Expert: Leisha Hiester - 7/22/2009

Question
I am experimenting with stoneware glazes with crystal overlays.  However I am getting a high level of cracking in the cooling process (40%) despite cooling verrrry slowly.  They are being fired in an Axner Super kiln to cone 10, dropped to 2200 and held 3 hours and then cooled naturally.  Kiln is closed until 150f is reached.  Can you recomend changes in the glaze or firing process that may reduce this problem?

Answer
Steve,
How about this from the real experts at CeramicsArtsDaily; hope it's helpful. Leisha


How to Correct Five Common Ceramic Glaze Defects


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 couldn't resist), today Robin Hopper gives some expert pointers on how to solve five of the most common pottery glaze problems (such as crawling, shown above). - Jennifer Harnetty, editor.

Five Common Ceramic Glaze Faults and How To Fix Them

by Robin Hopper

Crazing

Crazing is the most common 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

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.
 


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Take advantage of Robin Hopper's years of experience in the studio with the Robin Hopper Collection: The Ceramic Spectrum, Functional Pottery and Making Marks.

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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 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 flaws 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

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.
 
To see example images of each of these glaze defects, hop online!  

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