Pyrophyllite is a low expansion mineral. The individual particles expand less on heating (and therefore contract less on cooling) than other refractory particles (like silica). Thus, during firing, pyrophyllite particles counter the shrinkage of the plastic components surrounding them (assuming that particles remain whole and undissolved to exert their influence on the surrounding matrix). Pyrophyllite-refractory clay combinations potentially have excellent volume stability and resistance to deformation at high temperatures. Pyrophyllite has a second expansion-lowering mechanism: Its presence in stoneware and porcelain bodies encourages the development of more mullite; this both increases firing strength and reduces thermal expansion. In this case, the individual particles of pyrophyllite react and transform from their original state. In clay bodies, substitution of part or all of the silica will decrease thermal expansion while maintaining the level of maturity. In electrical porcelain, large amounts (up to 95%) may be used in bodies for optimal electrical properties and high frequency characteristics (where zero porosity is not critical).