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. We (Lakeside Pottery) know of cases where the "pinging" sounds of newly developed crazing lines go for many years.
Generally, crazing is considered a glaze defect because the vessel can be significantly weaker than an uncrazed pot. Craze lines can also harbor bacteria or germs. Therefore, dinnerware pottery should be uncrazed ware.
There are cases where Crazing is a desired effect like in the pictures on the left (click to enlarge). The effect in the top picture was accomplished by melting and fusing glass on the pot. For more about firing glass with pottery, visit Combining Glass with Pottery . The effect on the bottom picture is by using a raku setup with a crackle white glaze that cracks by the rapid cooling before entering the reduction / smoke chamber.
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.