blue glaze. Fired at cone 6 oxidation.

Safety Data Sheet

Click here for web view.

Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508