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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.

Key phrases linking here: fireclay - Learn more


A refractory naturally occurring secondary clay. Fireclays are generally refractory because they contain high concentrations of Al2O3 and low concentrations of fluxes (like Na2O, K2O, CaO, MgO). Kaolins actually qualify as super-duty fireclays because of their purity and high melting point, however they are not used for other reasons (poor plasticity and working properties, higher cost). Most fireclays are typically quite plastic, an advantage because they can support the addition of grog and fillers and still function well in the forming process. Ball clays, likewise, are refractory and most would easily qualify as a fireclay (but, like kaolin, they are more expensive). Many ball clays are actually refined fireclays (with iron particulates removed and ground to 200 mesh). A high quality materials can be made using a three-part mix of kaolin:silica:ball clay, it can support 25% grog without problem and fires white and very refractory.

Fireclays provide an economical material for the manufacture of many types of fire brick and refractories (typically light duty). Their particulate impurities and buff or brown burning behaviour is not an issue in these applications. Plastic fireclays that can tolerate high percentage grog additions, while still producing good working properties, are particularly useful.

The "quality" of fireclays is obviously measured by their ability to withstand heat. In our experience, a good fireclay can withstand heat to produce 10-12% porosity at cone 10R (as measured using our SHAB test). Such materials exhibit relatively low fired shrinkage, e.g. 3-4%, with added grog that shrinkage can be cut to half. These can be quite practical for refractories to service to 2350F (1300C) or more. There are actually not many commonly-used materials that can do this, it is more common for fireclays to have 4-6% porosities at cone 10 (5-7% fired shrinkage). Such materials need only a small feldspar addition to be a stoneware. Most fireclays are of this type. There are a class of "false fireclays", ones that have zero zero porosity at cone 10, they are actually vitreous stonewares. These are actually quite common, and are most often used as ingredients in stoneware clay bodies.

The true service temperature of a fireclay is best confirmed by its PCE value. A fireclay with a PCE of 30 is said to be a super duty. Such materials commonly contain 35% or more Al2O3. Siliceous fireclays have the lowest Al2O3 content, less than about 30%. Aluminous fireclays can have very high alumina, even 60%.

Related Information

How can we tell if a clay is really a fireclay without doing a PCE?

Two set of fireclay test bars

While a PCE test is certainly desirable, few people or companies have the ability to do this test. But the much-more-accessable SHAB test produces physical testing data that is perhaps even better. The last column of numbers (titled "ABS", for absorption) prove that the clay on the right, my code number L4380, is very refractory. Even at cone 10R it has 11.3% absorption, and its firing shrinkage is only 3.9%. How do I know cone 10 11.3% porosity is impressive for a fireclay? Because I have tested many other fireclays using this same procedure. What about the clay on the left, L4378? In comparison, its cone 10R porosity of 6.4%, making it quite a bit more vitreous. But not enough to qualify it as a stoneware (which would have 1-2% absorption and 6-7% firing shrinkage). Additionally, many common fireclays also exhibit same level of maturity as L4378. So is it a fireclay or a stoneware? A 10% feldspar addition would convert it to stoneware, so there is merit in calling it a "stoneware material". But that is "in comparison to" the very refractory L4380. But, if we were compare L4378 to any of the false fireclays commonly sold, then it could certainly be termed a fireclay in comparison.

Is Lincoln 60 really a fireclay? Simple physical testing says...

Materials are not always what their name suggests. These are Lincoln #60 Fireclay test bars fired at cone 10 reduction (top) and from cone 11 down to 6 oxidation (top to bottom). This clay already has stoneware density at cone 7 (3% porosity as indicated by our SHAB test). It vitrifies progressively from there upward (less than 3% porosity at cone 7 to near zero% by cone 9 oxidation. Maximum firing shrinkage happens at cone 8 and by cone 10 it is expanding (indicating decomposition has started) and it is bloating by cone 11 (melting is sealing the escape of gases of decomposition). Is Lincoln #60 a really fireclay? Absolutely not! But at cone 6 it is a credible plastic stoneware, all by itself!

Skagit Fireclay PSD test

Particles from each category in a particle size distribution test of Skagit Fireclay

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.

Jordan Fireclay fired test bars

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

PBX Fireclay fired test bars

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

IMCO 400 Fireclay fired bars

Cone 10R top, 11 oxidation and downward below that. This material, although called a "fireclay", is more fine grained and much more vitreous than what would normally be considered a refractory fireclay. Is it similar to Lincoln Fireclay (from California).

M-4 Fireclay fired bars

Cone 10R top, cone 11 oxidation downward below that.

Lignite can be big trouble

Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging. This is a coarse clay, but if it were incorporated into a recipe of a stoneware, glaze pinholing would be likey.


Glossary Refractory
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.
Glossary Firebrick
In the ceramic industry, these are the bricks used to build kilns. This term grows out of their ability to withstand high temperatures that would melt or deform structural bricks.
Glossary Secondary Clay
Clays form by the weathering of rock deposits over long periods. Primary clays are found near the site of alteration. Secondary clays are transported by water and laid down in layers.
Glossary Crucible
In ceramics, potters make crucibles to melt frits, stains and other materials. Crucibles are made from refractory materials that are stable against the material being melted in them.
Materials Fireclay
Tests Pyrometric Cone Equivalent
Make a pyrometric cone out of a clay or material to see what temperature it bends at
By Tony Hansen
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