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M390

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

M390 fires very dark 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 slick. Care and attention during drying are a must, especially with larger pieces, 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 pieces (i.e. handles) use slip with a low water content and apply as much pressure and lateral movement as possible during the joining process.

Firing

M390 is high in iron and thus burns quite dark as it matures in the kiln. At cone 3-4 it is a very warm toasty red that variegates to a deep red by cone 5. At cone 6 the body darkens to a deep red brown. At cone 7 the color lightens somewhat as the body begins the melting process. We try to maintain the porosity at about 1.75-2.0% at cone 6 to hold onto the red color. If you fire higher than cone 6 some of the red color will be lost.

Because of the very high iron content the fired color progression is volatile (the color change from cone 5 to 6 is more dramatic than expected). We recommend cone 4-5 for the warmest color and cone 6 for the best strength and interaction with glazes. If the dark color is not important consider using M350.

Glazing

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 them (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 (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 M390 fires to red at cone 4-5, keep in mind that the color will darken considerably under a transparent glaze because the latter fluxes the surface of the clay. If you need a red clear glazed result you will have to work at low fire between cone 06-02.

Glaze Recipes

You can develop a compatible glossy or matte base for this body from our suggested starting point base recipes available on our Internet web site at http://digitalfire.com/education/glaze/cone6.htm. Information is given on how to fit the glaze to your body and how to customize it it for colors, opacity, speck, variegation, etc. For slip decoration, be careful to match drying and fired shrinkage of the slip with the body since low temperatures generate little glass to adhere the slip.

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: 3.5-5.5%
 Cone 5: 2.0-3.0
 Cone 6: 1.3-2.3
 Cone 7: 1.0-2.0

Chemical Analysis

 BaO       0.3
 CaO       0.3
 K2O       2.3
 MgO       0.7
 Na2O      0.1
 TiO2      0.7
 Al2O3    16.5
 P2O5      0.2
 SiO2     68.2
 Fe2O3     3.4
 MnO       0.0
 LOI       7.3%

Plainsman Clays Ltd.
Box 1266, 702 Wood Street, Medicine Hat, Alberta T1A 7M9
Phone: 403-527-8535 FAX: 403-527-7508

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