No crazing out of the kiln. So it is good. Right? Wrong!

Thursday 30th October 2014

The side of this white porcelain test mug is glazed with varying thicknesses of VC71 (a popular silky matte), then fired to cone 6. Out of the kiln there was no crazing, and it felt silky and wonderful. But a 300F/icewater test was done and then it was felt-pen marked and cleaned with acetone. This is what happened! This level of crazing is bad, the dense pattern indicates a very poor fit. Then why was it not crazed coming out of the kiln? The glaze is apparently elastic enough to handle the gradual cooling in the kiln. But what the kiln did not do, time certainly will. This recipe has 40% feldspar (a big high-expansion KNaO contributor), that much in a cone 6 glaze it a red flag to crazing. Coupled with that was low Al2O3 and SiO2, another tip-off.

Pages that reference this post in the Digitalfire Reference Database:

IWCT 300F:Ice Water Crazing Test, Understanding Thermal Expansion in Ceramic Glazes, Crazing, Co-efficient of Thermal Expansion

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