Solving Glaze Defects Blisters, Craters and Pinholes

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 firing or decorating 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 materials is not fully removed from the body, then gas will form during the glaze or decorating firing, forming bubbles or blisters. These may pop to become craters or pinholes.

These defects can occur because:

  1. 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.

  2. The kiln was heated so quickly that there was not enough time for the carbon to burn out.

    Carbon which is only partially burned will continue to oxidize during the glaze or decorating firing causing defects.

  3. The ware was underfired. That is, there was not enough heatwork.

    When the body is underfired, it is weaker and it's expansion may no longer fit the glaze.

  4. There was not enough venting of the fumes in the kiln.

    If the gases produced during firing are not removed from the kiln, they may deposit onto the surface of the glaze or affect the color of 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.

Most critical for good results are proper firing practices. We recommend the following:

  1. Bring air into the kiln and make sure it circulates around the ware especially during bisque firings:
    • use setters and stilts to improve air flow around the ware
    • use half shelves to improve air flow through the kiln
    • adequately vent the kiln
    • position ware to take best advantage of air flow in the kiln

    Use a downdraft vent like the Orton KilnVent to bring a controlled amount of air into the kiln and circulate it throughout the kiln. This helps remove fumes and even out the temperatures in the kiln.

  2. Control the firing.

    Fire slower, especially below 1200°F (650ºC). 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.

  3. Use witness cones to verify heatwork.

    Underfiring can occur due to burned out heating elements, an improperly adjusted Kiln-Sitter®, a controller thermocouple which is drifting or differences in heating within the kiln. Witness cones gives a true reading of the heatwork the ware received.

    Witness cones placed throughout the kiln show differences in heat distribution.

  4. Vent the kiln to remove gases and prevent them from redepositing on ware. Only downdraft venting removes the gases from the kiln.

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

Want to learn more?

Read more about glaze and body fit, heat distribution and measuring heatwork in the Orton Firing Line and Technical Tips publications. Published 8 times a year, each issue is packed full of articles to help you learn more about firing. Members of the Orton Firing Institute receive these publications at no charge. Single copies are available to non-members at a per issue rate.

For information on Orton products, see your Orton dealer or distributor. For information on the Firing Institute or publications, contact: Orton
Orton Firing Institute
P.O. Box 2760
Westerville, Ohio 43086

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