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Sculpture

Highly grogged highly plastic sculpture body. This body is built on a 50% base of kaolin and ball clay along with 20% of a fine red earthenware with the remainder a high quality grog.

Process Properties

This body is a major departure from traditional Plainsman style native grog bodies in a number of ways that relate to its working properties.

-It employs imported refined minerals and has a matrix of fine smooth clay rather than a coarse stoneware. This results in a body that is not susceptible to water splitting and is more plastic. While traditional Plainsman sculpture bodies have other advantages, they do split when exposed to water during the sculpting process.

-This body is very heavily grogged. The grog particles are all coarse, there are no fines. In many other sculpting bodies, much of the grog complement is actually a fine powder.

-The nature of the grog is much different. The particles are flat and lay down on contact with your hands. This produces a remarkably smooth feel, so much so that it throws extremely well lacking the abrasive feel of other grogged bodies. It is possible to make extremely overhung and exaggerated shapes.

On one hand the grog terminates cracks, reduces shrinkage and vents internal water but on the other it reduces the dry strength and provides surface imperfections where cracks can start. Couple this with the highly plastic and fine nature of the clay matrix and you can see why some caution is needed to make sure that pieces dry evenly.

Firing

From a firing point of view this body is also a significant departure from typical scupture bodies: it fires much more vitreous. In addition it employs a low fire red (rather than refractory red) to achieve both its tan to brown burning color and maturity.

Without the grog this body base is highly plastic and vitrifies at cone 8. However with the significant grog complement it provides a stable matrix for warp-resistant firing above cone 8. However since it has a vitreous matrix there is need to support pieces well when firing to cone 10. Also be aware that at higher temperatures, smaller increases in firing temperature will produce more than the expected increase in warping. The benefit of the more vitreous nature is that it creates a stronger and more durable fired product that is much less susceptible to freeze-thaw failure.

While the body matrix does vitrify at higher temperatures, the grog particles are refractory. Thus you will be able to measure porosity, but remember that this porosity is primarly in the grog.

Since this body employs a low burning red to achieve color it will fire brown rather than red in reduction.

Since the body contains a high proportion of fine plastic clays it is important to dry pieces slowly enough to avoid cracking. While the grog reduces drying shrinkage and speeds water venting, the fine clay matrix is still less permeable and will hold onto its water longer. In addition, early stages of firing must be slow, especially on pieces with thick sections. If firing is too fast at the start internal steam pressure will fracture the peice. Ideally the kiln should be candled (held around boiling point) for a day or more previous to firing larger pieces.

Glazing

Although glazing will not be common, you will find that the more dense matrix of fine clay will produce less glaze defects. At higher temperatures the body fires a darker tan to brown, so light colored glazes will be darkened to some extent.

Physical Properties

 Drying Shrinkage: 5.0-6.0%
 Dry Strength: n/a
 Drying Factor: C110
 Water Content: 20.0-21.5% Mar 99
 Dry Density: N/A

Sieve Analysis (Tyler mesh):

     +48     (300 microns): 20.0-23.0%
   48-65 (300-210 microns): 3.0-4.0
  65-100 (210-149 microns): 0.05-0.15
 100-150 (149-106 microns): 0.05-0.1
 150-200  (106-75 microns): 1.0-2.0
 200-325   (75-45 microns): 3.5-4.5

Fired Shrinkage:

   Cone 8: 6.0-6.5%
   Cone 10: 5.0-5.5
  Cone10R: 5.0-5.5

Fired Absorption:

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

Chemical Analysis

 CaO       0.3
 K2O       1.3
 MgO       0.5
 Na2O      0.2
 TiO2      1.0
 Al2O3    24.2
 P2O5      0.1
 SiO2     62.4
 Fe2O3     2.4
 MnO       0.0
 LOI       7.6%

News

Since this body deviates from what is considered normal in scupture clays we emphasize that you should test it well. The benefits are clearly defined but these have associated risks that must be managed.
Logo Plainsman Clays Ltd.
Box 1266, 702 Wood Street, Medicine Hat, Alberta T1A 7E5
Phone: 403-527-8535 FAX: 403-527-7508 		Top

Revised: 10/29/07, Author: Tony Hansen