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Firing Defects
Joe Molinaro/Eastern Kentucky University
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*Listed below are some common firing defects which appear after the
glaze-firing process. The student is advised to use this only as a
guide. Additional information about these, and other firing defects
can be found in many ceramic texts.
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BLACK CORING Blackened center of clay walls caused by reducing
iron-bearing clays too much. This can also make the clay brittle.
BLISTERING OR A more severe form of pinholing where bubbles, CRATERING
blisters and crater show the bare clay. Over-firing or firing too
quickly are often the cause of this problem.
BLOATING Bubbles in the surface of the clay which are often caused by
insufficient oxygen available to help remove all the carbonaceous
material from the clay. Also, overfiring, too low a bisque and clay
impurities can cause this problem.
BUBBLES Can be caused by the glaze firing too fast. Gases are trapped
in the glaze.
CRAWLING The contraction of the fused glaze causing bare, unglazed
areas. Common causes are due to improper glaze application, dust or
oil on bisque, over-fired bisque ware, thick glaze, or pots that are
too damp.
CRAZING A network of cracks in a glaze. Caused when a glaze contracts
more than the clay during the cooling cycle. Adjustments in the glaze
formula, such as increasing the silica or alumina, or decreasing the
feldspar or other soda or potash materials can prevent this. Proper
heating and cooling cycles can also reduce crazing.
DULL SURFACE A dry surface of glaze. Often caused by underfiring the
kiln, thin glaze application or glaze high in matt agents. DUNTING
Cracking in the clay which often occurs from either heating or cooling
the kiln too quickly. Opening the kiln too early should be avoided.
Also, dunting can be the cause of too much silica in the clay body.
EXPLODING Happens during the early stages of the firing and are often
caused by the water being removed too fast. Slow heating of the kiln,
especially in the early stages of firing, can prevent this.
FRACTURING Breaking of the ware because the glaze is applied too thick
on the inside and thin, or not at all, on the outside.
PEELING The contraction of a glaze, slip, engobe or underglaze during
the firing which can cause it to flake off. Applying these too thick
can ensure peeling.
PINHOLING Tiny holes in the fired glaze surface. Can be caused by
incorrect firing, poor clay preparation or improper glaze application.
RUNNING Glazes that move downward on the ware during firing. Glazes
applied too thick or overfiring the kiln can cause running.
SHIVERING Glaze expanding more than the clay. Lowering the silica or
increasing the alkaline oxide amount will help prevent this.
WARPING Happens when there is a variation in the fired shrinkage or
clay density as well as poor stacking in the kiln (overhanging pots in
the flame path). Prevention is achieved by constructing with even wall
thickness, slow firings and correct kiln stacking.
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