High fire, sandy, medium plastic, semi-vitreous, grey burning speckled, native body for reduction fired stoneware.
H431 is the body of choice for people looking for a textured light burning body with a more natural earthy speckled fired appearance and reduction speckling. However, if you use H431 we recommend that you consider carefully its limitations as outlined below. For functional applications, H550 is a better choice.
H431 is a balance of high amounts of very plastic ball clays and bentonites with a very significant amount of sand and silty clay to cut plasticity. H431 thus has a very sandy feel during throwing. This sand generates more than the usual amount of slip with the associated loss of fine particles from the surface. The open structure of H431 also means that extra water can be absorbed during throwing so it will not stand up as well to over-working on the wheel (as is normal with students).
The sand and silt content in H431 also means that it is more susceptible to water-splitting during throwing or hand building. The abundance of larger particles provides openings into which water can penetrate and split the soft clay. Thus it is very important that you use water sparingly during throwing and make sure that it does not stay long on points where the clay is under stress (i.e. the outside of the belly on a vase). With H431 it is practical to develop modified handle pulling techniques and use slip rather than water during throwing. In addition it is wise to immediately sponge away excess slip squeezed out of joins (often such splits are misinterpreted as drying cracks).
These fired test bars (left to to right) compare H431, H550 and H435 at cone 10R (top) and cone 11, 10, 9 and 8 oxidation.
The most striking effect that this body will have on the fired glaze is the formation reduction speckle. H431 contains significant amounts of iron stone concretion particles which melt vigorously in high temperature reduction and blossom on the bare surface or bleed up through glazes (it has the most speckle of any of our non-brown burning bodies). Other than the degree of specking, the appearance of the fired body in high temperature oxidation and reduction is quite similar.
Since H431 contains bentonite and highly plastic ball clay it cannot be bisque or once fired as fast as other bodies or cracking and disintegration can occur. It is thus important to fire slowly through the 100-200C range where water turns to steam and needs to vent out of the clay (it is best to pre-heat your kiln overnight on low).
If you must refire any ware to fix imperfections, remember that due to the high sand content, fired H431 ware requires slower heating and cooling through quartz inversion temperatures.
H431 bisque ware should be fired to about cone 06 and even though it is dense it is still quite absorbent and takes glaze fairly well. The open structure of the bisque means that glazes will tend to pinhole during drying as water-displaced air escapes from minute surface holes.
H431 will not perform well in thermal shock situations (i.e. teapots) if the glaze does not fit well. This body has a fairly high fired strength which can be significantly impacted if ware has a glaze which does not fit properly. Do not judge your glaze as fitting because there are no visible signs of trouble when the ware is removed from the kiln, use an ice water:boiling water immersion test to make sure.
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: 6.0-7.0% Dry Strength: n/a Water Content: 19.0-20.5% Drying Factor: C120-C130
Sieve Analysis (Tyler mesh):
+48: 0-0.2% 48-65: 0.0-0.5 65-100: 3.0-5.0 100-150: 8.0-12.0 150-200: 4.0-7.0 200-325: 6.0-10.0
Cone 8: 4.0-5.0% Cone 10: 4.5-5.5 Cone 10R: 4.5-5.5
Cone 8: 4.5-6.5% Cone 10: 2.5-3.5 Cone 10R: 1.5-2.5
A cone 10R vase made from H431. Tony Hansen.
Safety Data SheetClick here for web view.
We Are Rationalizing Our Product Line
Plainsman manufactures bodies by grinding and pugging clays that we mine (native bodies) and by batch mixing bagged minerals and materials that we import (refined bodies). We stock about 10,000 boxes of 50+ clays (some in multiple stiffnesses) and need to reduce the warehousing and production burden of small-run bodies and remove obsolete and legacy products. For bodies being discontinued: We have migration paths and can assist with issues. Some changes involve increased cost. In certain cases you might consider having us custom-mix a body so you can continue to get it, but please work with us on trying to adapt to alternatives first.
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508