A1 Speckled Bentonitic Fireclay


A2 is a coarsed particled refractory and iron stained ball clay with enough bentonite to significantly slow drying and increase drying shrinkage. It is heavily contaminated with ironstone concretions, when pulverized in our equipment these break down into particulates many of which cause specking in firing (very pronounced reduction).

Plainsman A1 stockpile. The ironstone concretions are clearly visible around the bottom edges. This is the top-most layer of mined, many fossils found in the area of the quarry are from this layer.

Process Properties

Drying shrinkage is around 10%. Water needed to prepare pottery stiffness is about 27%, far higher than a typical stoneware or fireclay. The 42 mesh ground material has a very coarse texture. It would thus be normal for this be added to existing recipes to darken color, increase plasticity and add fired speckle.


The fired porosity is around 5% for cone 9, 10, 11 and 10R. While not a fireclay this is more refractory than a high temperature clay body. Since it has low water permeability heavier pieces made from A1 (or A1 containing bodies) must be completely dry before firing (force dried at 250F for an extended period).

Plainsman A1 Fired test bars. The amount of speckle in these bars is minimal because the clay was prepared by slaking the raw lumps (thus no ground ironstone is present).


A1 bowl by Zach Quin during 2023 Medalta residency. He processed the clay from lump to plastic form.

Safety Data Sheet

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702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508