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L215

Description

Low temperature, smooth, highly plastic, red-burning, talc-containing terra cotta body. L215 is made from Plainsman M2, a natural and very clean red burning clay that we mine in Montana.

Process Properties

Starting in May 2018 we have changed the recipe. Formerly it contained 10% pyrophyllite (Pyrax) (in addition to 10% talc). We switching the pyrophyllite to ball clay. This slightly reduces crazing of glazes and greatly increases plasticity. It feels very smooth and you will be able to make thinner and lighter ware than ever before. It will dry very hard. This change slows the drying (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

Firing

L215 Fired Bars (cone 3, 2.5, 2, 03, 03.5, 04 top to bottom)

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).

If you are dip-glazing it is important to bisque lower, 1750F, otherwise it will not absorb water fast enough to make the process efficient.

To get the best defect-free surface please consider using a hold-rise-drop-hold firing schedule. For 04-03 consider using the C03DRH schedule.

Glazing

L215 contains 10% talc, this increases thermal expansion making it fit common glazes better (less chance of crazing).

Commercial brush-on glazes are formulated to fit high-talc bodies (like L213, L212). Talc bodies have high thermal expansion, this puts the squeeze on the glazes (reducing crazing). But this can go too far, glazes can be under excessive compression and shiver off contours or press outward from the inside of ware, cracking it (especially if ware is unglazed or engobed on the outside). We cannot guarantee that any glaze made by another manufacturer will not craze or shiver. Do stress testing (in boiling water:ice water, the BWIW test). Do not assume food safety of brightly colored glazes in your kiln without a leach test (e.g. GLLE test). Consider using transparent or white liner glazes for food surfaces.

Many find that commercial brushing and dipping glazes dry very slowly and drip badly. And they often do not fit well (craze or shiver). And they are expensive. With a little effort you can make your own of both types. And add stain and opacifier powders to make almost any color. There is no need to search for a recipe, we have done it for you. You can mix your own glaze from a pre-mixed powder you buy from us or weigh out your own ingredients. However, at low temperature, there is no one-glaze-that-fits-all bodies. To solve this we have created a three-recipe solution, these have thermal expansions that span the entire range of low fire bodies. And they fire crystal clear (in a drop-and-hold firing). Our Zero3 K transparent glaze fits low talc bodies (like L215) and low fire stoneware (like Zero3), Zero3 H fits high talc bodies (like L213, L212) and Zero3 L fits zero talc bodies (like Buffstone). We recommend cone 04-03 using a drop-and-hold firing schedule (for defect free surface and good transparency). Do not trust electronic controllers to be accurate, we verify using cones and manually program to compensate for error. If you need to fire lower or higher than 03, be sure and do testing first.

You can blend the K, H and L glazes to get the best fit. We recommend stress testing by boiling-water-into-ice-water (and vice versa) to bring out any crazing or shivering. If crazing occurs, blend in some of the L. If shivering occurs, blend in some of the H. In this way these glazes can be optimally fitted to almost any clay body.

It is also practical to make your own brushing glazes, ones that will work identically to the commercial ones mentioned above. Like the dipping versions, you can blend to tune the thermal expansion to fit your clay body. You will be surprised at how easy it is and how much money you can save.

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).

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).

Glaze slurry consistency and quality: A secret to achieving even glaze coverage is controlling the thixotropy and specific gravity of the slurry, both in freshly mixed and stored batches. A glaze of the right specific gravity and having a slightly gelled condition goes on to bisque ware evenly, does not drip and dries in seconds. Always screen glazes when first making them (80 mesh). Be alert to any particulate that may appear after storage (e.g. precipitates) and screen again if needed.

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 formula 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 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).

Thermal Expansion

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.

Physical Properties

 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

Fired Shrinkage:

 Cone 04: 0.75-1.25%
 Cone 02: 4-5
  Cone 2: 5-6

Fired Absorption:

 Cone 06: 12-14%
 Cone 04: 10-12
 Cone 03: 7-8
 Cone 02: 4-5
  Cone 2: 2-3

Gallery

L215 with underglaze decoration and G2931K clear glaze. Fired at cone 03 in 3 hours cold-to-cold.

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.

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: plainsman@telus.net