A closeup of the rim and a transparent-glazed cup made from a high-iron clay (Plainsman Redstone) and fired at cone 6. Iron-bearing clays tend to gas more on firing and can generate many more bubbles in glazes for this reason (a buff stoneware would be almost free of bubbles with this glaze, Plainsman M340 transparent). Thus, it is best to use opacified glazes with this type of body.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.