These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.
Pages that reference this post in the Digitalfire Reference Library:
Where Do I Start?, G2934 - Matte Glaze Base for Cone 6, IWCT 300F:Ice Water Crazing Test, Crazing, Feldspar Glazes, Functional, Calculated Thermal Expansion, Find and Improve a Cone 6 Clear Ceramic Glaze
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.