Plainsman Fireclay

Description

A moderately refractory light colored lightly iron speckled plastic clay blend specifically formulated to closely match the working and fired properties of Standard AP Green Fireclay. Plainsman Fireclay (PFC) is a blend of refractory materials (a ball clay, a stoneware, a kaolinized sand and a kaolin).

Traditional fireclays tend to be pure, unblended coarsely ground raw clays that are mostly used for non-pottery purposes. There are more than 20 common ones available in North America. Physical properties, contamination and consistency factors that are very important to potters are not often well maintained in these materials. Plainsman Fireclay, on the other hand, is tightly controlled for uses relating directly to pottery, it is processed using the same equipment and methods as other Plainsman clay bodies and native materials.

You could fashion a buff burning stoneware clay body using 90%+ of this material plus a little feldspar to increase maturity (and cut plasticity). The 'balance' and 'character' of Plainsman Fireclay compared to industrial minerals (like kaolin, ball clay, quartz and feldspar) is good reason for its use in non-whiteware bodies.

The physical properties of most fireclays are neither published by their manufacturers or well understood by most people in the ceramic community. Various products on the market are quite different in plasticity, fired character, impurity content, particle size distribution, etc. Plainsman Fireclay was formulated to match the physical properties of the now defunct AP Green Missouri Fireclay as closely as possible (since it was the most widely used material).

The chemistry of fireclays used in pottery applications is not typically as important as the physical properties. However if you intend to use this material in refractory, pay attention to the comparative alkali content and do some tests to verify suitability. We have actually incorporated some less refractory ingredients into the blend to more closely match the firing characteristics of AP Green Fireclay below cone 12.

Process Properties

Most fireclays are fairly coarse grained powders and the grains tend to be harder non-clay particles. However some materials (like AP Green Fireclay) are inherently very fine yet have many coarser particles that are simply agglomerates of unground clay that break down in water. While Plainsman Fireclay is a member of the former more common type and has a wide distribution of particle sizes, it is comparatively a little finer than most fireclays in the larger particle size ranges, thus bodies with a lot of this material will still feel relatively smooth.

Like most fireclays, this material is quite plastic. However, the nature of its plasticity may be different than other fireclays, it produces a body that has a more ball-clay-like character (as opposed to kaolin-like). Some fireclays tend to make bodies more difficult to center and neck in during throwing, you will find this material contributes to excellent plastic qualities.

If your body is less plastic using Plainsman Fireclay (unlikely) you can add a little bentonite to the recipe (1% or less should be enough). If it is more plastic then you might need to exchange some ball clay for kaolin in the recipe or select less plastic substitutes for other clay ingredients.

PFC contains 10% of a very fine white kaolinized sand. This additive gives us control of maturity and plasticity. Sensitive throwers might feel this sand in high-fireclay bodies or notice it in the slip produced. However this sand can provide a channel for water penetration into the plastic clay in some formulations; if you experience water splitting during throwing (vertical cracks) then we recommend exercising more care to avoid leaving water on surfaces that are being stretched (e.g. bellies on vases). For a simple comparison test between versions of your clay body using different fireclays, balance cigarette shaped pieces of clay horizontally on your finger and put a few drops of water on top and compare the amount of time it takes for a split to begin.

Plainsman Fireclay has a slightly higher drying shrinkage than many other fireclays. This may be because it is more plastic, but not necessarily. That means you may need to be a little more careful to dry pieces evenly in plastic bodies with substantial amounts of fireclay.

Firing

Plainsman Fireclay fired bars. Cone 10R top. Cone 8 to 10 (bottom upward).

By itself, Plainsman Fireclay burns to a buff-yellowish color at cone 10 with an evenly distributed population of fine iron speckles. The color is modified toward tan in areas where soluble salts from within the body are left on the surface during drying.

