It is especially true in the ceramics world that one person’s fault is another person’s fancy–especially when it comes to glaze defects. Many ceramic artists deliberately create faults in their glaze surfaces to achieve a particular aesthetic.
But, of course, there are some cases in which a glaze must be perfect for reasons of safety or hygiene. So just in case glaze defects are driving you “craze-y” (sorry, I just couldn’t resist), I thought I would share this excerpt from the late Robin Hopper’s book, The Ceramic Spectrum. Read on to get some expert pointers on how to solve five of the most common pottery glaze defects (such as crawling, shown at above). – Jennifer Poellot Harnetty, editor.
Crazing is the most common glaze defect, and normally the easiest to correct. In both crazing and shivering the eradication of problems relies on matching the thermal expansion characteristics of both body and glaze. In practice, the most effective ways to correct crazing are:
1. increase the silica, in body or glaze
2. decrease the feldspar, in body or glaze
3. decrease any other material containing sodium or potassium
4. increase the boron
5. increase the alumina, i.e. the clay content
6. increase lead oxide.
Shivering is the most problematic of glaze defects, particularly in any functional ware, since slivers of glaze from the edges can drop into food or drink. These slivers are usually small, sharp, jagged-edged pieces that detach themselves from the body. It is the most dangerous of glaze faults. Shivering is the reverse of crazing, therefore the remedies are the opposite as well:
1. decrease the silica in either the body or the glaze
2. increase the feldspar, especially sodium feldspar or nepheline syenite, or other alkaline bearing materials.
Crawling is caused by a high index of surface tension in the melting glaze. It is triggered by adhesion problems, often caused by bad application. It occurs where a glaze is excessively powdery and does not fully adhere to the surface of the clay. This can be alleviated by the addition of a small amount of gum to the glaze batch. Crawling is more common in matt glazes than in fluid ones; sometimes the problems of crawling can be reduced by the addition of a small amount of extra flux. Crawling can also occur when one glaze is applied over another, particularly if the first is allowed to dry out completely before the second application. Some fluxes, particularly zinc and magnesium, are likely to cause crawling when used in excess. Calcining all or part of the zinc can help this problem.
Pitting and Pinholing
These are the most annoying and difficult glaze defects to cure. They can be caused by a badly controlled firing cycle, the glaze composition, or can originate with the body, particularly highly grogged clay bodies. The following remedies should be tried to cure pinholing or pitting:
1. lengthen the firing cycle
2. apply the glaze less thickly
3. add more flux to the glaze to make it more fluid
4. decrease the content of zinc or rutile in the glaze
5. where zinc is used, try calcining half or all of the zinc content
6. increase the maturing temperature of the glaze
7. hold the kiln at the glaze maturing temperature for a soaking period of up to two hours
8. cool the kiln slowly.
Blisters and blebs are usually the result of either an excessively thick application of glaze or incomplete clay preparation, wedging, blunging, etc. Sometimes, however, these faults can be due to overfiring or to the use of soluble fluxes in the glazes. The following fluxes might cause this problem: borax, boric acid, potassium carbonate, magnesium sulfate, and sodium carbonate. If these materials are present in a problem glaze, it would be well to replace them with other fluxing agents, or fritted materials.
This post was excerpted from the late Robin Hopper’s popular book The Ceramic Spectrum, which is available in the Ceramic Arts Shop.