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.
H570 fires whiter and throws better than other popular whiteware bodies manufactured in North America. It also generates less slip on throwing yet the surface lubricates well with water (there is less mud to clean up after use). Another key advantage is drying performance. Compared to a high ball clay body, H570 dries better and its dry strength is better. The reason for these behaviors is that it uses more kaolin and less ball clay than competing products (their ratio creates good particle packing).
Common sense drying should prove successful using H570. Dry ware evenly, avoid letting any part of a piece become drier than another at any stage of the drying process.
H570 fired bars. Cone 10R top. Cone 8 to 11 oxidation (bottom upward).
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.
We recommend developing a base glossy and matte glaze that fit H570 well (do not craze or shiver) in your circumstances and which suspend and apply well (consider visiting http://digitalfire.com/4sight/recipes and trying 1947U glossy and 2571A matte bases). Avoid high-feldspar glazes (having more than about 35%), they often craze. From there you can add opacifiers, variegators, speckle and colors. Often it is helpful to identify the additions in other recipes and transplant them into your base. Some recipes rely on high melt fluidity to encourage crystalization and variegation, view these with suspicion and test them well (also test the additionless versions). Consider getting an account at http://insight-live.com so you can organize a methodical development program and adopt better methods of testing (e.g. melt fluidity, thermal stress, slip-fit tests). Finally, learn how to adjust the glaze slurry to the best specific gravity and then properly gel it, this will enable you to glaze ware more evenly and with much less problems. A number of the recipes at http://albertaslip.com and http://ravenscrag.com work well on H570.
For slip decoration, be careful. Slips must be drying, firing, thermal expansion and quartz inversion compatible with the body (glazes only need thermal expansion compatibility) or failure will occur at the slip-body bond. First make sure that the glaze fits (does not craze) on the body (without slip). Then match the drying of the slip and body. Usually the slip needs to be sticky to adhere well, so it needs lots of plastic clay (some slips have up to 10% bentonite for this reason). It will thus have a higher drying shrinkage than the body, this is often tolerable because the majority of the shrinkage occurs early. Mix your slip and dewater it to pugged consistency. Roll it and the body into thin 1/8" slabs, then lay one on top of the other and roll that down to 1/8" to create a bi-clay sandwich. Cut 1/2" wide by 6" long bi-clay bars and dry them watching how they curl. Expect some curve from straight (but not too much). Then fire these bars. If there is a drastic change you have a misfit in firing shrinkage which sets up tension. If it curves toward the slip this is best because the compression of the glaze will push it back to equilibrium. If they curve too much during firing, adjust the flux content of the slip and redo the test. Matching thermal expansion is generally a matter of making sure there is adequate silica in the slip recipe, usally around 20% is appropriate. A mismatch will become evident in heat-shock testing. Likewise, quartz inversion compatibility is best when the slip and body have a similar silica content.
The chart shown was produced from a specimen fired once to cone 10 reduction in the Plainsman lab and tested in an Orton dilatometer. If you fire to a different temperature, employ different heatup or cooldown rates, or glaze-fire more than once the thermal expansion in your ware may be different than this chart indicates. This chart is intended for comparison with those of other bodies more than as a statement of an absolute behavior. Remember also that calculated thermal expansions cannot be done on bodies (because they do not melt) and for glazes they are always relative, meant to be compared one with another within the same chemistry system. Also, note that the measured value shown here is an average extrapolated from a long curve that is not linear.
Thermal Expansion Chart. Average: 5.7.
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
Sieve Analysis (Tyler mesh):
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
CaO 1.1 K2O 1.3 KNaO 0.1 MgO 0.3 Na2O 1.7 TiO2 0.6 Al2O3 23.9 SiO2 62.4 Fe2O3 0.5 LOI 8.2%
Safety Data SheetClick here for web view.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508