|Monthly Tech-Tip |
I bisque fired these at cone 06 and glaze fired them at cone 04. Left piece outside glaze: Mayco C300 matte, inside: Spectrum 700. The inside glaze is clearly under compression, notice how it is pushing the crack apart (a spiral network of cracks is growing around the piece). The right piece is glazed with G2931H, but despite its higher thermal expansion it too is under enough compression to fracture the piece (notice the cracks beginning at the bottom). The presence of significant amounts of silica sand in the clay is the reason for this, quartz raises the thermal expansion of the clay. That produces this problem of glaze compression. It could be possible to use this clay pure as a terra cotta by screening out more of the silica sand.
Evaluating a clay's suitability for use in pottery
Would you like to be able to use your own found-clays in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do this.