This problem was suffered by a potter moving from Europe to Canada. In Europe she used lead based glazes and got smooth defect-free surfaces. But in Canada she began using our Gerstley Borate based glazes and had many problems adapting to them. We used glaze chemistry to both adjust recipes to source boron from frits instead of GB and to reduce their melt fluidity. She also adjusted firings: Higher bisque and glaze firings and a drop-and-hold firing schedule (like C03DRH). And she adopted a finer particle-sized terra cotta clay body. These measures enabled her to eliminate the issue.
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
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