Techno File: Glaze-to-Ware Bond

There are many recipes for glazes that are designed to crawl. Unfortunately, glazes may crawl when that is not the artistic intent. Here’s a troubleshooting guide to fix crawling.

Definitions

Glaze Crawling: A fired glaze that has pulled apart, leaving islands of thickened glaze with the exposed bare clay surface between them. Crawling occurs while the glaze is molten and results when the surface tension of the liquid glaze is stronger than the bond between the glaze and the clay it has been applied to.

Specific Gravity: A unitless measure of the density of a substance divided by the density of a reference fluid. For liquids and solids the reference fluid is almost always water.

Surface Tension: The attraction force between molecules at the surface of a liquid. This tensile force works against the attraction force between the molecules of the liquid and the surface the liquid is on.

Science

Glaze shrinkage is the fundamental cause of glaze crawling. Keeping this in mind makes it much easier to arrive at the best solution to a particular crawling problem.

Glaze shrinkage is a two-step process. First, the applied glaze shrinks as it dries. Step two is the glaze shrinkage that occurs when the glazed ware is fired. Except with single firing (when glaze is usually applied at the leather-hard stage), the ware itself does not shrink as glaze that has been applied to it dries. Thus, drying shrinkage of a glaze on a surface that isn’t shrinking necessarily puts the glaze in tension. If this tension is too great, the glaze surface will crack. In severe cases, the surface of the dried glaze will appear like a miniature version of cracks in dried mud. Fine cracks on the surface of the dried glaze are often difficult to see. Under strong side lighting, examine the glaze surface carefully with a 10X magnifying glass to search for visible cracks.

Glaze cracking upon drying falls into one of two categories, either glaze-to-ware bond or excessive tension in the glaze itself. An ideal application produces a bond between the ware and the dried glaze that is sufficiently strong to hold the glaze in place during any further decoration as well as during handling and firing.

Problematic drying shrinkage is most often the result of a combination of factors that can include the glaze recipe, its chemistry, or its application. An entire book could be written on the possible sources of these problems. Too much clay in the recipe, along with glaze chemistry and application that permit or cause the glaze to be too thick are the most common culprits.

Firing shrinkage is seldom the cause of glaze crawling, except in single firing or with recipes that are created with the intention of achieving a crawling effect. Additions of more than 10% magnesium carbonate are common in glazes intended to crawl. Note: One way magnesium carbonate leads to crawling is that it must make a high-surface tension melt that overcomes the liquid to substrate (ware in this case) bond. Ball clays, which consist of smaller clay particles than kaolins and thus shrink more; nepheline syenite, a sodium-rich mineral; and magnesium carbonate were combined by Lana Wilson and others to produce consistent crawling glazes. Wilson’s Ball Crawl glaze (60% nepheline syenite, 22% magnesium carbonate, 18% OM-4 ball clay) adds the effect of a high-shrinkage clay to the effect of magnesium carbonate on the melt. She still used 22% of magnesium carbonate, so it’s a traditional magnesium carbonate crawl glaze in that respect.

Boron is the most common flux in low-fire glazes; potassium and sodium are the major high-fire glaze fluxes. As the graphic shows, all reduce glaze surface tension and thus reduce the tendency for glaze crawling. Adapted from: The Potter’s Dictionary of Materials and Techniques by Frank and Janet Hamer.

A) Crack caused by shrinkage of the drying glaze. B) As the glaze starts melting in the kiln surface tension draws the molten glaze away from the crack. C) Higher glaze surface tension than the glaze-to-body bond pulls the glaze away, exposing bare clay. Adapted from: Glazes—for the Self-Reliant Potter by James Danisch and Henrik Norsker.

Studio Takeaway

The easiest glaze crawling problem to fix is the one caused by firing a kiln with work that has damp glaze. Make sure the glaze surface is bone dry (does not feel cool to your cheek) before firing.

Dust, oils, or other soiling of the ware surface can be, but seldom are, the cause of a poor glaze-to-ware bond. Simply rinsing or dunking ware in a tub of water should be sufficient to remove dust that may have accumulated on the surface. If a piece has been handled a great deal, or with hands to which an oily moisturizer has been applied, washing the piece with warm, soapy water may be necessary.

Rinsing porous bisqueware is a good way to simultaneously get rid of dust that can cause glaze crawling and reduce the thickness of the applied glaze. If the ware is already damp before glazing, it doesn’t absorb as much water from the glaze. That causes a thinner glaze coat to be left on the ware.

Crawling can be problematic when applying two coats of glaze. Ideally the second coat will have less water (be denser), have less clay so it shrinks less, and dry faster than the first coat.

Excessively thick glaze application on a dry ware surface is a frequent cause of crawling. It’s also often a factor in pinholing and other surface faults. To discuss all the possible reasons a glaze will go on too thick would use much more space than is available here. That said, here is a good rule of thumb to maintain proper glaze thickness: the glaze slurry should have a specific gravity between 1.4 and 1.6 and lose its wet sheen within 10 seconds or less after being applied to ware.

the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.

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