Five Steps to Stop Glaze Shivering

by Jeff Zamek

Note: The following is excerpted from the Ceramics Monthly of October 1995. For a reprint of the full version, call 413 527 7337 or write fixpots@aol.com.

Though statistically not as common as other defects, shivering is a severe glaze problem that occurs when a glaze is under too much compression. The fired glaze looks like a paint chip peeling off the underlying clay body. When shivering is very severe (glaze under extreme compression), it can tear or break the underlying clay body, causing the whole pot to crack apart upon cooling.

Shivering can develop as the piece cools or sometimes years later! Essentially when the glaze is under extreme compression, it might "buckle'' at any given time.

Shivering and crazing are at opposite ends of the same basic problem: The glaze and clay body do not fit when cool. Crazing happens when the glaze is under extreme tension. [See "Eight Steps to Stop Crazing" in the April 1995 CM.] Interestingly, ceramic materials fail ten times faster under tension than compression. Correspondingly, crazing (glaze under tension) is ten times more prevalent as a glaze defect as compared to shivering. Recognizing and understanding a problem are the first steps in solving any glaze defect.

Shivering can occur at any temperature range, in oxidation or reduction. Frequently, when a glaze does shiver or peel off the fired clay surface, it is on the pot's edges or raised areas. The chip size can range from 1/16 inch to more than 2 inches in size. With any correction, the goal should be to have the clay and glaze cool at a compatible rate, with the glaze coming under slight compression as it cools.

Although shivering is classified as a glaze defect, it can be corrected through adjustments to the glaze recipe, the clay body recipe or a combination of the two. Several points must be considered before attempting to fix a glaze shivering defect: Clay bodies containing too much free silica can cause shivering; fireclays, as a group, are known to have randomly high levels of free silica. Fine grog high in silica can also cause shivering, especially if burnishing has brought it to the clay surface in the forming process.

The following five steps are recommended to correct shivering. The first involves adding high-expansion materials (feldspars or other alkali-bearing materials) to the clay body and/or glaze. Decreasing low-expansion materials (flint) from the clay body and/or glaze also must sole the problem. Sometimes a combination of both methods will be necessary.

1. If only one glaze is shivering on the clay body, try additions of 5, 10 or 15 parts potash feldspar to that glaze.

2. Decreasing the flint in a glaze by 5 or 10 parts will also adjust the clay body/glaze fit.

3.Occasionally, adding feldspar/frit and removing flint will be necessary to stop shivering.

4.If many different types of glazes are shivering on the same body; correct the problem by adding 5, 10 and 15 parts feldspar (or other alkali-bearing materials) to the clay recipe.

5. A decrease of 5 or 10 parts flint in the clay body may also correct glaze shivering.

In most instances, shivering can be corrected by additions of feldspar, frit or other high-expansion materials to the glaze. If the problem persists, the solution is to adjust the clay body recipe or change to another clay body altogether.

Note: The preceding was excerpted from the Ceramics Monthly of October 1995. For a reprint of the full version, call 413 527 7337 or write fixpots@aol.com.

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Eight Steps to Stop Crazing

by Jeff Zamek

Note: The following is excerpted from the Ceramics Monthly of April 1995. For a reprint of the full version, call 413 527 7337 or write fixpots@aol.com.

It's important to understand glaze theory if you want to solve any glaze problems. There is no shortcut to knowledge in ceramics. What most potters would prefer, however, is a step-by-step guide for solving common problems. When you're lost, you don't need to know how to build a road; you need to know which road to take to reach your destination. This guide is meant to help potters who are troubled by crazed glazes find their way.

Crazing is a network of lines or cracks in the fired glazed surface. It happens when a glaze is under tension. A craze pattern can develop immediately after removal from the kiln or years later.

The strongest, most stable glazes are under compression.

Although crazing is considered a glaze defect, it can also be corrected by adjusting the clay body. A glaze adjustment might not be possible if it is under so much tension that there is no room in the recipe for correction. The goal should be to have both glaze and body shrink at a compatible rate, with the glaze coming under slight compression.

Before starting, let's consider the following points:

If the craze pattern is tight (lines spaced less than 1/8 inch apart), the degree of difficulty in eliminating crazing is increased; the closer the lines, the harder the fix.

If the clay body has a high absorption rate (over 4%) after firing, chances of correcting the crazing are also low.

If you have tried several corrections with no success and the result you want is fairly common (i.e., clear gloss, satin matt, etc.), try another glaze recipe.

If the glaze is unique and cannot be changed, try another clay body (perhaps a simple Cone 06-04 white clay made from 50 parts ball clay, 50 parts talc and 3 parts whiting; the whiting helps keep a lot of glazes from crazing).

With these four points in mind, you are now ready to take a corrective step, or a combination of steps to solve glaze crazing.

1. Crazing can often be eliminated simply by applying a thinner glaze coat. With some glazes, a thinner coat is not an option, but often a slight decrease in glaze thickness will stop crazing.

2. Add increasing amounts of flint (without changing the amounts of the other ingredients) to the recipe; the finer the mesh, the better.

3. Fire the glaze kiln to the correct cone over a longer period of time.

4. Fire one or two cones higher, but only if the glaze will not be adversely affected. By firing higher and/or longer, the glaze and clay body might fit better.

5. Add flint (200 mesh) to the clay body. Increase the flint content by increments of 5%, 10% and 15%.

6. Slow cool the glaze kiln. Do not open the door until temperature is below 200C (390F). You should be able to unload the kiln bare handed.

7. If you're using a low-fire body and the glaze is crazing, try bisque firing one or two cones higher.

8. If you're using a fritted low-fire glaze and it's crazing, try using a frit with a lower coefficient of expansion.

While the eight steps listed are not the only ways of correcting crazing, they have consistently shown good results.

Note: The preceding was excerpted from the Ceramics Monthly of April 1995. For a reprint of the full version, call 413 527 7337 or write fixpots@aol.com.