No crazing out of the kiln. But an ice-water test did this.

Thursday 30th October 2014

The side of this white insight-live.com/glossary/71">porcelain test mug is glazed with varying thicknesses of V.C. 71 (a popular silky matte used by potters), then fired to cone 6. Out of the kiln, there was no crazing, and it felt silky and wonderful. But after a 300F/icewater IWCT this happened (it was felt-pen marked and cleaned with acetone). The glaze was apparently elastic enough to handle the gradual cooling in the kiln. However, the recipe has 40% feldspar and low Al2O3 and SiO2, in a cone 6 glaze these are red flags for crazing.

No matter what anyone tells you, glaze fit can rarely be fixed by firing differently (that just delays it). If someone needs to cool their kiln slowly to prevent crazing it simply means the glaze does not fit - its needs to be adjusted to reduce its co-efficient of thermal expansion.

Pages that reference this post in the Digitalfire Reference Library:

IWCT 300F:Ice Water Crazing Test, Understanding Thermal Expansion in Ceramic Glazes, Is the V.C. 71 pottery glaze a true matte? , V.C. 71 silky matte cone 6 pottery glaze recipe - It is known to craze yet is popular, Co-efficient of Thermal Expansion, Glaze Crazing, Glaze Crazing

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.