Test bars of different terra cotta clays fired at different temperatures

Wednesday 8th May 2013

Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.

Pages that reference this post in the Digitalfire Reference Database:

SOLU Soluble Salts, CDRY Drying Factor/Water Content/Solubles, The Majolica Earthenware Process, Terra cotta, Maturity, Vitrification, Firing Shrinkage, Majolica

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.