Mid-temperature, finely ground and smooth, plastic, semi-vitreous, medium brown burning, native body for functional ware.

M350 is the material of choice if you need to make functional ware from a brown burning body. It is processed to 100 mesh particle size and is intended to produce ware with a clean, unblemished glaze surface.

M350 is very similar in composition to M390 and shares the very fine and smooth natural character and plasticity. Its lighter color will be suitable for all but very specialized glaze effects requiring a dark burning body. M350 contains 5 different clays each of which is quite balanced on its own and it is a body over which we have a lot of consistency control.

Process Properties

M350 has medium plasticity and feels slick on the wheel and generates significant slip during throwing. While you will find it dries well for smaller items, as with any other fine-grained material, care and attention in drying are necessary in 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 entire piece. If you need to attach elements (i.e. handles) use slip with a low water content and apply as much pressure and lateral movement as possible during the joining process.

If you need an all-around coarser material, M375 is another possibility, however it can produce pinholing in some glazes and will tend to produce rough glaze surfaces where grog exposes through thinly glazed sections (i.e. mug lips).


These M350 fired bars show the progression of color from cone 4 (bottom) to cone 8. Cone 5 still has the leather tan color, but it not really vitreous enough for functional ware, cone 6 is better. Cone 8 is over fired.

M350 fires to a leather brown color at cone 6. In the cone 4-5 range there are pinkish and more variegated tones in the brown color. At cone 7 the body burns to a dense grey brown. If this M350 is fired beyond cone 7 it will begin to bloat, thus we recommend cone 5 for the warmest color possible, and cone 6 for better hardness and functional strength. We try to maintain this body at about 1.0-1.5% porosity at cone 6, thus is it more vitreous than M390. M350 does work in reduction at cone 4-6 whereas M390 is too vitreous because of its high iron and M340 because of its talc complement.

M350 is fine-ground and thus fires to a homogeneous color compared to M332, which burns to a darker more earthy variegated surface. M390 has a similar character but fires to much darker shades.

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).


M350 is quite fine and fires to a homogeneous fired surface for most glazes. Since it is a dark stoneware, the iron in the clay will bleed into glazes and colors and mute them to some extent so that glazes will not be as vivid as they would be if used on porcelain.

M350 is high in silica and will accept most typical cone 6 glazes without producing crazing. However, crazing is possible on M350 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.

Although M350 fires to a reddish tone at cone 4-5, keep in mind that the color will darken considerably under a transparent glaze because the glaze fluxes the surface of the clay advancing its color to that of a hotter firing.

Caution About Clear Glazes

Clear glazes often do not work on dark bodies. The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body (M390) is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition.

Left mug: The outside glaze adds 4% iron to G2926B (the glaze was not screened, so iron particles are agglomerated and acting as a fining agent, removing the bubbles). Right mug: The whole thing is glazed with GA6-B Alberta Slip base glaze. These amber glazes have an added benefit: The color darkens over dark burning bodies (to almost black).

Glaze Recipes

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).

This body is a great candidate for the engobe process, we recommend the L3954B recipe.

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).

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: 6.0-7.0%
 Dry Strength: n/a
 Water Content: 20.5-21.5%
 Drying Factor: c120
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

   48-65: 0.0-0.1%
  65-100: 0.1-0.5
 100-150: 2.5-3.5
 150-200: 4.5-6.5
 200-325: 7.0-10.0

Fired Shrinkage:

 Cone 4: 4.0-5.0
 Cone 5: 4.5-5.5
 Cone 6: 5.0-6.0
 Cone 7: 5.5-6.5

Fired Absorption:

 Cone 4: 4.0-6.5
 Cone 5: 3.5-5.0
 Cone 6: 1.5-2.5
 Cone 7: 1.0-2.0

Chemical Analysis

 BaO       0.3
 CaO       0.2
 K2O       2.2
 MgO       0.7
 Na2O      0.1
 TiO2      0.7
 Al2O3    17.2
 P2O5      0.1
 SiO2     68.5
 Fe2O3     2.6
 MnO       0.0
 LOI       7.5%


M350 vs M390 at cone 6. The liner glaze is Alberta Slip GA6-A base, outside is GA6-C Alberta Slip rutile blue. By Tony Hansen.

M350 mugs with GA6-C Alberta Slip rutile blue glaze outside and GA6-B Alberta Slip base inside (left front). Fired to cone 6 using C6DHSC firing schedule.

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