Left: G1916Q transparent fired at cone 03 over a black engobe (L3685T plus stain) and a kaolin-based low fire stoneware (L3685T). The micro-bubbles are proliferating when the glaze is too thick. Right: A commercial low fire transparent (two coats lower and 3 coats upper). A crystal clear glaze result is needed and it appears that the body is generating gases that cause this problem. Likely the kaolin is the guilty material, the recipe contains almost 50%. Kaolin has a 12% LOI. To cut this LOI it will be necessary to replace some or all of the kaolin with a low carbon ball clay. This will mean a loss in whiteness. Another solution would be diluting the kaolin with feldspar and adding more bentonite to make up for lost plasticity.
Pages that reference this post in the Digitalfire Reference Library:
L3685U - Cone 03 White Stoneware/Engobe, Firing: What Happens to Ceramic Ware in a Firing Kiln, G1916Q - Low Fire Frit 3195 Glossy Transparent, Glaze Bubbles, 1950F-Body decomposition causes glaze bubbles, 200C-Decarbonation
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.