High temperature Grolleg porcelain. P700 is our most vitreous cone 10 white body, it is the closest thing we have to a true translucent porcelain body. It is a mix of 50% Grolleg kaolin with feldspar and silica. We also add micro-fine bentonite to improve its plasticity.
P700 is a smooth and slick fine grained body. We add more bentonite than is commonly done for Grolleg porcelains so this is very plastic (if not too soft)! Of course, this means you must take precautions when drying (dry ware evenly).
P700 fired bars. Cone 10R top. Cone 6-11 (bottom upward).
P700 vitrifies to a very pleasant silky surface and is translucent on thin pieces. Compared to our other porcelains it burns much cleaner (to a distinctly blue-white tone at cone 10 reduction firing compared to their off-white or ivory colors).
This is a zero-porosity porcelain so warping during firing can be an issue. Avoiding shapes that lack structural strength (i.e. straight sided cylinders, goblets with flared bases, severely overhung, flared or non-curved profile bowls). Be careful to use kiln shelves that are flat. Kiln wash is needed to prevent pieces from sticking to the shelf during firing.
Coleman Porcelain, P600 and P700 at cone 10R. P700 and Coleman have the same recipe except for one important difference: The former has 10% more feldspar and 10% less kaolin (meaning that cone 10R puts Coleman Porcelain well beyond the point at which it reaches zero porosity). It thus fires more translucent and a little whiter but is much more subject to warping and plucking. P700 is more plastic (so you can make thinner ware). P700 has a slightly higher drying shrinkage (so more care is needed during drying).
P700 employs only kaolin (rather than a kaolin:ball clay mix of most porcelains). Notwithstanding this, it fits most common mix-yourself glazes. However attention to fit is still important, even for commercial bottled glazes.
If you wish to use slips or engobes, make them from a base of P700 for the best possible drying shrinkage/fired shrinkage match.
P700 with pure Alberta Slip as a glaze. Fired at cone 10R.
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. Some recipes rely on high melt fluidity to encourage crystallization and variegation (often because of inadequate SiO2 and Al2O3 or containing Gerstley Borate 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.
Consider using our G1947U glossy or G2571A matte base recipes, just add colorants, opacifiers, variegators (you will find links to much more information and pictures about these). If you have a recipe that is troublesome, consider transplanting its opacifiers, colorants and variegators to these bases instead. http://ravenscrag.com and http://albertaslip.com also have many recipes that work well on porcelains.
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).
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: 5.0-5.5% Water Content: 23.5-24.5% Drying Factor: D542
+100: 0.0-0.1% 100-150: 0.1-0.3 150-200: 0.2-0.8 200-325: 3.0-5.0
Cone 8: 7.5-8.5% Cone 10: 8.0-9.0 Cone 10R: 8.5-9.5
Cone 8: 0.2-0.8% Cone 10: 0.0 Cone 10R: 0.0
CaO 0.2 K2O 3.2 MgO 0.3 Na2O 0.6 TiO2 0.0 Al2O3 23.9 SiO2 64.6 Fe2O3 0.5 LOI 6.6%
P700 cone 10R Mug by Tony Hansen. Alberta Slip (pure) outside, G1947U transparent inside.
H570 vs. P700. Clear glazed. Cone 10R.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508