Print

H435

Description

High fire, moderately sandy, plastic, semi-vitreous, grey burning lightly speckled, general purpose native material body for reduction fired stoneware.

H435 has been manufactured for many years. It has about 15% fine sand and is quite high in silty stoneware clay; these are balanced by our native plastic stonewares and some of our high coal, super-plastic ball clay. H435 is semi-vitreous and has been used by people who want something a little finer than the very coarse H431. However, for functional applications, we recommend the much stronger (fired strength) and smoother H550.

Process Properties

H435 contains significant amounts silty clay and sand which give it a lower drying shrinkage and quite sandy texture during throwing. H435 contains a high coal ball clay which imparts a dark grey blackish color to the pugged. This ball clay significantly masks the feel of the sand during the throwing process.

The open structure of H435 also means that extra water can be absorbed during throwing making it more susceptible to water-splitting (the sand provides openings into which water can penetrate). Thus it is beneficial to 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). Since H435 generates significant amounts of slip during throwing, try to develop a technique where the slip produced during centering can be employed later as a lubricant during throwing.

Firing

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.

H435 fires to a variegated speckled stone-grey color at cone 10R. It has more speckle than H550. In oxidation firing there is some very fine specks and the body retains a stoney light grey color through the cone 9-11 range. Although H435 may appear stable at cone 11 sometimes we cannot guarantee successful results in this range.

Glazing

Although it has a low overall thermal expansion H435 is easy to adapt non-crazing glazes to. It contains plenty of high quartz plastic clays. Thus a glaze which crazes on a porcelain body will usually fit well on H435.

H435 is quite dense in the bisque state compared to porcelains, thus it does not absorb water as fast or evenly. This may lead to some pinholes after the glaze has dried but these are no cause for concern as they heal during normal firing. We suggest that you do not bisque this body any lower than cone 06 because it does contain some carbonaceous material that needs to be burned away before the glaze firing if possible.

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: 5.5-6.5%
 Dry Strength: n/a
 Water Content: 19.5-20.5%
 Drying Factor: c120-
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +48: 0-0.5%
   48-65: 1.5-3.5
  65-100: 5.0-10.0
 100-150: 3.0-6.0
 150-200: 7.0-11.0
 200-325: 8.0-12.0

Fired Shrinkage:

   Cone 8: 3.5-4.5
  Cone 10: 4.5-5.5
 Cone 10R: 5.0-6.0

Fired Absorption:

   Cone 8: 4.0-6.0%
  Cone 10: 3.0-4.0
 Cone 10R: 2.0-3.0

Chemical Analysis

 BaO       0.5
 CaO       0.3
 K2O       2.1
 MgO       0.5
 Na2O      0.1
 TiO2      0.7
 Al2O3    17.4
 P2O5      0.0
 SiO2     69.4
 Fe2O3     1.3
 MnO       0.0
 LOI       7.8%

Gallery

H435 mugs fired at cone 10R.

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