Glaze Defects and Their Remedies

There are many things that can go wrong with a glaze. Even after testing and compensating for the composition of local ingredients when using recipes from the Glaze Page or other countries, lots of things can go wrong. Here is a description of the most common glaze defects and some suggestions on how to fix them.

Crazing
One of the most common glaze defects is crazing, although in some cases crazing is a sought after effect, in which case it's called a Crackle Glaze. Crazing will occur when the glaze doesn't fit the clay body. The glaze shrinks more than the clay during the cooling, thus causing the effect. Crazing can occur after a piece has been fired. Often one will hear a bowl or platter making a so-called pinging noise long after firing. Crazing can also occur over centuries, as can be seen in many older wares, that would have looked OK initially.

Remedies:

• Add silica to the clay body
• Add silica to the glaze
• Add alumina to the glaze
• Soak the kiln

Pinholing
When small dots of unglazed or depressed areas appear in the glaze surface, this is called "pinholing". Pinholing occurs when gases in the glaze and clay bubble up to the surface. The gas bubbles pop and a 'hole' appears, which doesn't fuse over.

Remedies:

• Fire the glaze higher
• Soak the kiln
• Wipe all dust off bisque ware
• Spray bisque ware lightly with water prior to glazing

Blistering
When a glaze has a 'bubbled', lava-like surface, this is called blistering. Some so-called 'lava glazes' are specifically formulated to blister. Normally this effect occurs when gases released from the glaze become trapped in the glaze matrix before they can escape and the glaze smooth over. This can be due to underfiring or overfiring.

Remedies:

• Fire the kiln slower
• Reduce kiln atmosphere less

Crawling
The glaze pulls together and forms separate droplets on the surface of the clay body. Crawling occurs when the glaze contracts too much during the heating cycle and doesn't get sufficient a chance to smooth over. Another cause may be due to bad adhesion of the glaze, e.g. when glaze was applied to a very dusty pot.

Remedies:

• Substitute some of the clay content of the glaze with kaolin
• Substitute glaze fluxes such as zinc oxide and dolomite with their calcined form
• Clean off any dust with a sponge prior to glazing

Shivering
Shivering occurs when a glaze contracts less than the clay body. Parts of the glaze crack and move against each other lifting off from the clay surface. This can be so bad that pieces of glaze will actually jump off the surface.

Remedies:

• Reduce silica in the clay body
• Reduce silica in the glaze
• Reduce alumina in the glaze

Free glaze recipes are available at the Glaze Page!

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