As already implied, when substituting fireclays in body recipes, you should make test bars to compare the body using the two. Note specifically any differences in the following:

  • Maturity: Almost all clay bodies employing fireclays will also contain feldspar, thus you can add or remove a few percent to compensate if the body is more or less vitreous.
  • Speck development: If you use fireclay to impart speckle in reduction bodies, then pay close attention to this in fired tests. If anything, the Plainsman material is likely cleaner, and there will be fewer 'large rouge specks' (which are a constant source of aggravation in many fireclays). There will likely be a higher population of finer specks.
  • Dunting: Plainsman Fireclay may be higher in free silica, therefore you might consider removing some silica powder from your body if dunting becomes a problem.

While PFC by itself is quite refractory it will not necessarily produce a less vitreous body than other less refractory fireclays (adjust with feldspar as needed to compensate). We recommend measuring maturity by doing porosity tests (since the 'apparent' maturity of a clay body can be misleading; a body which appears more vitreous can actually be less so and vice versa).

Soluble Salts: Like other fireclays, this material dries with some iron-bearing soluble salts left on the surface. These tend to impart a brownish coloration to some areas where they concentrate (the fired character of traditional fireclays are partly a product of this). These deposit differently depending on the potter's process and natural variations in the raw material. If you need to eliminate these, then add about 0.3% barium carbonate for the A2 portion in your clay body recipe.

Thermal Expansion: Plainsman Fireclay has a relatively high free silica content and will thus contribute to a higher thermal expansion in clay bodies compared to AP Green fireclay (crazing glazes will craze less, shivering glazes will shiver more). In some cases the higher silica may be an advantage, for example common high feldspar glazes that typically craze on most bodies will be better on bodies based on this material. In other cases the higher expansion could increase dunting problems in bodies containing inadequate feldspar or recipes with significant additions of ball clay and silica. For example, it is common to find bodies which are mixtures of fireclay, ball clay and other stoneware clays (like Goldart). In these cases you may find that substituting some or all of the ball clay for kaolin will not only improve glaze fit but impart other improvements.

Glazing

PFC is not air floated, it contains particles you can feel. These generate gases as the kiln is fired and can cause pinholes or blisters. For example, there are particles of calcium sulfate, these generate sulfur during firing. Rutile glazes, for example, are well known for the blistering problem. If your glazes are affected you may need to do a drop-and-soak firing. Do this by firing to temperature (e.g. cone 10), soaking long enough to even out the kiln, then drop 100 degrees and soak there (glazes stay fluid at much lower temperatures than most people realize). This enables the increasing melt viscosity of the glaze melt to overcome the surface tension that may hold bubbles in place. They will break and the glaze will heal.

Physical Properties

 Drying Shrinkage: 6.5-7.5% (APGF is 6.0-7.0)

Sieve Analysis (Tyler mesh):

                      PFC        APGF
 +48 (300 microns): 0.1-0.5    0.0-0.5
   48-65 (300-210): 0.5-1.5    0.0-0.5
  65-100 (210-149): 1.0-3.0    0.0-0.5
 100-150 (149-106): 2.0-4.0    0.0-0.5
 150-200  (106-75): 4.0-6.0    0.0-1.0
 200-325   (75-45): 7.0-10.0   0.0-1.0

Fired Shrinkage:

             PFC        APGF
   Cone 6: 5.0-6.0%   6.5-7.5%
   Cone 8: 5.5-6.5    7.0-7.5
  Cone 10: 6.5-7.5    7.5-8.5
 Cone 10R: 6.0-7.0    7.5-8.5

Fired Absorption:

              PFC      APGF
   Cone 6: 5.0-6.0%   2.5-3.5
   Cone 8: 4.0-5.0    2.0-3.0
  Cone 10: 3.0-4.0    1.5-2.5

Chemical Analysis

 CaO       0.2
 K2O       2.1
 MgO       0.2
 Na2O      1.0
 TiO2      0.7
 Al2O3    27.0
 P2O5      0.0
 SiO2     58.1
 Fe2O3     2.0
 MnO       0.0
 LOI       8.7%

Compared to Others

Vs. Plainsman Buff Fireclay: This differs from the discontinued Plainsman Buff Fireclay in that it is a little smoother (having less sand), not quite as plastic and not quite as refractory. This material is actually Buff Fireclay with some ball clay and sand replaced with a smooth stoneware clay.

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