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What is Crazing?
Crazing presents itself as a series of lines or cracks in the fired glazed surface. The craze pattern can develop upon removal from the kiln or even years later. Crazing happens when a glaze is under tension. The most stable glazes are under slight compression. While crazing is classified as a glaze defect, it can also be corrected by adjusting the clay body. The goal is to adjust the glaze and clay body to cool at a compatible rate with the glaze coming under slight compression. Listed below are some important considerations:
- If craze lines are tightly packed and close together (spaced less than 1/8 in. apart), the chances of eliminating the lines by this method are decreased. The glaze will have to be recalculated by using glaze calculation software. This is a significant indication of the degree of difficulty in fixing the problem. “The closer the lines, the harder the fix.”
- If the clay body has a high absorption rate (more than 4%) after firing, the chances for correcting the crazing are low.
- If several corrections have been tried with no success and the glaze is common (i.e., gloss transparent, satin matt, matt, gloss blue, black, brown, etc.), try another glaze formula in the hopes of arriving at a better glaze fit on the clay body.
- If the glaze is unique and cannot be changed, try another clay body. (A common low-fire white, c/06 - c/04 clay body formula is: ball clay 50, talc 50, whiting 3. Whiting in the clay body prevents crazing in many glazes.)
After considering the above points, go on to a correction (or a combination of corrections) to solve glaze crazing.
1. Crazing can often be eliminated through a thinner glaze application. With some glazes, a thinner coat is not an option, but often a slight decrease in glaze thickness will stop crazing. However, this does not address the actual cause of the problem.
2. Add increasing amounts of flint to the glaze formula, the finer the mesh the better. Most flint used in glazes comes in 200, 325 and 400 mesh. Finer grind sizes might be available on special order from a ceramics supplier. Also, try fused silica, a calcined silica with a very low shrinkage rate. The low shrinkage rate helps stop crazing. Most glaze formulas have some room for increasing the flint without the glaze becoming opaque or dry when fired. Try additions of 10, 20 and 25 units of measure (i.e., if the glaze has 50 grams of flint, increase flint to 60 grams, 70 grams and 75 grams). Do not change the other glaze materials.
3. Fire the glaze kiln to the correct cone over a longer time. During the last quarter of the glaze firing cycle, try stretching out the firing by two or three hours. This will give the clay body the best chance to “tighten up” or reach its maturity, which will help in achieving a good glaze fit.
4. Fire the glaze kiln one or two cones higher, but only if the glaze will not be adversely effected. By firing higher and/or longer, the glaze and clay body might be adjusted into a better fit. Remember, what has to change is the rate of shrinkage in the clay body, glaze or both, which results in the glaze being under slight compression. However, if the clay body is already over-fired or on the edge of its maturity range, firing higher will cause more crazing in the glaze.
5. Add flint (200 mesh) to the clay body. Increase flint by 5, 10 and 15 units of measure. Flint found in clay bodies remains a crystalline solid that has different characteristics than flint in a glaze, but it will still work to stop crazing in a glaze.
6. Slowly cool the glaze kiln. Do not open the kiln door until the temperature is below 200°F. The kiln should be cool enough to unload without gloves. Waiting for the kiln to cool will cause no problems, while fast cooling increases the chance of crazing. If the pots are “pinging” when the kiln door is open, the glaze is under stress and is more likely to craze.
7. If you’re using a low-fire body and the glaze is crazing, try bisque firing one or two cones higher than the glaze firing temperature. This might bring the clay body/glaze into a better fit.
8. If the glaze contains frit and is crazing, try using a frit with a lower coefficient of expansion. The ceramics supplier or the manufacturer of the frit will have the coefficient of expansion rates for each frit. Materials with low coefficients of expansion (flint) are less likely to cause crazing. Other high coefficient of expansion materials can also cause crazing in the glaze. These can be substituted with lower coefficient of expansion materials, but this process is best accomplished through a more complex glaze calculation method.
Frequently a combination of methods will work, depending on the severity of the craze problem. Being flexible in your thinking is important while evaluating the test results. If the results show that the craze lines moving further apart, continue with the corrections. If the craze lines are getting closer together, or staying the same, try something else. While these eight steps are not the only corrections for glaze crazing, they have consistently shown positive results.