Close-up of blisters. (Photo courtesy of Jeff Zamek.)

Blisters, craters and pinholes are related glaze surface defects. They show up as a rough, grainy or bubbled surface on the ware and appear after the glaze or decoration firing.

What Causes these Defects?

This family of problems can be caused by many different factors, including:

• dust and contamination in the glaze
• air bubbles in the glaze
• air trapped in the slip
• improperly mixed slip
• a dirty kiln

Most commonly, however, the problems are related to gases coming from the body, glaze or kiln atmosphere.

What Happens During Firing?

Clays and glazes contain organic materials. When heated, these burn out of the body, forming gases such as carbon, sulfur and water. If the carbon in the organic materials is not fully removed from the body, then gas will form during the glaze or decorating firing, causing bubbles or blisters. These may pop to become craters or pinholes. These defects can occur because:

• There was not enough air in the kiln during firing for the carbon to properly burn out. Any combustion process requires air. Without air, oxidation cannot occur.
• Carbon monoxide formed by the oxidation of carbon has not been adequately removed from the kiln. If the gases produced during firing are not removed from the kiln, they may deposit onto the glaze surface or affect the glaze color.
• The kiln was heated so quickly that there was not enough time for the carbon to burn out. Carbon that is only partially burned will continue to oxidize during the glaze or decorating firing, causing defects.
• The ware was under-fired (did not receive enough heat-work). When the body is under-fired, it is weaker and its expansion may no longer fit the glaze.

How Do I Solve Glaze Defects?

To make sure that glaze defects do not occur, it is important to properly mix glazes and slips, and to use good pouring (slip) and application (glaze) techniques. Proper housekeeping for the kiln and workplace should be observed. Straining glaze through nylon often helps remove any lumps.

Proper firing practices are critical for good results. Bring air into the kiln and make sure it circulates around the ware, especially during bisque firings. Use setters, stilts and half shelves to improve air flow around the ware and through the kiln. Adequately vent the kiln, and position ware to take best advantage of the air flow in the kiln. Use a downdraft vent to bring a controlled amount of air into the kiln and circulate it throughout the kiln. This helps remove fumes and even out the temperature in the kiln.

Control the firing and fire slower, especially below 1200°F (650°C). Slow down the firing by adjusting switches to lower settings or soak/hold at a temperature to allow carbon to burn out. Use an automatic controller to set heating rates and hold times.

Use witness cones to verify heat-work. Under-firing can occur due to burned-out heating elements, an improperly adjusted KilnSitter^TM, a controller thermocouple that has changed, or differences in heating within the kiln. Witness cones give a true reading of the heat-work the ware received. Witness cones placed through the kiln show differences in heat distribution.

Vent the kiln to remove gases and prevent them from re-depositing on ware. Only downdraft venting removes the gases from the kiln.

If good firing and venting practices are observed, problems with glaze surface defects can be controlled.

Editor's note: This information has been provided by The Edward Orton Jr. Ceramic Foundation. For additional information, visit www.ortonceramic.com.