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H443

Description

High temperature, slightly textured, plastic, semi-vitreous, iron-brown speckled, general purpose, native body for dark iron reduction fired stoneware.

H443 is a classic mottled-brown reduction speckled stoneware. Unlike H440, it generally fires with no red hues. H443 is not a vitreous body, it stops short of complete maturity to retain a variegated earthy color. It is best suited for decorative pieces large or small (vases, bowls, planters, etc.). It is not suitable for functional ware.

Process Properties

H443 is a reddish brown in pugged form and made totally from a blend of Plainsman native stoneware materials. It has good plasticity (exellent if it is stiffer) and exhibits only a slight sandy texture during throwing (by virtue of kaolinized sand in its recipe). H443 has an excellent distribution of clay particle sizes in the plus 325 mesh range and these are a factor in its fast drying and high dry density and strength. However, even though H443 dries well it does have a fairly high drying shrinkage and thus care and attention are necessary to be sure larger pieces, especially flat plates, shallow bowls, and sculptural ware are dried evenly.

Firing

These test bars (left to to right) compare H440 and H443 at cone 10R (top) and cone 10, 8 and 8 oxidation.

H443, like other iron reduction bodies, depends on stopping short of being vitrified to achieve the characteristic warm brown coloration. H443 has been formulated so that cone 10R is at its transition point from a toasty brown to a dark brown. When fired just right the surface is a patchy network of darker brown vitrified areas beginning to invade the lighter colored matrix. If over-fired or over-reduced the surface will burn a dark solid brown. If you wish to fire at a lower reduction temperatures, good color is possible as low as cone 8.

H443 contains some iron stone concretion particles that melt vigorously in high temperature reduction and blossom on the bare clay or bleed up through glazes. The degree to which these speckles melt and develop is dependent on the amount of reduction imposed.

H443 can be fired in oxidation, but its color is dramatically lighter (a leather to greenish brown from cone 6 to 10) and the speckle is much finer. If overfired H443 will tend to blister and bloat sooner in oxidation because the coarser particles in the clay are much more active in producing gaseous by-products during decomposition.

H443 and H440 are not the right bodies to be using for functional ware. It is a decorative iron stoneware for heavy visual pieces. Even though these glazes fit, the mug on the left completely shattered with just the light tap of a hammer.

Glazing

Since H443 is a dark stoneware it readily bleeds its iron into glazes and colors and significantly colors them. This will especially be the case in reduction firing, brightly colored glazes will not be as vivid as they would be if used on porcelain. On the other hand, this effect will enhance the appearance of earthtone and variegated glazes.

H440 has a reasonably high porosity so the fired body will tend to absorb water. Thus it is important that glazes not be crazed to assure that ware is water tight and will not be susceptible to moisture expansion. Strength can be significantly impacted if glazes do not fit properly (use an ice water:boiling water immersion test to make sure).

Glaze Recipes

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).

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: 6.0-6.8%
 Dry Strength: n/a
 Water Content: 20.5-21.5%
 Drying Factor: C120
 LOI: 6.5-7.5
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +48: 0.0-0.5%
   48-65: 1.0-3.0
  65-100: 5.0-7.0
 100-150: 3.0-5.0
 150-200: 4.0-7.0
 200-325: 7.0-10.0

Fired Shrinkage:

   Cone 8: 5.2-6.2%
  Cone 10: 5.5-6.5
 Cone 10R: 5.5-6.5

Fired Absorption:

   Cone 8: 4.5-6.0%
  Cone 10: 3.0-4.0
 Cone 10R: 2.5-3.5

Chemical Analysis

 BaO       0.5
 CaO       0.4
 K2O       1.6
 MgO       0.5
 Na2O      0.2
 TiO2      0.8
 Al2O3    18.9
 P2O5      0.0
 SiO2     66.1
 Fe2O3     2.5
 MnO       0.0
 LOI       8.6%

Gallery

A cone 10R planter made from H443. In this firing it has a reddish tone (the color varies according to reduction). By Tony Hansen.

Safety Data Sheet

Click here for web view.

Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net