The glaze is simply insufficiently melted. In cone 10 reduction the iron acts as a flux and it melts very very well, so there are no pinholes. But here it simply does not have enough fluidity to heal the disruptions caused as gases escaping from the body below bubble up through it.
Glaze Pinholes, Pitting
Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.
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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