• Materials

Fireclay

Alternate Names: Fire Clay

Notes

'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

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).

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.

Jordan Fireclay fired test bars

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

PBX Fireclay test bars fired over a wide temperature range

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

Different runs of Plainsman Red Fireclay

These are FireRed, a red fireclay. The top bar on each is cone 10R, the next one down in 10 oxidation. The third one down is cone 8 where the red color is holding.

Pine Lake fireclay lab test bars

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

Skatgit Fireclay test bars

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

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.

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By Tony Hansen