M390
Description
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 (top).
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 60 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.
Firing
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.
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).
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 certain colors (compared to porcelain). On the other hand, this effect will enhance the appearance of earthtone and variegated glazes. For functional surfaces consider using our L3954B engobe.
M390 is high in silica and will accept most typical cone 6 glazes without crazing. However, crazing is possible if a glaze is high in sodium (i.e. from soda feldspar or nepheline syenite) or is very low in silica or alumina.For functional ware please check glaze fit using a boiling water:ice water immersion test. Please contact Plainsman if you need help to adjust your glaze.
Caution About Clear Glazes
The GA6-B Alberta Slip base is the preferred liner glaze and works well as a transparent on this body. Here is why.
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 the L3954B 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 brushing glazes and dipping glazes is practical (especially with our guidelines). Be careful using recipes from the web, common sense recipe limits apply. The best approach is to begin with good transparent bases that do not craze, cutlery mark or leach. Our G2926B glossy and G2934 matte can be purchased as powders or mixed from their recipes. G2934 has significantly lower thermal expansion, it will almost certainly never craze. The two can be mixed, not only to fine-tune matteness, but also thermal expansion. For example, an 15:85 matte:gloss mix gives a silky matte for us. The gloss is dominant so a 50:50 mix is glossy, but with lower thermal expansion than the glossy by itself.
Consider also making glazes based on Alberta Slip (especially the GA6-B amber base) and Ravenscrag Slip.
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. It can be colored with stains or whitened with zircopax. It can be applied thickly as an engobe or thinly as a slip.
If you want to develop and mix your own glazes and engobes consider getting an account at insight-live.com. You can organize a methodical development program and adopt better methods of testing (e.g. melt fluidity, thermal stress, slip-fit tests).
This is G3806C clear on M390. There is still some clouding, but it is better than other transparents we have used. An even better solution is Alberta Slip GA6-B amber-clear base. Or, you can add some iron oxide to G2926B, it acts as a fining agent.
Casting Recipe
M390 has soluble salts that prevent the action of deflocculants so it cannot be slip cast (it gels in response to additions of Darvan or sodium silicate). With an addition of C-Red clay it is possible to convert our M370 casting body into one that fires the same color as M370, it is documented under our code number L4558. You can also mix one based on Redart, it is documented under our code number L4005D. As noted elsewhere, 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.
Thermal Expansion
We do not supply thermal expansion values. If a chart is supplied here, please view it only as a way to compare one body with another. Please note that, although you may calculate the thermal expansion of a glaze, this cannot be done for clay bodies since they do not melt. The best way to fit glazes to clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down, fire a glazed piece and thermal stress it (using an IWCT test, 300F into ice-water). If it still crazes, repeat the process.
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: 3.5-4.5% Cone 5: 3.5-4.5 Cone 6: 4.0-5.0 Cone 7: 4.5-5.5/pre>Fired Absorption:
Cone 4: 4.5-5.5% Cone 5: 3.0-4.0 Cone 6: 2.0-3.0 Cone 7: 1.0-2.0Chemical 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%Gallery
M390 with GA6-C Alberta Slip rutile blue. Fired at cone 6 oxidation. Follow the firing curve instructions for that glaze to get this blue, cobalt-free color.
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.
M390 mugs glazed with the GA6-C Alberta Slip rutile blue recipe (outside) and GA6-B Alberta Slip base recipe (inside). By Tony Hansen.
