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.
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.
Fired from cone 8-11 and 10 reduction (bottom to top).
Fired to cone 10R (top) and 7,8,9,10 oxidation (from bottom to top).
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.
Cone 10 reduction (top), cone 10 down to 6 oxidation below that (top to bottom).
Cone 6 to 10 oxidation (top to bottom) fired shrinkage and porosity testing bars.
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.
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).
Out Bound Links
Overview of quartz hazards in the ceramic industry and process
Pyrometric Cone Equivalent A measure of how refractory a material is. The measure is done by making a small cone from the material and firing it till it bends. A typical stoneware clay body, for example, might have a PCE of cone 20.
A refractory naturally occurring secondary clay. Fireclays are refractory because they contain high concentrations of Al2O3 and low concentrations of fluxes (like Na2O, K2O, CaO, MgO). Kaolins actually qualify as a super-duty fireclay because they contain almost no fluxing oxides (and are thus very ...
In Bound Links
Hawthorne Fireclay, Hawthorn, HAWTHORN 35m
Idaho I Fireclay, A.P. Green Idaho 1
Idaho II Fireclay, A.P. Green Idaho 2
Maryland Ball/Stoneware Clay
AP Green, A.P., A. P., APG Fire, APGFC
Red Mesa Clay, Vista Red