Crazing looks like a spiderweb of thin lines in the glaze over a piece of fired porcelain or pottery. You may find it on antique dinnerware, but it can also show up on contemporary pieces as a defect in the firing process or a deliberate technique to enhance the appearance of decorative pottery.
Causes of Crazing
Dinnerware is glazed to provide a non-permeable surface on bowls and dishes so food particles and liquids won't be absorbed by porous clay. Some glazes are part of the color patterns, and some are clear, painted over the design to fire into a glass-like finish in the kiln. Crazing happens when the glaze shrinks faster than the clay as the fired piece cools. It looks like very fine hairline cracks and will stain into more visible cracks if food stuff gets into breaks in the glaze surface. The crazing may be evident when the piece comes out of the kiln or develop years later as moisture gets into the clay and expands it, "breaking" the glaze. Reasons for the crazing could be:
- The kiln is opened too soon -- thermal shock -- before the clay inside has cooled sufficiently.
- The bisque -- the unfired hard piece of clay that is then glazed and fired -- was not fired long enough or at a high-enough temperature.
- The bisque and the glaze are incompatible; bisque and glaze react against each other and won't fuse.
- Dinnerware is placed in the dishwasher, where repeated heating and cooling can damage the bonds between clay and glaze.
- The underside of a piece is not glazed or only partly glazed -- dry-footing -- allowing moisture to enter the piece and expand it under the top glaze.
- An excessive glaze coat was applied to the piece.
- The natural aging process of glazed clay pieces can result in crazing.
Crazing is an intentional decorative technique applied by some potters. Art pottery and bespoke pieces may melt and fuse glass on pottery to create a fine web of crack lines. Raku pottery is a specific firing technique that can use a crackle white glaze on clay that is removed from the kiln and rapidly cooled, then plunged into a reduction chamber with flammable materials that burn, smoke and "destroy" the finish in interesting and sometimes unpredictable ways.
Concerns About Crazed Dinnerware
The inherited dinnerware or the collectibles with crazing may not belong on the dinner table. Old glazes contained lead, and really old dinnerware may have been lead-glazed for color or gloss before anyone knew it was toxic. Crazing is cracks and can leach that lead out into food, especially when the plates or bowls are used to serve acidic foods. The web of crazed lines may also harbor bacteria and be difficult to sterilize without further damaging the piece or its finish. When prized pieces show signs of crazing, retire them to a place of honor in the sideboard or on the wall.