Middle temperature, smooth, very plastic, white-firing, semi-vitreous, refined material body for functional stoneware.

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

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

M370 is our must popular white burning material. Thus, unlike our P300 porcelain, M370 is semi-vitreous (it is somewhat porous at cone 5-7). Also, it employs high quality ingredients and utilizes multiple clays and feldspars to minimize effects of material changes. Compared to similar competitor bodies, M370 is whiter (because it uses more kaolines and less ball clays). It is white enough that M370 powder is a good base for slips to be painted on the ware assuring the best drying and fired-fit possible. M370 has a similar high temperature sister body named H570.

Process Properties

Since M370 is less vitreous than a zero-porosity porcelain it does not require as much feldspar in the recipe. This enables us to maximize the clay content and create a body of 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 with a low water content 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 fires more dense than a stoneware but it does not reach zero porosity as do true porcelains. It 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).

There is an important caution with M370: It is quite high in free quartz. This is an asset to achieving glaze fit but also means that you should not cool 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 continue even if you attempt to slow-cool 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, you will find that glazes and colors are quite bright on it in comparison.

We provide liner and base glazes documented at our glazes page (e.g. M370 Transparent, Matte Base). We manufacture these but also provide the recipes if you want to mix your own (each has links to a page showing the recipe). If you still get crazing the M370 glossy recipe page also has a link to a low expansion version.

For slip decoration be careful to match the drying and fired shrinkage and thermal expansion of the slip with the body. We recommend using a slip or engobe recipe based on M370 itself. Add a tiny amount of Frit 3110 (e.g. 2%). This improves adherence to the fired body and brightens up added colorants. Be careful about adding any more than that as it will overly affect fired shrinkage and thermal expansion compatibility with the body and the slip may become too translucent. Add 2% VeeGum or Bentonite (the extra stickiness helps it adhere well to leather hard ware). VeeGum requires more water but it gels and applies better; even with extra water needed it should not crack on drying (rather, its high stickiness imparts excellent drying and hardening properties). If the slip is too translucent consider adding a little Zircopax to opacify it. Add your stains to that. Different percentages will be required for different colors, the darkest colors around 5%, the lightest up to 20%. If high stain percentages are needed it may dry out the slip so extra frit might be needed.

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 (yeilds 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 more if needed. Tune the amount of deflocculant such that the slurry gels after 30 minutes or so. This recipe produces a slurry of 1.77 specific gravity after 1 hour of propellor mixing in our lab. 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.

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.

Safety Data Sheet

Click here for web view (format adheres to Globally Harmonized System of Classification and Labelling of Chemicals - GHS)

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