September 2021: The entire industry is phasing out of the use of talc, it is happening suddenly. That means we must change this body. You can find more information on our work at this page.
The plasticity of the new version (see recipe change info below) has greatly increased plasticity, it feels very smooth and you will be able to make thinner and lighter ware than ever before. It will dry very hard. Drying is now slower (giving students more time to work on their projects). Of course the greater plasticity is accompanied by a drying shrinkage increase, so care is needed to avoid cracking (do what is needed so the pieces dry evenly throughout the process).
L215 fires to a light red at cone 06 developing strength similar to white burning low fire bodies. In truth, even though most people fired to cone 06, at that temperature bodies are weak and porous. Cone 04 is somewhat more dense and strong but by cone 02 it is almost a stoneware. A reasonable compromise is cone 03, most low fire glazes and underglazes can easily survive to that point. If you must fire to 06 you must use commercial clear, paint on glazes to get transparent (our G2931K clear is cloudy at 06 and 04).
This is the lowest vitrifying body we make, for dip-glazing you may need to bisque lower so it has enough absorbency. We normally bisque other bodies at 1850F, but for this we do it at 1750F.
Commercial brush-on glazes are made by many companies, they are typically formulated to fit high-talc bodies (like L213, L212). These bodies have high thermal expansions to reduce the likelihood of crazing. However, when glazes have to high thermal expansion, they can come under excessive compression and shiver off contours or press outward from the inside of ware, cracking it. We cannot guarantee that glazes made by another manufacturer will not craze or shiver. Do stress testing (in boiling water:ice water, the BWIW test). Regarding toxicity: Do not assume food safety of brightly colored glazes in your kiln without a leach test (e.g. GLLE test). Consider using a transparent or white liner glaze for food surfaces.
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).
When clear-glazing an important issue is glaze thickness. The mug on the left was double-dipped (so suspended bubbles are present in the handle recess, thumb-hold and along its edges). The glaze needs to be thick enough so that it feels glassy smooth but thin enough to avoid the bubbles. Normally, if applied the thickness of the one on the left, it would be completely milky, filled with micro-bubble clouds. Why has it not done so here? Because it is fired at cone 03 (using G2931K glaze and the C03DRH firing schedule). An added benefit is that the body is so much stronger than it would be if fired at cone 06 or 04. And the underglazes work fine.
Left: L215 bisque fired at 1750F, glazed with G2931K and glaze fired at cone 04. Right: Same but glazed fired at cone 03. The 03 glaze is more transparent (because it has fewer micro-bubbles suspended in the glass) and the body is much stronger. Cone 03 is also more tolerant of getting the glaze on too thick (it does not go cloudy).
For slip decoration and engobes be careful to match the fired shrinkage of the slip with the body. Where we do not recommend a specific engobe recipe use a one based on the porcelain itself. Add 2% VeeGum or Bentonite (the extra stickiness helps it adhere well to leather hard ware). Be careful about adding fluxes (e.g. frit), this increases fired shrinkage (the mismatch with body can cause flaking) and can compromise opacity.
The difference between a slip and an engobe. L3685U slurry was applied to the insides of these L215 mugs. But on the left it is a "slip", on the right an "engobe". Why? The left mug only has a thin layer (applied by painting a gummed version on at leather hard stage). On the right a gelled slurry was poured into the leather hard piece, poured out and the rim dipped (creating a much thicker layer with more power to impose its own drying and firing shrinkage). So it is much more important that the latter be compatible with the underlying body (flaking off at the rim is the first sign of poor fit). The EBCT test is used to measure how compatible the body and engobe are.
I ordered 12 colors of Amaco Velvet underglazes on Amazon. They arrived a few days later. Some needed a little water, some were a little thin, but in general they painted on well. I brushed on some L3685U white slip first (the velvet white would also work), then strokes of each of the colors. I bisque fired to 1750F (needed to achieve sufficient porosity) and then dipped each in G2931K clear glaze and fired to cone 03 (C03DRH schedule), 03 (1950F) is required to get crystal-clear results and durable ware. The colors are bright considering they have only been applied in a one-coat stroke.
Twelve Amaco underglazes on L215 with G2931K dipping clear fired at cone 03. Colored strokes are one coat.
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 are developing a casting version of L215 with matching engobe and glaze. Learn more about it here. It fires to a similar color and the recipe is flexible so you can adjust plasticity vs casting rate.
Starting in May 2018 we changed the recipe. Formerly it contained 10% pyrophyllite (Pyrax), we switched it to ball clay. The body has always contained talc, free from the expansion-reducing effect of the Pyrax, the talc will now bring the thermal expansion more in line with other common low fire bodies (and it will fit commercial glazes better). If your glazes were crazing this will reduce it. If they formerly fit this could introduce shivering.
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: 6.5-7.5% Dry Strength: Very high Water Content: 20.5-21.5% Drying Factor: C130
Sieve Analysis (Tyler mesh):
+48: 0.5-1.0% 48-65: 1.0-2.0 65-100: 0.5-1.5 100-150: 1.0-2.0
Cone 06: 12-14% Cone 04: 10-12 Cone 03: 7-8 Cone 02: 4-5 Cone 2: 2-3
Buffstone and L215 with underglaze decoration and G2931 weigh-and-mix-yourself clear dipping glazes. But they are not the same. The G2931L glaze (left) has a lower thermal expansion (to slow crazing on Buffstone). The G2931K glaze fits L215 because the latter contains some talc (which raises its expansion). The L215 mug has survived a 300F-to-ice-water thermal shock without crazing or shivering. If you switch the glazes the Buffstone mug would craze it and the L215 would shiver it off in flakes.
L215 with G1916Q+2% iron oxide (given the piece a redder hue), fired at cone 04-05 fired using the 04DSDH schedule. The piece is glazed on the bottom and fired on a three-pointed-stilt. Strength is excellent.
G1916Q + 2% iron oxide on L215 fired at cone 03, 05. Both were fired using the 04DSDH schedule. The cone 03 version has a deeper variegated color and the body is stronger, but there are tiny dimples on the surface, almost too small to see (because the glaze firing is significantly higher than bisque, decomposition in the body is generating bubbles). The cone 05 version, right, is like glass.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508