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H555

Description

High temperature, smooth, functional, medium plastic, vitreous, light grey-buff burning body made from a mix of refined and clean native clays for reduction and oxidation fired functional stoneware. H555 is a 50:50 blend of our native materials (like those used in H550) and of refined industrial minerals (like ball clay, kaolin, feldspar, silica). It thus features some of the robust drying and working properties of the former while also displaying the more refined and vitreous fired character of the latter.

Process Properties

H555 is one of Plainsman's most pleasant throwing bodies. Although smooth and fine, it is not as slick as bodies made entirely of refined materials. It dries relatively fast and has very high green strength.

H555 has a lower drying shrinkage and thus it dries with less cracking. However, since it is fine grained, extra care and attention in drying are rewarded when making larger pieces, especially flat plates and shallow bowls (i.e. use slip containing an aggregate like molochite, focus drying on evenness rather than speed, use as much pressure and lateral movement as possible when joining, make ware with an even cross section, etc.).

Firing

H555 fired test bars. Cone 10R top. Cone 8 to 11 oxidation (bottom upward).

H555 fires to a pleasant light grey-buff in reduction and buff-white in oxidation. It is semi-vitreous (burns to about 1% porosity at cone 10). It burns significantly whiter than H550 (our buff burning native stoneware) and the speckle population and size is much lower. By cone 7 porosity is about 4% so we do not recommend this body for middle temperature functional ware.

H555's high fired strength and homogeneous surface makes it an excellent compromise for fine functional stoneware. But remember that while it is more resistant to warping than our porcelains it is more prone to deforming on overhung or extreme shapes than our native stonewares.

Glazing

H555 does not bleed iron or interfere with glaze melts and thus encourages clean results. However if you use earthtone glazes that are at their best on iron or speckled bodies, consider trying H550 or H443. If you use white or transparent glazes there will be some scattered small back speckles, especially if the glaze cover is thin.

H555 is easier to fit glazes to than our porcelains but can require a little more effort than our stoneware bodies. Crazing is likely with glazes high in sodium and potassium or very low in silica or alumina. Thus you should watch out for high feldspar low silica/kaolin glazes (these are quite common). High fired strength is one of the important features of this body and this can be severely impacted by a glaze which is under excessive compression or tension. Use a boiling water:ice water immersion test to make sure your glazes fit well. Please contact Plainsman if you need help to adjust your glazes.

H555 with Ravenscrag celadon glaze. Fired at cone 10R.

Glaze Recipes

Consider using our standard G2571A matte and G2947U glossy base glazes as starting points. Information is given on adding colorants, opacifiers and variegators to make any effect you want. You will also find excellent recipes made from Ravenscrag Slip and Alberta Slip (on their respective websites, Ravenscrag.com and AlbertaSlip.com). For slip decoration, be careful to match drying and fired shrinkage of the slip with the body.

Thermal Expansion

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 (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.

Thermal Expansion Chart. Average: 5.7.

Physical Properties

 Drying Shrinkage: 5.5-6.5%
 Dry Strength: n/a
 Water Content: 21.5-22.5%
 Drying Factor: C120
 Dry Density: n/a

Sieve Analysis (Tyler mesh):

     +48: 0.0-0.1%
   48-65: 0.0-0.2
  65-100: 0.0-0.3
 100-150: 0.1-0.5
 150-200: 1.5-2.5
 200-325: 5.0-9.0

Fired Shrinkage:

   Cone 8: 6.0-7.0%
  Cone 10: 6.5-7.5
 Cone 10R: 7.0-8.0

Fired Absorption:

   Cone 8: 1.0-2.0%
  Cone 10: 0.5-1.5
 Cone 10R: 0.5-1.5

Chemical Analysis

 CaO       0.6
 K2O       2.0
 MgO       0.5
 Na2O      0.3
 TiO2      0.7
 Al2O3    20.9
 P2O5      0.0
 SiO2     65.5
 Fe2O3     1.1
 MnO       0.0
 LOI       8.3%

Gallery

Safety Data Sheet

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Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net