5% tin oxide in a transparent boron cone 6 glaze
This is a 45:27:28 Gerstley Borate:Kaolin:Silica base on a dark firing clay body. But tin oxide will do that in almost any cone 6 clear glaze (use one that fits your clay body).
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
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