A high temperature, smooth, plastic, semi-vitreous, white to grey-white burning, ball clay:kaolin body for oxidation and reduction fired functional stoneware. H570 is less white and less vitreous than our other other porcelains. But it is better drying, less prone to warping in the kiln, easier to fit glazes to and potentially more consistent (because it utilizes three clays to minimize effects of individual changes). The combination of working and firing properties make it a very good choice for functional ware for the majority of people.
B-Mix cone 10.
H570 exists to compete with other highly plastic whiteware bodies made in North America. It is highly plastic and throws very well (this is because it incorporates plastic #6 Tile kaolin, a ball clay and significant bentonite (4%) in the recipe). Another benefit of this is that it generates less slip on throwing yet the surface lubricates well with water.
Drying: Porcelains do not dry as well as stonewares or bodies with particulates. You will get the best results if the clay is not too soft, ware is not too thick, contours are smooth, wall thicknesses are even, joins are few and done with thick slip, the degree-of-wetness in all parts of a piece is kept equal throughout all stages of drying. Large pieces are best made on plaster bats so the bottom can stiffen with the walls. The worst drying performance will occur with thick ware made from very soft clay, the use of non-absorbent bats, where vessel walls are thick at the bottom and thin at the lips or edges, walls are of uneven thickness with lots of joins or abrupt angles (giving cracks a place to start) and where drying is uneven (e.g. lips and edges are permitted to stiffen early on while lower sections remain soft). Large, flat plates are the most difficult shapes to dry, it may be necessary to stretch the time out to a month or more to achieve the even drying needed for success.
Although H570 does burn white and speck-free, it does not reach zero-porosity at cone 10. However it does fire more dense in reduction than in oxidation. H570 has a fired surface and strength more similar to stoneware than porcelain and is thus better suited to firing conditions that are not totally consistent. Compared to vitreous stoneware or porcelain, lids will display less tendency to stick to the lips of containers during firing and overfired ware (or extreme shapes) will be less likely to warp, bloat, or glue themselves to shelves.
However there is an important caution: H570 is quite high in free quartz. This is an asset to achieving glaze fit but also means that you should not cool the kiln through quartz inversion temperatures (e.g. 1000-1050F) too quickly or dunting cracks could occur (especially in large bowls and plates). You will note from the expansion curve (smooth at 220C 428F) that H570 does not appear to form significant cristobalite despite the fact that it is semi-vitreous. Remember also that cooling your kiln too quickly through any stage may set up temperature gradients within pieces that will continue even if you attempt to slow-cool during certain ranges.
H570 will preserve bright glaze colors without the iron-bleeding problems associated with stonewares. However, if you wish to use glazes with earthtone shades, consider one of our darker burning materials. As noted, H570 is high in raw quartz (25% addition plus that in the 10% ball clay) and it is not vitreous. That means the quartz can impose the higher thermal expansion needed to prevent crazing in glazes (compared to our P600 and P700). However some competing popular whitewares can have 50%+ ball clay plus a 25% quartz addition. These formulations, although undesirable for various reasons, do enable people to employ very high feldspar recipes without crazing. If you are switching from one of these consider trying Plainsman P580 or contact us for help adjusting or adopting more balanced glaze recipes.
H570 has some porosity so water penetration is demonstrated. Make sure your glazes are fitted to the clay by stress-testing ware (using the boiling water:ice water method) to bring out delayed crazing. You can find base matte and glossy glazes on our website.
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: 6.0-7.0% Dry Strength: n/a Water Content: 22.0-23.0 Drying Factor: C120 LOI: 5.0-6.0% Dry Density: n/a
65-100: 0.1-0.3 100-150: 0.1-0.4 150-200: 1.0-2.0
Cone 8: 6.0-7.0% Cone 10: 6.5-7.5 Cone 10R: 6.5-7.5
Cone 8: 2.0-3.0% Cone 10: 1.0-2.0 Cone 10R: 0.3-0.8
The decline of the ceramic industry in North America has impacted the price, availability and quality-for-ceramics of raw materials from which porcelains are made (especially ball clays and kaolins, which have seen increases in soluble salts, foreign particles and iron specks). In addition, we do not have filter-pressing, pre-mixing and stainless-steel pugging equipment (these would drastically increase prices). While you cannot make Wedgewood-quality ware it will be far whiter and cleaner than our stonewares. Notwithstanding that, be aware that transparent glazes carry a chance of isolated specks using our standard porcelains, it is better to use white and colored glazes on these. If you absolutely need a clean, white, translucent porcelain consider using our premium products, Polar Ice and P700 (they cost more but you get much more). We are also developing engobes for low, middle and high temperature ranges, using these you can apply a porcelain-like surface of almost any color at leather hard stage and completely hide the underlying stoneware. These engobes are so opaque that a white one can completely mask a black body underneath using only one coat. So, if you can master their use (there is lots of documentation here) many new design opportunities will offer themselves.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508