This was a fast firing. The glaze is G2934, a silky matte. The one on the left is Plainsman Coffee Clay. It contains 10% raw umber, that generates plenty of gases during firing. The center one is Plainsman M390, not normally difficult to fire defect-free. The right one, M332, should be the worst, but is the best!
Plainsman Cone 6 Slow Cool (Reactive glazes)
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.