Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in production either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers.
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.