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Alternate Names: Fire Clay


'Fireclay' is a generic term that in the simplest terms refers to a refractory clay (one which can be fired to a high temperature without deforming or melting). Typically fireclays are plastic and have significant iron impurities. Light duty fireclays have a PCE of about 27 and super duty materials can melt as as high as cone 32.

The obvious use for fireclays is to make bricks and shapes for the structural elements in kilns and furnaces. These clay can be mixed with other materials to introduce air space in increase the insulating value of the product.

Fireclays are useful in many types of ceramics including brick, certain types of tile and sculpture and pottery clays. They impart plasticity and particle size distribution to the body and counter the early melting of any low temperature clays in the mix. For vitreous fireclay based bodies, considerable feldspar content is necessary.

Hundreds of different kinds of fireclays are available. However they are not normally interchangeable in body recipes since they vary drastically in plasticity, particle size, fired color, thermal expansion, and mineralogy.

Related Information

A fireclay that is not really a fireclay!

This is a Lincoln 60 fireclay drying disk (that has been fired to cone 10R). It has near zero-porosity and is dense and very strong. It is like a stoneware clay, quite vitreous.

Skatgit Fireclay test bars

Fired from cone 8-11 and 10 reduction (bottom to top). A refractory material.

Pine Lake fireclay lab test bars

Fired to cone 10R (top) and 7,8,9,10 oxidation (from bottom to top). A refractory material.

Different runs of Plainsman Red Fireclay

These are Fire-Red, a red refractory low plasticity clay mined in Manitoba, Canada. Red fireclay are not common. The top bar on each set is fired to cone 10R, the iron imparts deep color but the matrix is still fairly pours. The next one down in each set is fired to cone 10 oxidation. The third one down is cone 8, the red color is holding (it shifts to brown between around cone 9).

PBX Fireclay fired test bars

Cone 10 reduction (top), cone 10 down to 6 oxidation below that (top to bottom). A refractory material.

Jordan Fireclay fired test bars

Cone 6 to 10 oxidation (top to bottom) fired shrinkage and porosity testing bars.

What does Hawthorne Firelclay look like when fired?

This is a Hawthorne Fireclay sample from 1997, these test bars are made to measure fired shrinkage and porosity. Top bar: Cone 10R. Proceeding down from there is cone 11, 10, 8, etc (oxidation). Drying shrinkage is 4.5%. Firing shrinkage is about 8% at cone 11 going down to 7% at cone 6, it is thus very stable across a wide range. Porosity is likewise, 3% at cone 11 slowly rising to 5% by cone 6. So this material is already fairly vitreous by cone 6 yet still stable at cone 11.

Reduction and oxidation color difference in a cone 10 red fireclay

Plainsman FireRed fireclay fired to cone 10R. This shows the effect of reduction where the body is exposed to the kiln atmosphere (very dark burning) and where it is not (inner foot ring).


Oxide Analysis Formula
Materials Plainsman Red Fireclay
Materials Chamotte
Materials Cedar Heights Fireclay
Materials Plainsman Fireclay
Materials Christy Plastic Fireclay
Materials Clayburn Fireclay
Materials Denver Fireclay
Materials FM Fireclay
Materials Greenstripe Clay
Materials Grefco Fireclay
Materials Hawthorne Bond
Materials Idaho 1 Fireclay
Materials Idaho 2 Fireclay
Materials Imco 400 Fireclay
Materials Imco 800 Fireclay
Materials Jordan Fireclay
Materials Kaiser Denver Fireclay
Materials Kaiser Missouri Fireclay
Materials Lincoln 60 Fireclay
Materials M-4 Fireclay
Materials APG Missouri Fireclay
Materials Monmouth Fireclay
Materials Narco Fireclay
Materials PBX Fireclay
Materials Pine Lake Fireclay
Materials Red Mesa Fireclay
Materials Skagit X Fireclay
Materials T19
Materials NAT Dry Milled Fireclay
Materials Ceramic Fireclay
Materials Industrial Fireclay
Materials Fireclay A/S
Materials Whitfield 202 Fireclay
Materials Hallam Fireclay
Typecodes Fireclay
Fireclays are non-kaolin non-ball clay materials similar to stoneware clays but lacking fluxing oxides. Many fireclays have a PCE of 28 or more.
Typecodes Fireclay
Fireclays are non-kaolin non-ball clay materials similar to stoneware clays but lacking fluxing oxides. Many fireclays have a PCE of 28 or more.
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Glossary PCE
Glossary Fireclay
In the ceramics industry, clays that are resistant to deforming and melting at high temperatures are called fireclays. Kiln bricks are often made from fireclay.
By Tony Hansen
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