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Craters & Blisters

Glaze Surface Defects


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Blisters, Craters and Pinhole problems?  

Some glaze defects can be very difficult to diagnose.  Blistering appears as a pronounced, sharp-edged burst bubble and looks like a crater on the fired glaze surface, often revealing the underlying bisque pottery or clay body and sharp edges around the blister.  Pinholes result in tiny craters with soft edges; blisters result in slightly larger craters with sharp edges.  Go to our pinholes article if you believe you may have this issue.

Blisters can also show up as a rough, grainy, blistered, bubbled, cratered or surfaces with "holes" or bald patches in the glaze following firing.  Sometimes the glaze or fired piece looks like it is covered in large bubbles similar to honeycomb.  

If there is blistering over your underglaze decoration then potential causes can be:

Potential causes:

  • Insufficient drying of the piece between glazing and firing.
  • Excessive underglaze or glaze application, or both. Underglazes and glazes move through a boiling phase prior to smoothing out to a glossy glaze.  If application of the glaze/underglaze is too thick then gasses may get trapped.
  • trapping gasses.  Check the viscosity of your clear glaze.
  • Over firing (the most common types of blisters are apparently caused this way).
  • Rapid firing cycle (this does not allow the glaze to mature).
  • Rapid cool down.
  • Insufficient soak time (increase soak time up to 30 minutes and consider holding the soak at a slightly lower temperature).
  • Too much direct heat (move pottery away from edges of batts/shelves and away from the direct heat of elements).  Usually, the blisters will appear on one side of the pot or fired item if too much direct heat is the cause of the problem.
  • Incompatibility of glazes.  If the blisters appear only on overlapping glaze surfaces, then it could be due to an incompatibility of glazes.
  • Kiln issues.  If the blisters are only present in only one part of the kiln, then it could be due to irregularities in the kiln firing e.g. heat problems.  Try firing with Pyrometric cones on each shelf to see if this highlights any heat problems or irregularities.  There may be differences in temperature from top to bottom in their kiln. 
  • To repair blistering, you could experiment by slowly re-fire your glazed ware (i.e. increase the cycle time).

Orton Ceramics have produced a PDF on Firing Tips  CLICK HERE to view

We like this explanation from about.com: 

Blistering is a glaze defect similar to pinholing and pitting. As the glaze melts, gases vaporize out of the clay and glaze. In essence, blisters are the result of the glaze being frozen in the midst of boiling.

In most instances, this out-gassing is completed and the glaze flows back together to form a smooth surface by the time the kiln has reached temperature. However, too much gas or too viscous a glaze may interfere with all the gas being released in time for the glaze to smooth back out. In such cases, blistering is likely.

Reasons for Blistering

The most common reason for blistering is an overly thick application of glaze, especially if the glaze has pooled at the bottom of the pot. Firing too hot or too quickly at the end of the cycle can also contribute to glazes blistering.

Certain ceramic materials are also prone toward blistering. Lead is especially sensitive to atmosphere and any lack of oxygen in the kiln will cause it to gray out the colors, blacken, and blister. Borax, potassium carbonate (and carbonates in general), magnesium sulfate, soda ash, and fluorine can produce large amounts of out-gassing.

http://pottery.about.com/od/glazeproblems/tp/blister.htm