Clear Choices: Not All Clear Glazes Are the Same
PART TWO OF A SERIES • by PETE PINNELL
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very potter has a favorite clear glaze recipe, but often just one. Clear glazes can seem kind of boring (compared with other glazes), so most people find one that meets the minimum requirements (doesn’t craze or run) and then don’t give it much more thought. However, clear glazes can be just as varied as any other kind of glaze. In this article, I’m going to present three successful clear glazes, each of which has very different qualities. These aren’t necessarily the “best” glazes at anything — they each have specific strengths and weaknesses. I’m presenting these to provide you with my thought process for evaluating them, as much as to provide you with the glaze recipes themselves.
I then translated that formula into a recipe made from
contemporary materials. Surprisingly, it worked quite well and looked a lot like those Chinese examples from the 18th century. Once I got out of school, I ended up rounding the ingredient percentages (to make the glaze easier to mix) and found that it worked just the same. It’s a recipe that many potters use today. From a chemical standpoint, this formula is what potters might call a “high lime” glaze. Most of the fluxing is done with calcium oxide (“lime” in traditional potter’s language, since potters used to use “quicklime” or “burnt lime” for this), with potassium oxide and sodium continued on next page
Pete’s Clear Glaze Cone 9-12 oxidation or reduction 25% 35 20 20
Potash Feldspar* Silica Whiting Grolleg China Clay TOTAL
100%
* I've used Custer Feldspar (both old and new), G200, and G200 HP. All work, but they have slightly different qualities. for Pete’s Celadon, add:
Tin Oxide Barium Carbonate Barnard Clay
1% 2% 3%
CLAYTIMES·COM n SPRING / SUMMER 2016
The first glaze is one that many potters know as “Pete’s Clear”. I formulated this glaze when I was a graduate student in the early ’80s. I had long admired the clear glazes on Qing Dynasty (1644 to 1912) Chinese porcelain, and discovered that there were published chemical analyses that had been done from shards. I collected a number of analyses, discarded those that seemed anomalous, and averaged the others.
Image 1: Pete’s Clear Glaze. The left tile is fired to cone 6 in oxidation and is melted, but not yet mature. The three right-hand tiles are fired to cone 10 in reduction and have a progressively thicker glaze application (single-, double-, and triple-dipped). As you can see, the glaze doesn’t cause any smearing of the underglaze, but it does obscure the lines as it becomes thicker.
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