These are the inside uppers on two mugs made from the same clay with the same clear glaze. The one on the left was fired in a large electric kiln full of ware (thus it cooled relatively slowly). The one on the right was in a test kiln and was cooled rapidly. This glaze contains 40% Ferro Frit 3134 so there is plenty of boron and plenty of calica to grow the borosilicate crystals that cause the cloudiness in the glass. But in the faster cooling kiln they do not have time to grow.
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Borate glazes, those fluxed with the oxide B2O3, are the most common type used in ceramic industry and hobby for low and medium temperatures.
|Oxides||B2O3 - Boric Oxide|