M340 has medium to high plasticity and feels smooth (having a slight texture).
There is some distribution of particle sizes in the plus 200 mesh range, these provide channels for faster drying than other bodies you may have used. You should have few problems drying smaller pieces, but care and attention are necessary when 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 drying of the entire piece to effect a more even process.
M340 fires from a straw color at cone 4 to patchy stone-grey-buff at cone 6 to grey by cone 8 (the greying begins by cone 7). M340 is best used at cone 6. This temperature is a functional compromise between the maximum vitrification of cone 8 (where it will also tend to warp or bloat) and the higher porosities in the cone 4-5 range. We typically add 2-3% talc flux to maintain fine control over the body's fired maturity at cone 6 (and reduce the incidence of quartz inversion cracking problems).
To get the best defect-free surface please consider using a hold-rise-drop-hold firing schedule. For example, at cone 6 start with the PLC6DS schedule for porcelains. For stonewares, the best results will come with the C6DHSC schedule.
M340 is quite fine particled in its natural state and takes glazes very well, producing fine homogeneous surfaces. It is high in silica and will craze fewer glazes porcelains. 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 (little clay or silica). As a general rule, unbalanced glazes containing high feldspar and little kaolin or silica are usually a problem. For functional ware check glaze fit using a boiling water:ice water immersion test.
Since M340 does contain some iron oxide, brightly colored glazes will tend be muted compared to porcelain. This can be handled by using a well fitted slip between body and glaze or opacifying the glaze more.
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 transparent or white liner glazes 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). Because ware is not crazed out of the kiln does not mean it will not 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).
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).
If you want to develop and mix your own glazes and engobes consider getting an account at http://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).
M340 has soluble salts that prevent the action of deflocculants (rather than thin, an M340 slurry will turn into a gel in response to additions of Darvan or sodium silicate). We have developed a casting body that is of similar color and maturity (made from refined materials). We do not produce this as a product, but you can find its recipe and information on mixing it yourself here. It 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.
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 (using an IWCT test, 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.
Drying Shrinkage: 6.0-7.0% Dry Strength: 800 psi Water Content: 20.0-21.5% Drying Factor: C120 Dry Density: 2.0
+48: 0.0-0.1% 48-65: 0.4-0.8 65-100: 1.5-2.5 100-150: 1.5-2.5 150-200: 4.0-6.0
Cone 4: 4.0-5.0% Cone 5: 4.5-5.5 Cone 6: 5.0-6.0 Cone 7: 5.5-6.5
Cone 4: 4.0-5.5% Cone 5: 2.5-4.0 Cone 6: 1.5-2.5 Cone 7: 1.0-2.0
M340 at cone 6. Kathy Ransom.
A very deep and rich blue (with no cobalt). This is M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt Umber instead of the normal 10% Zircopax) was applied inside and partway down the outside (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is GA6-C Alberta Slip rutile blue.
Amaco PC-20 Blue Rutile (three coats) vs Ravenscrag Blue G2917 on M340. Fired at cone 6.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508