Two low fire transparent highly fritted glaze recipes for pottery

These work well from cone 04 up, they are better than any commercial transparent brushing glaze we have used. And far better than glazes made using raw sources of boron (like ulexite, colemanite, Gerstley Borate). These glazes have lower thermal expansion and do not craze on any body we have tried (yet are ultra gloss and ultra-clear). They are G1916QL1 and G3879C. We developed them for use on the dolomite-based (rather than talc-based) L4410L low-temperature art clay body. These are a good demonstration of the technical and economic sense it makes to use highly fritted glazes at low temperatures. Having a good base glaze is the key to adopting low temperatures for your production. You would likely agree that no stoneware glaze has melting patterns like these shown in this melt fluidity test! These recipes and all details about their development and adjustment are openly available.

Related Pictures

Here is my setup to make brushing glazes and underglazes by-the-jar

To make a low SG version of G2934BL I have already weighed out a 340g batch (it contains 5g each of Veegum and CMC gum to gel the slurry and slow the drying). I use 440g of water initially (adjusting that according to experience in brushing behaviour). After shake-mixing all the powder in the plastic bag I pour it into the water on low speed and finish with 20 seconds on high speed. This produces a low specific gravity brushing glaze, it just fills this 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel and the CMC for slower or faster drying. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Since each glaze recipe responds differently and changes differently over time, good notes are essential. We are working on dozens of these at any given time, each is code-numbered in our group account at Insight-live.com. This is so worthwhile doing that I make quality custom labels for each jar!

Medium fire white bodies used at cone 04. Is that smart?

Low-temperature white talc casting bodies have always been porous, 10-12% absorption is typical - pieces are basically just bisque-fired. People just pretend everything is fine! A stoneware potter might be horrified by this - until he/she sees the bright colors and brilliant glazes! If a dolomite body alternative is not available to you or your supplier is making a talc body that is no longer white there is an alternative: Use a middle-temperature white burning body. Why is this even possible? Their porosity at cone 05 is also 10-12% - but won't commercial glazes craze? Yes. So just make your own. This is G1916QL1 applied thickly on Polar Ice, P300, M370 (and L213 talc body), these are all plastic bodies but they can be adjusted for casting (removal of the bentonite). None crazed out of the kiln or after a BWIW test. I did this again with a thinner glaze application and at cone 03 (100°F hotter) and the results were even better. G1916QL1 can be mixed as a dipping or brushing glaze. But how can you guarantee no crazing? Fire even hotter, perhaps cone 02. Lower the thermal expansion further (as per instructions on its web page). And increase the silica in the body recipe at the expense of kaolin or ball clay (compensate with whatever bentonite is needed to get good casting properties). For a good cone 6 porcelain recipe for casting try L3778G. All of this can also be done with a plastic body, for that try L3778D.

Learn to mix any of your glazes for these three application methods

Potters are used to making dipping glazes that they weigh out and mix from recipes. Hobbyists commonly use bottled commercial brushing glazes. Did you know that a dipping glaze can be turned into a brushing glaze by the addition of Veegum (or Veegum CER) and water? Do you know what a base-coat dipping glaze is? Here is a quick overview: Dipping glazes need to go on to bisque ware evenly, be thixotropic enough to hold on at thickness and drain and dry quickly. But they don't need to dry hard. Brushing glazes need a cohesive slurry that dries slowly and hardens well on drying. They also must adhere to the body really well so that multiple layers can be applied (individual layers go on thin). Base-coat dipping glazes are in between, they need to dry fast enough and gel well enough to make application by dipping possible (although less practical) and they need to adhere well enough to tolerate another layer without letter go of the body.

White clay with red slip vs terra cotta clay with white slip

Both of these mugs are fired at cone 04 using the C04PLTP schedule. The one on the left is made from L4410P (Plainsman Snow) with Amaco V-303 Terra Cotta underglaze brushed on the outside base. It is cover-glazed with Spectrum 700 brush-on clear. The mug on the right is made using a red terra cotta body (L4170B with 10% 65 mesh silica sand and 3% bentonite added to convert it from a casting to throwing body). The white engobe brushed on the upper outer surface is L3685Z2 (it is blender mixed for brushing with a 1.5% Veegum and 1.5% CMC gum additions). It paints on to the leather hard clay in one coat and almost no brush strokes. The clear cover glaze is G1916Q dipping (a glaze you can make yourself). The terra cotta piece on the right has two obvious advantages: It fires much stronger and the engobe fires whiter. This method of "white engobe on terra cotta" produces a surface of porcelain-whiteness on a body that fires at cone 04. And, it has much higher strength than white burning bodies. Plainsman L215 and L210 terra cottas could also be used with this engobe.

Links

Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
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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