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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 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 - 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 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.
Glaze clouding is a universal issue in ceramics. Terra cotta bodies demonstrate this best. Pretty well all transparent glazes, even commercially available ones, can cloud. This example is G2931K, it can be beautifully crystal clear. But the thickness of application is the key to achieving that (as thickness increases this happens). We ball milled it to see if that would help, but as you can see, that has not impacted the problem. This is a dipping version so that is part of the reason why it is easy to get it on too thick. One of the advantages of brushing glazes is the ability to carefully control thickness,
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Glossary |
Transparent Glazes
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. |
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Troubles |
Clouding in Ceramic Glazes
There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass |
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