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.
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.
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.
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).
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.
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).
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.
Drying Shrinkage: 6.0-7.0% Dry Strength: n/a Water Content: 20.5-21.5% Drying Factor: c120 Dry Density: n/a
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
Cone 4: 4.0-6.5 Cone 5: 3.5-5.0 Cone 6: 1.5-2.5 Cone 7: 1.0-2.0
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%
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508