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
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:
increase the silica, in body or glaze
decrease the feldspar, in body or glaze
decrease any other material containing sodium or potassium
increase the boron
5. increase the alumina, i.e. the clay content
increase lead oxide.
Shivering
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:
decrease the silica in either the body or the glaze
increase the feldspar, especially sodium feldspar or nepheline syenite, or other alkaline bearing materials.
Crawling
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 matte 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:
lengthen the firing cycle
apply the glaze less thickly
add more flux to the glaze to make it more fluid
decrease the content of zinc or rutile in the glaze
where zinc is used, try calcining half or all of the zinc content
hold the kiln at the glaze maturing temperature for a soaking period of up to two hours
cool the kiln slowly.
Blistering
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 Network Shop.
Search the Daily
Published May 15, 2024
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
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:
Shivering
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:
Crawling
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 matte 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:
Blistering
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 Network Shop.
**First published in 2013
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
Related Content
Ceramic Artists
Functional Pottery
Ceramic Sculpture
Glaze Chemistry
High Fire Glaze Recipes
Mid-Range Glaze Recipes
Low Fire Glaze Recipes
Ceramic Colorants
Ceramic Glazes and Underglazes
Ceramic Raw Materials
Pottery Clay
Ceramic Decorating Tools
Ceramic Kilns
Making Clay Tools
Wheel Throwing Tools
Electric Kiln Firing
Gas Kiln Firing
Raku Firing
Salt Firing and Soda Firing
Wood Kiln Firing
Ceramic Decorating Techniques
Ceramic Glazing Techniques
Handbuilding Techniques
Making Ceramic Molds
Making Ceramic Tile
Wheel Throwing Techniques