Porcelain is a dense ceramic popular in the decorative arts. Crazing is a web of small or large cracks across the surface of the porcelain. The cracks are in the glaze and not the actual porcelain so the piece remains intact. Crazing in porcelain is caused by a glaze imbalance during the firing process, extreme age or because the piece was buried under the ground. Crazing can be prevented in several ways depending on how it occurred in the first place.
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Choose the right kind of glaze for the right kind of clay. There are a number of different recipes for both glaze and clay. It may take a little experimentation to reduce or eliminate crazing from the custom made mixes.
Fire the porcelain in a bisque fire before final firing. A bisque fire is a preliminary firing where the water content of the clay is chemically altered creating a stronger clay. There is no glaze on the pottery during a bisque fire.
Refire a crazed piece to heat up and redistribute the glaze. Set the kiln at a higher temperature and then fire for longer than the original firing. The hope here is that the glaze will melt down and the crazing lines will disappear.
Leave glazed pottery pieces in the kiln until they have completely cooled. Removing the glazed porcelain when the surface is still hot and putting it into contact with cold air can cause crazing.
Make glazes that consist of at least ten per cent Alumina. This can be in the form of feldspar or clay.
Paint a thinner layer of glaze onto the pottery. In a lot of cases crazing occurs on thickly applied glaze because the density reduces the elasticity of the glaze. When thinly layered glaze crazes it is most likely an issue with the clay underneath the glaze.
Coat both the inside and the outside of the porcelain with glaze. This creates a balance in the compound and prevents crazing. This method is more likely to succeed on thin porcelain items of less than a quarter inch thickness.
Tips and warnings
- Experiment with glaze and clay compounds on tiles rather than actual porcelain pieces. It is better to lose a four inch porcelain tile to crazing than a custom ordered vase.
- Kilns are extremely hot and should only be used by qualified competent adults.
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