H450 is suitable for many types of functional and decorative stoneware. Unlike our other high temperature buff stonewares, this material contains no fine sand, it is completely smooth. Where ever possible H450 is preferable H550, H435 and other more expensive white stonewares or porcelains.
H450 has the dry strength and working character of a classic Plainsman native body and will stand up to product handling during manufacture very well. Even though H450 is smooth to the touch it still has a good distribution of particle sizes in the plus 325 mesh range (it draws from the illite, ball clay and kaolinite mineral families). As with any Plainsman native body, H450 has a higher dry shrinkage to plasticity ratio than most refined bodies and anyone who is used to using our bodies is aware of the care and attention to drying needed in making larger pieces, especially flat plates and shallow bowls.
These fired test bars compare H450 (left) with H550. They have been fired to cone 10R (top) and 10, 9 and 8 oxidation.
H450 has a higher porosity that H550 because it utilizes a larger amount of Idaho kaolin than any other body we make (it also contains some fine refined silica powder). Buff bodies made from Plainsman native materials exhibit attractive grey-to-buff variegations at a porosity threshold around 2% (H550 normally fires a solid grey at about 1.6-1.8% porosity at cone 10R). Thus H450 does not fire quite as strong as H550, but for many it is a better compromise between fired maturity, resistance to fired warping, aesthetics and working properties.
H450 is the cleanest burning native material we make. In oxidation H450 burns to a much lighter grey buff than it does in reduction. It will display color variegation in most reduction kilns fired below cone 11. The nature of the variegation relates to radiation of vitreous darker grey color from points where fluxing begins. As firing proceeds these darker grey areas invade the entire buff-colored surface.
Since H450 contains some free quartz, some care is required when heating and cooling it through quartz inversion temperatures, especially if ware is being refired.
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). 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).
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 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: 6.3-6.8% Water Content: 22.0-23.5% Drying Factor: C120
+48: tr 48-65: tr 65-100: 0.5-1.5 100-150: 1.0-2.0 150-200: 5.0-7.0 200-325: 7.0-9.0
Cone 8: 5.5-6.5% Cone 10: 6.5-7.0 Cone 10R: 6.0-7.0
Cone 8: 3.0-4.0% Cone 10: 1.5-2.5 Cone 10R: 2.0-2.5
BaO 0.3 CaO 0.2 K2O 1.5 MgO 0.4 Na2O 0.1 TiO2 0.7 Al2O3 19.0 P2O5 0.0 SiO2 68.2 Fe2O3 1.2 MnO 0.0 LOI 8.4%
Cone 10R mugs. Plainsman H550. The liner glaze is pure Ravenscrag Slip GR10-A and the outside glaze is a 50:50 mix of Ravenscrag and Alberta Slips (GR10-E). By Tony Hansen.
Plainsman H450 (buff stoneware) mugs fired at cone 10R with pure Alberta Slip on the outsides, G1947U transparent (left) and pure Ravenscrag Slip liner glaze. By Tony Hansen.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508