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High temperature, slightly sandy, plastic, semi-vitreous, grey-buff burning, general purpose, native body for reduction and
oxidation fired functional stoneware.
H550 is widely used by production potters and small companies for all types of functional and decorative stoneware. For many years
H550 has been a standard among Plainsman customers for cone 10 reduction functional ware. We maintain the porosity at about
1.5-1.8% for cone 10R, this is the lowest safe level that avoids bloating. See H450 for a smoother, lighter burning, but not quite
as vitreous material.
H550 has a fairly plastic but slightly sandy texture and generates plenty of slip during throwing. H550 has a good distribution of
particle sizes in the entire plus 200 mesh range (it draws from the illite, ball clay and kaolinite mineral families) to give
relatively fast drying and high green strength. However, since H550 has a fairly high dry shrinkage, care and attention in drying
are still necessary in making larger pieces, especially flat plates and shallow bowls. If you want a smoother, more plastic body,
please use H450 instead.
These test bars (left to to right) compare H431,
H435 and H550 at cone 10R (top) and cone 10, 8 and 8 oxidation.
H550 is fairly vitreous at cone 10R, often close to 1% porosity (this is the most mature of our bodies made completely from our
materials). H450 is slightly less mature and a little more stable in the kiln, it is also a little cleaner and lighter burning.
In oxidation H550 burns to a much lighter grey buff than it does in reduction. It generally fires solid grey in reduction kilns
(cone 10R is past its transition point from reduction light-buff to grey buff). Cone 10R finds H450, on the other hand, at the
transition point, so it has more color variegation (where the darker vitrifying grey areas invade the buff-colored surface).
Since H550 contains some free silica, some care is required when heating and cooling it through quartz inversion temperatures,
especially if ware is being refired.
This mug is made from H550 and glazed with Ravenscrag Bamboo
(outside) and GR10-C Ravenscrag Talc Matte (see the Cone 10 glazes page at ravenscrag.com).
Since H550 contains more than 70% SiO2 you can expect it to accept most typical cone 10 glazes without crazing them, glazes which
craze on porcelain bodies will often fit well on H550. It is still possible to get crazing with glazes high in sodium and
potassium or very low in silica or alumina. Thus you should watch out for high feldspar low flint/kaolin glazes (these are quite
common). We recommend a boiling water:ice water immersion test to make sure glazes fit well. Please contact Plainsman if you need
help to adjust your glazes.
You can develop a compatible glossy or matte base for this body from our suggested starting point base recipes available on our
Internet web site at http://digitalfire.com/education/glaze/cone10.htm. Information is given on how to fit the glaze to your body
and how to customize it it for colors, opacity, speck, variegation, etc. For slip decoration, be careful to match drying and fired
shrinkage of the slip with the body since low temperatures generate little glass to adhere the slip.
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.
Drying Shrinkage: 6.0-7.0%
Water Content: 20.5-21.5%
Drying Factor: C120
Dry Density: 2.0
Sieve Analysis (Tyler mesh):
Cone 8: 4.0-5.0%
Cone 10: 4.5-5.5
Cone 10R: 5.0-6.0
Cone 8: 3.5-5.0%
Cone 10: 2.5-3.5
Cone 10R: 1.5-2.5
We continue to monitor this body closely for maturity. We want it to be as vitreous as possible but also to have a margin against
bloating. In 2006 we added H450 to our line of bodies, it is smoother and a little less vitreous that H550.
||Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX: