This was a fast firing. The glaze is G2934, a silky matte. But that does not mean it is pinhole-prone, it has good melt mobility. The clay on the right is Plainsman Coffee Clay. It contains 10% raw umber, that generates plenty of gases during firing. The centre one is Plainsman M390, not normally difficult to fire defect-free. The left one, M332, should be the worst, but is the best! What is needed to fire these without pinholes? The drop-and-hold and slow-cool C6DHSC firing schedule. It is extra effort to program your kiln controller, but well worth it. If you don't have a kiln controller then by a little experimentation you can develop a switching pattern to produce the same effect.
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.