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.

Related Pictures

An ultra-clear brilliantly-glossy cone 6 clear base glaze? Yes!

I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the G3806E and G3806F.

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.

Links

Recipes	G3806C - Cone 6 Clear Fluid-Melt transparent glaze A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others.
Recipes	G2926B - Cone 6 Whiteware/Porcelain transparent glaze A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
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
Glossary	Melt Fluidity Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.

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