Starting with code beginning "P6983" (March 2022) L215 has a new recipe. The change was to remove the 10% talc and commercial ball clay, replacing them with our 3D silty clay and BGP, a low temperature plastic terra cotta (we are removing talc from many other clay bodies also). You should not see an impact on glaze fit or fired color. The body remains highly plastic and smooth (but not as smooth as L210). It now fire more stable in the cone 2-4 range, producing a red stoneware.
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 surface after trimming a mug
While many people fire to cone 06, at that temperature bodies are weak and porous and commercial glazes don't really melt well enough. Cone 04 is more dense and considerably stronger. By cone 02 L215 is almost a stoneware, however it also generates more gases and some glazes will bubble. Cone 04 is thus the best compromise (with cone 03 being the practical maximum). Low fire glazes melt and bond better the higher you can fire.
L215 Fired Bars (cone 4, 2, 02, 04, 06 top to bottom)
For dip-glazing adjust bisque temperature to have an absorbency needed. We normally bisque other bodies at 1850F, but for this, we do it at 1750F.
Since L215 burns red brightly-colored glazes will be somewhat muted. If you are seeking the classic terra cotta red appearance through a transparent glaze, the glaze will need to be applied as thinly as possible (to avoid clouding). Not all commercial transparent glazes are created equal, it may be better to try ones from different vendors to get the best results.
Although cannot guarantee that commercial glazes will not craze or shiver we do test multiple types for fit. 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.
Although commercial bottled products are expensive, they are convenient. It is practical to make your own dipping or brushing base glazes that dry faster and fit better (do not craze or shiver). Adding powdered stains is also possible. There is lots of documentation on how to make and mix our G1916Q and G3879 and recipes. Each of them has variations to enable tuning fit any body. It is possible to mix them as a brushing glaze, base coat or dipping 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.
Spectrum 700 clear, 301 white (which is shivering), 743 red and 753 yellow on L210. Since L210 fires lighter than the traditional terra cotta red, bright-colored glazes work well on it. All of these work better on L210.
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 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: 6.5-7.5% 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 04: 0.75-1.25% Cone 02: 4-5 Cone 2: 5-6
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 mixed as a dipping glaze and applied on L215. These were fired at cone 03, 05 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