H435 is a legacy body. It has been replaced by other better ones (H550, H450 which are smoother, more vitreous and lower in fired speckle) but some people still like it for its coarse and more earthy nature (it has about 15% fine sand and is quite high in silty stoneware clay). H435 is not vitreous.
H435 feels quite sandy, especially during throwing. It contains a high-coal ball clay which imparts a dark grey color to the pugged material. Water can be absorbed during throwing making it 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 of 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.
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.
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 pinholes forming as the glaze dries. We not bisquing this body any lower than cone 06 (because of the carbonaceous material that needs to be burned away).
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 a transparent or white liner glaze 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). Even though ware may not be crazed out-of-the kiln it may 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).
Thixotropy: Many people mix their glazes the traditional way, just adding water until the slurry appears to be the right viscosity for dipping. However, if you want better application properties for one-coat dipping, consider creating a thixotropic slurry. Thixotropic glazes are creamy because they have been thinned and then gelled by the addition of a flocculant. They go on evenly, hang on without dripping and dry quickly. Achieving (and maintaining) this state involves targeting a specific gravity (usually around 1.43) and adding epsom salts (1-2g/1000g of powdered glaze).
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 one 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 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 thermal expansion values. If a chart is supplied here, please view it only as a way to compare one body with another. Please note that, although you may calculate the thermal expansion of a glaze, this cannot be done for clay bodies since they do not melt. The best way to fit glazes to clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down, fire a glazed piece and thermal stress it (using an IWCT test, 300F into ice-water). If it still crazes, repeat the process.
Drying Shrinkage: 5.5-6.5% Dry Strength: n/a Water Content: 19.5-20.5% Drying Factor: c120- Dry Density: n/a
+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
Cone 8: 3.5-4.5 Cone 10: 4.5-5.5 Cone 10R: 5.0-6.0
Cone 8: 5.5-6.5% Cone 10: 4.0-5.0 Cone 10R: 4.0-5.0
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508