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These Plainsman Midstone and Redstone cups are fired to cone 6 with M340 Transparent glaze liner (these are raw materials that body manufacturers incorporate into their products in fairly high percentages). Notice how many more glaze bubbles there are with the red cup. This is typical using other transparent glazes also. To get a bubble-free clear on this red burning body a glaze having a higher melt fluidity, lower melt viscosity, later melting and a fining agent is needed. And we found two: GA6-B and G3806C.

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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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