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M350

Description

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).

Firing


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. M350 does work in reduction at cone 4-6 whereas M390 and M340 are too vitreous.

M350 is fine-ground and thus fires to a homogeneous color compared to M332, which burns to a darker more earthy variegated surface.

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

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 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).


M350 with G2934 matte white glaze fired at cone 6

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 guidelines even beginners can make dipping glazes that go on silky smooth and evenly and dry in seconds). Be careful using recipes from the web. High-feldspar glazes (having more than about 35%) most 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).

The best approach is to begin with good transparent bases you understand, do not craze, cutlery mark or leach. Our G2926B glossy and G2934 matte are frit-fluxed bases with documentation on use and adding stain, color and variegator additions. G2934 has significantly lower thermal expansion. The two can be mixed to fine-tune matteness and thermal expansion. For example, a 20:80 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.

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. 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).

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

Fired Absorption:

 Cone 5: 3.5-5.0
 Cone 6: 2.5-3.5
 Cone 7: 1.5-2.2

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%

Gallery


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
Email: tim.lerner@plainsmanclays.com