Surface Defects: Blistering

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Last Updated: Thursday, 19 June 2014 19:43

Blistering is a glaze surface defect resembling crater-like objects. Blistering is a phenomenon in the glaze fire and is similar to pinholing, which was discussed in the last issue.

When your piece goes through the glaze firing process, the glaze melts to form a glass over the underlying clay body. During the melting process, the glaze literally will bubble and then boil as gasses vaporize. In successful fires, the firing program is long enough to allow out-gassing from both the clay body and the glaze to complete before the glaze cools. If the firing process ends too quickly or is too fast and stops during the boiling stage, blistering is the result. Essentially, the firing process ceased before the glass could smooth over and heal.

How to prevent blistering:

• For low fire work, Mayco recommends that bisque ware be fired 2 cones hotter than the glaze. For instance, clay should be bisque fired at cones 03/04 for a 05/06 glaze fire. This ensures that your bisque has been fired to a hotter temperature, so that when you bring your glaze up to temperature, your ware does not reheat to a level where additional out-gassing occurs.
• Blistering can also occur when glaze is applied too thick, trapping gasses. Be cautious when applying glaze where it may have a tendency to pool.
• Check out your firing program to make sure there is adequate length to allow the glazes to mature properly. Do not fast fire. You might consider adding a “hold” or “soak” in your firing program, which means keeping the kiln at temperature for an additional 10-15 minutes up to ½ hour.
• To repair blistering, you could refire your glazed ware at a longer temperature.

As always, we would recommend that you follow best practices whenever possible. If you have experienced problems or have technical questions, please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it..