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.
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.
- Your glaze has too much clay. Most glazes contain clay to help keep it in suspension and discourage running. If the glaze contains more than 20–30% clay, however, it is likely to shrink and if it shrinks too much, it will crawl. To avoid crawling, substitute calcined clay for most of the clay in the recipe.
- Magnesium carbonate encourages crawling, so limit magnesium to less than 15% of the recipe.
- Crawling can also occur when the glaze layer is very thick or when the piece is very dusty. Before glazing, wipe away any dust.
- Crawling will occur when a thick, wet glaze is applied over a glaze that is very dry. If the glaze is very thick, thin it with water. If you are using multiple layers of glaze, let the first glaze set but not completely dry before applying the next one.
- Crawling often happens when recently glazed ware is placed directly into the kiln for immediate firing. The glazes on the surface of the form dry unevenly and peel back as they are rapidly dried during the heating cycle of the firing. It is best to allow glazed work to fully dry prior to placing in the kiln for firing.
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.
- Use glazes that contain a lot of clay. A high-shrinkage clay such as ball clay is recommended. If the glaze contains 20–30% of such a clay, it is likely to shrink and crawl.
- Magnesium carbonate encourages crawling; use it in amounts over 15%.
- Thick, heavily applied layers of glaze will crawl. Mix glazes on the thicker side and apply them in thick layers.
- Sprinkle the bare, bisque-fired piece with dry clay powder to create a dusty surface. On such a surface, the glaze cannot attach itself to the clay body.
- Apply a wet glaze over a very dry glaze. Here, too, the glaze cannot attach itself to the clay body. Use multiple layers of glaze and let the first glaze dry completely.
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.
- Use glazes high in materials containing silica such as ground silica, feldspars, and frits. Silica strengthens the glaze and discourages contraction during the cooling process.
- Add small percentages of titanium or rutile, which strengthen the glaze and discourage crazing.
- Avoid materials with large amounts of sodium such as soda frits and soda feldspars. They contract in the cooling process, which encourages crazing. Use boron frits and potassium feldspars instead.
Crazing patterns can be pleasing to some ceramic artists, who try to create crazing rather than avoid it.
- Use glazes that are high in melting materials that contain sodium. Avoid materials that contain silica such as ground silica, feldspars, and frits. Also avoid materials that contain titanium or rutile because these can discourage crazing.
- Try accenting the visual pattern of crazing by rubbing a watery solution of India ink over the finished form. Then wipe away any ink left on the surface. The ink should settle into the crazing lines, making them more pronounced.
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.
- Avoid the use of highly fluxing materials such as frits, and increase the silica and alumina content of your glaze.
- Make sure your glaze recipes contains 45–75% feldspar and at least 8% clay. Feldspars begin to flux naturally in the mid- and high-fire ranges, but not in the low-fire range.
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:
- Make sure your glaze recipe contains little silica and alumina.
- Use less than 45% feldspar or frit and less than 5% clay.
- Make up the rest of the recipe (to add up to 100%) with such materials as magnesium carbonate (above cone 7), dolomite, and whiting. Exact proportions will need testing and depend on the exact temperature you are firing to.
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.