In the Studio: Glaze Faults, or Finds? Richard Zakin and Frederick Bartolovic
Appears in the Jan/Feb 2022 issue of Pottery Making Illustrated.
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To understand glaze faults, you must understand that while many ceramic artists deeply dislike unstable glaze surfaces and are distressed when pieces with such surfaces come from their kilns, many others seek out these characteristics.
While firing in the electric kiln generally produces stable, durable results, it is in the nature of ceramics that you will encounter unstable glazes at one time or another as you pursue your work.
Some common faults include crawling, crazing, and running.
Crawling
Glaze crawling is marked by the coagulation of the glaze on the surface of the form. The glaze pulls away, creating clumps of melted glaze in conjunction with bare spots revealing the clay underneath.
Avoiding Crawling
Achieving Crawling
Most ceramic artists who use crawling glazes apply them to a surface already prepared with a base slip or glaze. In this way the color revealed by the crawling is not that of the clay body, but rather of the base coating of slip or glaze.
Crazing
Crazing develops during the cooling period if the glaze shrinks more than the clay body. When this happens, the brittle glaze becomes stretched and stressed. It reacts to these stresses by pulling apart. The result is numerous, very fine, small cracks. The cracks create a web work of fine lines that form a pattern. Caution: Crazing weakens a glaze and makes it unsuitable for food use because the hairline cracks will harbor bacteria that cannot be washed away.
Avoiding Crazing
Achieving Crazing
Crazing patterns can be pleasing to some ceramic artists, who try to create crazing rather than avoid it.
Running
Running is the excessive flow of the glaze during the firing. Glazes can flow so much in an uncontrolled manner that they weld a piece of ware to the kiln shelf. Alternatively, running can be controlled with testing and experience. Ceramic artists who wish to achieve positive results with running glazes sometimes place their work in front of the kiln peep hole so they can watch the glazes melt at top temperature, and then turn the kiln off at the appropriate time.
Avoiding Running
Achieving Running
Some ceramic artists welcome the (un)controlled flow of the glaze during the firing. Glaze flow of this sort can be very striking as the piece can be wreathed in overlapping sheets of color.
You must exercise great care when using highly flowing glazes. These glazes may flow so much that they will fuse the piece to the kiln shelf, ruining both the piece and the shelf. If you use a highly running glaze, protect the shelves by placing the ware on a piece of broken kiln shelf or on a soft-brick firing pad (to absorb the flowing glaze). To encourage running:
Excerpted from Electric Kiln Ceramics: A Guide to Clays, Glazes, and Electric Kilns, Fourth Edition, by Richard Zakin and Frederick Bartolovic, published by The American Ceramic Society and available at https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqTEAQ.
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