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Dark bodies tend to have more carbon impurities and the burnout of these can generate gases that create bubbles in the glaze. Because of the dark background, the bubbles impart a muddy look. The body on the left is a finer particle size, so the lower thinner glazed section is a partial success, but the upper section is bubbling. The body on the right, although a more pleasant red color, is bubbling worse. Notice also that the warm color of the body is at least largely lost under the glaze. At its worst pinholes can appear.
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 L3954B 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 - they produce the micro-bubble clouding. 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 act as a fining agent (vacuuming up the bubbles)! Right: Alberta Slip GA6-B. It also fires as an amber-coloured glass, but on a dark body, this is an asset.
Glossary |
Glaze Bubbles
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. |
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