Six Easy Steps to Stop Glaze Shivering
November 5, 2008
Though not as common as some other glaze defects, shivering is a severe problem that occurs when the glaze is under too much compression as it cools in the kiln. When the glaze is in the liquid state during the firing process, it fits the clay body like warm honey; however, when the glaze cools, it compresses at a greater rate than the underlying clay body.
Shivering looks like a paint chip and can range from 1/16 in. to more than 2 in. Frequently, when a glaze peels off, it does so at the pot’s edges or raised areas of the pottery. With any correction, the goal should be to have the glaze and clay body cool at compatible rates, with the glaze coming under slight compression.
Shivering can happen when the pots are removed from the kiln or sometimes years later. It can occur in hydrocarbon-fueled kilns, such those fired by natural gas or propane, when a reduction atmosphere (excess fuel-to-air ratio in combustion) is introduced too early or too intensely in the firing, causing an unstable carbon bond between the clay and glaze layer. Glaze shivering can also take place in an oxidation electric kiln atmosphere at any temperature range.
In addition, clay bodies containing too much free silica can cause shivering; specifically, fireclays (as a group) are known to have randomly high levels of free silica. Fine grog that contains high levels of silica can also be a potential cause of shivering, especially if burnishing has brought it to the clay surface in the forming process. Sometimes tapping the suspect pot on its edge can induce the glaze to flake off. Thicker glaze applications are more likely to encourage shivering if any or all of the previous conditions are present. Conversely, a thinner glaze application can inhibit shivering, but it does not cure the underlying cause of the problem (too much compression in the fired glaze).
It is important to note that if one clay body and glaze shivers, all of the other pottery with the same clay body/glaze combination should be suspect of potential shivering.
Although shivering is classified as a glaze defect, it can be corrected through adjustments to the glaze formula, the clay body formula or a combination of both. The following six steps are recommended to correct shivering.
1. If only one glaze is shivering on the clay body, try additions of 5, 10 or 15 parts of feldspar or frit, whichever is contained in the original glaze formula (i.e., if the glaze has 10 parts feldspar, increase that amount to 15, 20 or 25 parts). Do not change the amounts of other materials in the glaze. Other alkali-bearing materials, such as frits, can be used to correct shivering, but keep in mind that frits can lower the melting point of a glaze. Adding too much of any flux or glass former will increase the chance of the glaze becoming glossy or running off vertical surfaces. The ideal fix is to put just enough feldspar or frit into the glaze to correct shivering without overloading the glaze with more flux than is needed.
2. Decreasing the flint in the glaze by 5 or 10 parts will also adjust the clay body/glaze fit.
3. Occasionally, adding feldspar/frit and removing flint will be a necessary to stop shivering. In rare instances, the same correction must be carried out in the clay body.
4. If many different types of glazes are shivering on the same clay body, correct the problem by adding, 5, 10 or 15 parts feldspar (or other alkali-bearing materials) to the clay body.
5. A decrease of 5 or 10 parts flint in the clay body may also correct glaze shivering.
6. If a commercial glaze shivers, it is often easier to find another glaze that looks the same, or use a different clay body with the expectation that an unrelated combination of glaze and/or clay will achieve a better fit.
Shivering can occur any time a glaze is under extreme compression. In the examples shown here, notice how the glaze peels away, exposing the underlying clay body.