Transparent glazes often work poorly on dark stoneware bodies

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.

Related Pictures

Thick application clouds a transparent glaze on a terra cotta clay

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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Is clear glazing a dark burning oxidation stoneware a good idea? Likely not.

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.

Our G2926B glaze may not work on dark burning clays

These two glazes, applied to the outsides of these mugs, both fire as brilliant glass-like super-transparents. But on this high-iron stoneware, from which both pieces are made, only one is working well. G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated as the body gases during firing. G2926B (right) works great on porcelain and buff stoneware but it cannot clear the clouds of bubbles coming out of this body (the bubbles are actually partially opacifying it). Even the normal glassy smooth surface has been affected. The moral: Potters need more than one base transparent recipe. Being able to host colors, opacifiers and variegators is nice, but sometimes just a transparent that works well is needed. An interesting trade-off of reactive melt-fluid glazes is that, while they develop more interesting surfaces, their lower SiO₂ and Al₂O₃ contents make them susceptible to crazing, settling of the slurry and cutlery marking.

Links

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.
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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