Fireclay denotes a silica-rich natural clay that can withstand a high firing temperature above the pyrometric cone equivalent (PCE) of 19
without melting, cracking, deforming, disintegrating, or softening. Typically, a good fireclay
should have 24 to 26 vol.% plasticity, and shrinkage after firing should be within 6 to 8 vol.%
maximum. Fireclays are mostly made of kaolinite, but some Fe2
O3
and minor amounts of
Na2O, K2O, CaO, MgO, and TiO2
are invariably present depending on the mineralogy and geology of the deposit, making it gray in color.
