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

These fired bars show the progression of color from cone 4 (bottom) to cone 8 (porosity and fired shrinage are also indicated, cone 8 is over-fired).

M390 fires dark red and is the material of choice for dark burning functional ware. It contains more of our native Redstone material than any other body. It is one of several bodies that we process to 100 mesh particle size and is intended to produce ware with a clean, unblemished glaze surface.

M390 is very similar in composition to M350 and shares the very fine natural smooth character. It is however more plastic. Its darker color tends to limit it to specialized glaze effects. M390 contains 5 different clays, each of which is quite balanced on its own.

Process Properties

M390 has high plasticity and feels smooth (possible very slight tooth). It generates significant slip during throwing if too soft. Care and attention during drying are a must, especially with larger pieces, flat plates, shallow bowls, and sculptural ware. Put 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.

21 mugs made from 1/2 box of M390, 10 kg (all scrap was reclaimed). These have a weight-to-capacity ratio of 1.05 (each 1 gram of fired body can contain 1.05 grams of water). All are lined with an engobe at leather-hard stage (and clear glazed inside at bisque).


M390 is a fairly high iron body and thus burns red to brown as it matures. At cone 3-4 it is a very warm toasty red. The red intensifies as it approaches cone 6 and turns brown by cone 7. We maintain the porosity at about 2.0% at cone 6 to hold onto the red color. We regard it as over-fired at cone 7 (although you might have success) but at cone 8 it will definitely be unstable (tending to bloat and warp). This instability occurs even though it has not reached zero porosity.

Transparent glazes will darken the color of the underlying body. If you fire to cone 7 keep in mind that accidental over firing could take it into the warping or bloating territory of cone 8. If you fire to cone 4, be aware that porosity is too high (and strength likely too low) for practical functional ware. Cone 6 is the best compromise of color, stability and strength.

M390 with Alberta Slip rutile blue. Fired at cone 6 oxidation. Follow the firing curve instructions for that glaze to get this blue, cobalt-free color.


M390 is quite fine and produces 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 significantly stain or mute certain colors (compared to porcelain). On the other hand, this effect will enhance the appearance of earthtone and variegated glazes.

M390 is high in silica and will accept most typical cone 6 glazes without producing crazing. However, crazing is possible on M390 if a glaze is high in sodium (i.e. from soda feldspar or nepheline syenite) or is very low in silica or alumina (generally, glazes containing high feldspar and little kaolin or silica are usually a problem for any body). 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.

Glaze Recipes

Transparent glazes: Although M390 fires red, keep in mind that the color will darken considerably under a transparent glaze (because the latter fluxes the surface of the clay). M390 is made from natural high iron stoneware clays and typical transparent glazes will likely fire milky (with clouds of micro bubbles). However under fluid melt transparents this clay looks alot better (our G3806C Panama Clear is an example). An even better solution is to use an amber-colored clear like GA6-A Alberta Slip base (the photo of the mug here shows it on the inside) or to add 3-4% iron to our standard G2926B clear (the iron particles agglomerate the bubbles and they escape).

G3806C clear on M390. There is still some clouding, but it is better than other transparents we have used.

There are many glazes made from Alberta Slip and Ravenscrag Slip that you can use on M390 (each of their sites have a cone 6 glaze recipe page).

Casting Recipe

M390 has soluble salts that prevent the action of deflocculants so it cannot be slip cast. We have developed a casting body that is similar to this one (made from refined materials). You can find information on it here.

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: 6.0-7.0%
 Dry Strength: n/a
 Water Content: 19.5-20.5%
 Drying Factor: c130
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +65: 0.0-0.2%
  65-100: 0.5-1.5
 100-150: 4.5-7.5
 150-200: 7.0-10.0
 200-325: 10.0-15.0

Fired Shrinkage:

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

Fired Absorption:

 Cone 4: 4.0-5.5%
 Cone 5: 2.5-3.5
 Cone 6: 1.5-2.5
 Cone 7: 0.5-1.5

Chemical Analysis

 BaO       0.3
 CaO       0.3
 K2O       2.3
 MgO       0.7
 Na2O      0.1
 TiO2      0.7
 Al2O3    16.3
 P2O5      0.2
 SiO2     68.3
 Fe2O3     3.6
 MnO       0.0
 LOI       7.2%


This mug is made from M390 and glazed with Alberta Slip rutile (outside) and GA6A Alberta Slip base (inside). By Tony Hansen.

These are all M390 clay fired to cone 6 (soaked 30 minutes at 2100F on the way down, then 100F/hr cool down to 1400F). Left: With L3954 white engobe inside, and midway down outside. The inside is glazed with G2926B Whiteware clear. The outside is the same glaze with 4% iron added (notice there are no bubble clouds with added iron). Center: Whole M390 mug glazed with G2926B clear (notice the bubble clouds). Right: White engobe applied as with the left mug, but the whole thing is glazed with GA6A Alberta Slip base glaze (but using Ferro Frit 3195 instead of 3134).

M390 bowl fired at cone 6 with Opal Blue glaze.

M390 horse under construction by Katie Stone at Medalta in 2014.

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