A cure for long-time Gerstley Borate sufferers

These are various different terra cotta clays fired to cone 04 with a recipe I developed that sources the same chemistry as the popular G2931 Worthington clear (50:30:20 GB:Kaolin:Silica) but from a different set of materials. The key change was that instead of getting the B₂O₃ from Gerstley Borate I sourced it first from Ulexite (G2931B) and then from a mix of frits (G2931K). All pieces were fired with a drop-and-hold firing schedule C03DRH. Fit was good on many terra cottas I tried (pieces even surviving boiling:icewater stressing). Where it did not fit I had thermal expansion adjustability because more than one frit was sourcing the boron. Frits are so much better for sourcing B₂O₃ than Gerstley Borate (the latter is notorious for turning glaze slurries into jelly!). Of course, a little glaze chemistry is needed to figure out how to convert a recipe from Gerstley Borate bondage to frit freedom, but there is lots of information here on how to do that.

Related Pictures

Gerstley Borate 50:30:20 glaze using Gillespie Borate instead

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This is the G2826A 50:30:20 GB:kaolin:silica base clear recipe. It is been used for decades as a base for all kinds of glazes. It starts melting early enough for use on low-temperature earthenware and is widely used in the raku process. Yet it is also common at middle temperatures (obviously care must be taken or it will run off ware onto kiln shelves when fired to cone 5-6). These tests were fired to cone 6 using the PLC6DS schedule.
The samples on the left use Gerstley Borate, on the right Gillespie Borate. The GBMF test tiles (lower left and right) reveal how much off-gassing is still happening on both when melting starts (they are full of bubbles). The GLFL test (centre) shows the melt flow of the two glazes, it is very similar (normal glazes do not run off the end of the runway like this). The two porcelain test tiles show it to fire crystal clear (there is some pooling since these were applied too thick). There is thus good reason to believe that Gillespie Borate will work well in this class of recipes.

Gerstley Borate 50:30:20 glaze. How could this have worked?

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This recipe, G2826A, a base transparent recipe having 50% Gerstley Borate plus 20% kaolin, is "jelly city". Even with 2.5g of Darvan deflocculant in this jar it is still thick enough to require pushing this tile down into it! Even then, it needs 5 seconds to build up enough thickness. And then does not even cover properly. People have suffered with this popular fluid-melt recipe for 50 years or more just to get the surface variegation it produces. They add all manner of colorants and opacifiers to it. And endure its incessant running onto kiln shelves, bubbling and clouding. It is time to just stop this "Dr. Jekyll and Mr. Hyde" of ceramic materials! And use different base transparents that employ frits to source the boron (B₂O₃). Same chemistry, just a better recipe: G2826A1. Then just add rutile or titanium to restore the variegation. Are you a masochist and still want to be punished? Then at least use the G2826A2 recipe with Gillespie Borate instead.

Tried and True recipes. Really?

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The cover of a book: 15 Tried and True Cone 6 Glaze Recipes

Books and web pages with flashy pictures are the centrepiece of an addiction-ecosystem to recipes that often just don't work. Maybe these are "tried" by a lot of people. But are they "true"? Most are so-called "reactive glazes", outside normal practice - to produce visual interest they run, variegate, crystallize, pool, break, tint, go metallic, etc. But this happens at a cost. And inside special procedures and firing schedules that need explaining. It is not obvious these are understood by the recipe authors or sharers. And these recipes are dated and contain troublesome and unavailable materials. We use frits now to source boron. Stains are superior to raw colorants, even in glazes like this. Many of these will craze badly. And many will not suspend in the bucket. And will run during firing. Reactive glazes have other common issues: Blistering, leaching, cutlery marking, fuming. Trying colors in differing amounts in different base recipes is a good idea. But the project is most beneficial when it shows color response in terms of quality recipes of contrasting chemistries. The point of all of this: Understand a few glazes and develop them, rather than throwing spaghetti against the wall hoping something sticks. Commercial reactive glazes are an alternative also.

A way to get crystal clear glaze on terra cotta: Lead bisilicate

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Boron and lead transparent glazes on terra cotta

The mug on the left is a brush-on version of a boron-based clear glaze for cone 05. Three coats were applied and the often-encountered clouding occurred. The one on the right is an 85:15 lead bisilicate:kaolin mix. Three coats were also applied. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully. And my lead testing kit passes it with no detectable lead release. Underglaze brushwork here we come! I have sought this effect for decades, this is it! Recent realizations about the slipware tradition in the UK (and their standard use of this same glaze) motivated me to get some of the frit. All I could get was a sample of the frit shards, but these milled down to a powder easily in our ball mill.

Of course, to be safe, I would still glaze the insides of pieces with a boron clear, likely as thin a layer as possible of G1916Q (with 2% added iron as a fining agent for the micro-bubbles). And, I will obviously fire these lead glazed pieces with the kiln exhaust system turned on.

Links

Media	Getting Frustrated With a 55% Gerstley Borate Glaze I show you why people love/hate this material and how I substituted it for Ulexite in this crazy recipe to make a far easier-to-use slurry that fires identical.
Materials	Ulexite A natural source of boron, it melts at a very low temperature to a clear glass.
Materials	Gerstley Borate Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How?

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