View Larger Image This white stoneware vase was glazed with Frasca Wood Ash Glaze over Green Dragon Matt Glaze.

May 14, 2008

From High Fire to Mid Range: Changing Kilns Leads to New Decorating and Glazing Techniques

by Harry Spring | Read Comments (3)

After high firing in a gas kiln for 25 years, Wickford, Rhode Island, potter Harry Spring was forced by circumstances to switch to electric. This was quite an adjustment, as Spring had come to depend on the serendipitous effects that are part of the magic of reduction firing. But, adjust he did. Today he shares with us some of the adjustments he made that have made switching to oxidation firing a fun challenge rather than a burden. –Jennifer Harnetty, editor.

Whoever said that oxidation glazes are boring?
Several years ago, I was forced through circumstances to use an electric kiln as my only firing source for my line of production stoneware. Since then, I’ve not only come to “put up” with electric firing and the challenges of a static kiln atmosphere, but also to appreciate the convenience of electronically controlled kilns, and the challenge of discovering ways of developing interesting and even exciting glaze effects.

I began by testing several commercial clays, and settled on a white stoneware (Miller 65) that was both durable and totally vitrified at Cone 6. Available through Laguna Clay Company, it’s good for throwing small-to medium-sized pieces, weighing 1 to 10 pounds.

I then experimented with ways to develop more interesting surfaces. I tried carving patterns into the clay to create places where the glaze could flow and pool. Another technique that I learned from a friend in California was to take slurry from my throwing bucket, run it through a 60- to 80-mesh sieve and apply it with an ear syringe for trailed-slip patterning.

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Next, I tried overlapping two and three glazes to create some movement on the surface. This worked wonderfully, but caused some irregularities where the glaze saturated the bisqueware and some running onto the kiln shelves when the glaze application was too thick. I found I could control the application thickness more easily and avoid running by spraying the second and third coats of glaze.

Now I use wood ash in and over my Cone 6 glazes to create visual interest, and the results are very exciting. I was fortunate to find a recipe that doesn’t require washing the ash before adding it to the glaze. Of course, this will make a glaze somewhat caustic, but I always wear surgical gloves when I glaze anyway, so this has not been a problem.

To prepare the wood ash, screen the dry ash (can come right out of your fireplace; any wood will do) through a 60- to 80-mesh sieve and add it to the glaze batch. Another way to achieve interesting effects is to simply sieve wood ash over the damp, newly glazed surface. I do this over a trash barrel. (CAUTION: ash is caustic, so always wear a mask and surgical gloves.) For the most dramatic effects, do both. Sieve the dry wood ash over the rim and shoulder of a pot that has just been sprayed with Frasca Wood-Ash Glaze over a dipped or sprayed base glaze, such as Green Dragon Matt Glaze Wood ash contains a good deal of calcium, as well as potassium, phosphorus, magnesium and sodium—all rather active fluxes in a glaze—so I limit the application of Wood-Ash Glaze to the top quarter of the pot.

Harry Spring operates Spring Pottery with his wife in Wickford, Rhode Island. To see more of his work, visit www.springpottery.com.