These are fired in cone 6 oxidation. They are all the same clay body (Plainsman M390). The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B clear glaze is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition. That being said, the other two glazes here are also early melters, yet they did not bubble. Left: G2926B plus 4% iron oxide. That turns it into an amber color but the iron particles vacuum up the bubbles! Right: Alberta Slip GA6-A using Ferro Frit 3195 as the melter. It also fires as an amber-coloured glass, but on a dark body this is an asset.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.