H431

High fire, sandy, medium plastic, semi-vitreous, grey burning speckled, native body for reduction fired stoneware.

H431 is the body of choice for people looking for a light burning body with a more natural earthy speckled fired appearance and reduction speckling. However, if you use H431 we recommend that you consider carefully its limitations as outlined below.

Process Properties

H431 is a balance of high amounts of very plastic ball clays and bentonites with significant amounts of sand and silty clay to cut plasticity. H431 thus has a definite sandy feel during throwing. This sand generates more than the usual amount of slip with the associated loss of fine particles from the surface. The open structure of H431 also means that extra water can be absorbed during throwing so it will not stand up as well to over-working on the wheel (as is normal with students).

The sand and silt content in H431 also means that it is more susceptible to water-splitting during throwing or hand building. The abundance of larger particles provides openings into which water can penetrate and split the soft clay. Thus it is very important that you use water sparingly during throwing and make sure that it does not stay long on points where the clay is under stress (i.e. the outside of the belly on a vase). With H431 it is practical to develop modified handle pulling techniques and use slip rather than water during throwing. In addition it is wise to immediately sponge away excess slip squeezed out of joins (often such splits are misinterpreted as drying cracks).

Firing

The most striking effect that this body will have on the fired glaze is the formation reduction speckle. H431 contains significant amounts of iron stone concretion particles which melt vigorously in high temperature reduction and blossom on the bare surface or bleed up through glazes (it has the most speckle of any of our non-brown burning bodies). Other than the degree of specking, the appearance of the fired body in high temperature oxidation and reduction is quite similar.

Since H431 contains bentonite and highly plastic ball clay it cannot be bisque or once fired as fast as other bodies or cracking and disintegration can occur. It is thus important to fire slowly through the 100-200C range where water turns to steam and needs to vent out of the clay (it is best to pre-heat your kiln overnight on low).

If you must refire any ware to fix imperfections, remember that due to the high sand content, fired H431 ware requires slower heating and cooling through quartz inversion temperatures.

Glazing

H431 bisque ware should be fired to about cone 06 and even though it is dense it is still quite absorbent and takes glaze fairly well. The open structure of the bisque means that glazes will tend to pinhole during drying as water-displaced air escapes from minute surface holes.

H431 will not perform well in thermal shock situations (i.e. teapots) if the glaze does not fit well. This body has a fairly high fired strength which can be significantly impacted if ware has a glaze which does not fit properly. Do not judge your glaze as fitting because there are no visible signs of trouble when the ware is removed from the kiln, use an ice water:boiling water immersion test to make sure.

Glaze Recipes

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.

Physical Properties

 Drying Shrinkage: 6.0-7.0%
 Dry Strength: n/a
 Water Content: 19.0-20.5%
 Drying Factor: C120-C130

Sieve Analysis (Tyler mesh):

     +48: 0-0.2%
   48-65: 0.0-0.5
  65-100: 3.0-5.0
 100-150: 8.0-12.0
 150-200: 4.0-7.0
 200-325: 6.0-10.0

Fired Shrinkage:

   Cone 8: 4.0-5.0%
  Cone 10: 4.5-5.5
 Cone 10R: 5.0-6.0

Fired Absorption:

   Cone 8: 5.0-7.0%
  Cone 10: 2.0-3.0
 Cone 10R: 2.0-3.0

Chemical Analysis

 BaO       0.4
 CaO       0.2
 K2O       2.2
 MgO       0.5
 Na2O      0.2
 TiO2      0.6
 Al2O3    15.9
 P2O5      0.1
 SiO2     71.7
 Fe2O3     1.4
 MnO       0.0
 LOI       6.8%

News

H431 can vary somewhat in speckle and surface soluble salts. It employs an extremely plastic and somewhat inconsistent iron-pyritic bentonite to source its reduction speckle. This material contains varying amounts of soluble salts.

Because H431 is so sandy we have to watch its tendency to water-split during throwing if the plasticity drops slightly. Generally, unless you need the fired color and sandy texture, we recommend you use our H550 body instead.

Plainsman Clays Ltd.
Box 1266, 702 Wood Street, Medicine Hat, Alberta T1A 7M9
Phone: 403-527-8535 FAX: 403-527-7508

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