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L212

Description

Low temperature, smooth, medium plastic, light cream to grey buff burning, refined talc earthenware. L212 is a mix of our cleanest native stoneware clays with 33% added talc. It is intended for good fit with commercial bottled glazes (i.e. Duncan, Mayco, Spectrum) and is a popular body for use in schools.

Process Properties

We have added a little ball clay and bentonite to improve and control the plasticity of L212 while it still retains the unique working properties and dry strength common to bodies in its family (H550, M340, Buffstone). Like classic ball clay:talc ceramic mixtures, this body has a unique and very pleasant smooth feel for throwing and modelling forms of all shapes and sizes.

L212 dries fairly quickly with minimal cracking. However it is fine grained so joins should be made using low water content slip and applying as much pressure and lateral movement as possible.

Firing

L212 bars fired at cone 04, 02, 2, 4 and 6 (bottom to top).

Although this body does not burn as white as L213 it is less expensive for us to manufacture. It fires best at cone 06-04. The color darkens to a yellow buff by cone 2, then to a grey buff by cone 4 and above. If you want to produce denser stronger ware you might consider firing L212 to cone 02 or higher. However we recommend you do this with caution because bodies of this type tend to melt suddenly. Although L212 might appear OK at cone 4, ware tends to be brittle at this temperature. Also, the volatility is not consistent; that is, L213 will melt at different temperatures for different runs.

Since L212 contains a significant amount of talc, it has a unique firing behavior. The talc significantly increases the thermal expansion of the ware. This is a disadvantage if ware will be used for functional purposes since it will not have as good thermal shock resistance. However the advantage is that contraction during cooling squeezes the glaze onto the ware and helps prevents it from crazing.

L212 is not made from white burning ball clay and talc as are typical ceramic slips. Thus it does not burn nearly as white.

It is important to realize that talc bodies of this type do not vitrify at cone 06-04. A typical thinly cast or thrown shape can often be torn apart with your bare hands.

Since L212 contains bentonite you must be careful not to fire too fast during early stages. Ball clay is very plastic and fine and thus does not vent water vapor quickly during the water smoking period of firing. We recommend that you candle the kiln overnight to give the ware a chance to dry thoroughly before starting the firing the next morning.

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

L212 underglaze decorated mug fired at cone 03. Outside glaze is G2931H clear. Inside is Crysanthos SG001 white.

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).

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.

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).

Buffstone, L212 and L213. Each has its own clear glaze. G2931L, G2931K and G2931H. L is low expansion (for zero-talc porous bodies like Buffstone). K is intermediate (for moderate-talc bodies like L215, Zero3 stoneware and porcelain). H is high-expansion (for higher-talc bodies like L212, L213).

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: 6.0-7.0%
 Dry Strength: n/a
 Water Content: 21.5-23.0%
 Drying Factor: C120
 Dry Density: 1.96

Sieve Analysis (Tyler mesh):

     +65: 0.1-0.5%
  65-100: 0.5-1.5
 100-150: 1.0-2.0
 150-200: 1.0-2.0
 200-325: 6.0-10.0

Fired Shrinkage:

 Cone 04: 1.5-2.5%
 Cone 02: 3.0-4.0
  Cone 2: 4.0-5.0

Fired Absorption:

 Cone 04: 11.0-14.0%
 Cone 02: 9.0-12.0
  Cone 2: 5.0-7.0

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