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 refractory). However they are not used for other reasons, not the least of which is that they are highly refined, in comparison, and therefore much more expensive. In addition, fireclays are typically quite plastic (which kaolins are not). This is actually an advantage because they can support the addition of grog and still function well in the forming process. Fireclays often contain particulate impurities (that need to be ground down) and enough iron to stain them somewhat when fired.
Is Lincoln 60 really a fireclay? Simple physical testing says...
Materials are not always what their name suggests. These are Lincoln Fireclay test bars fired from cone 6-11 oxidation and 10 reduction (top). The clay vitrifies progressively from cone 7 upward (3% porosity at cone 7 to 0.1% by cone 10 oxidation and reduction, bloating by cone 11). Is it a really fireclay? No.
Skagit Fireclay PSD test
Particles from each category in a particle size distribution test of Skagit Fireclay
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.
Pine Lake fireclay lab test bars
Fired to cone 10R (top) and 7,8,9,10 oxidation (from bottom to top).
An iron fireclay? Yes.
The natural Plainsman St. Rose Red clay before it is ground. This has about 6% iron oxide and is used to color high temperature throwing and sculpture bodies. It is quite refractory, very unusual for a clay this high in iron. It is from St. Rose, Manitoba.
Jordan Fireclay fired test bars
Cone 6 to 10 oxidation (top to bottom) fired shrinkage and porosity testing bars.
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