Why is that transparent glaze firing cloudy? The balls test us.

Tuesday 30th December 2014

G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and EPK) fired across the range of 1650 to 2000F (these were 10 gram GBMF test balls that melted and flattened as they fired). Notice how they soften over a wide range, starting below cone 010 (1700F)! At the early stages carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon; think of what a raw glaze might have! Of course, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (in this case 1450F), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible. Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue.

Pages that reference this post in the Digitalfire Reference Database:

G1916Q - Low Fire Frit 3195 Glossy Transparent, GBMF Glaze Melt Fluidity - Ball Test, Ferro Frit 3195, Ferro Frit 3110, Transparent Glazes, Glaze Bubbles, LOI, Soaking, Clouding in Transparent Glazes

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.