Common Causes of Glaze Defects on Clay

production process of pottery. Application of glaze brush on ceramic ware.
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Glazes do not always come out the way we expect or hope they will. Sometimes there are happy surprises, but we can also experience real frustration when we open the kiln to find glaze defects.

Why do glaze defects occur? There are seven main reasons.

  • 01 of 07

    Overfiring and Underfiring

    Raku ceramics
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    The most common reason for glaze defects is either through underfiring or overfiring.

    Underfiring results in a dry, scratchy glaze surface. Pots that have been underfired can be fired again to a higher temperature, which may salvage the glaze.

    Overfiring results in glazes that begin to run. The glaze coat may be thinner at the top of the pot and thicker at the bottom. Glaze may even run off the pot and drip onto the kiln shelf or other pots. Seriously overfired pots may show ​pinholing and pitting as the glaze reaches evaporation temperature.

    Overfired pots cannot be salvaged. For this reason, it is always best to use glazes that have a maturation range of two to three cones.

  • 02 of 07

    Clay Body Fit

    Like clothing on humans, glazes need to fit the clay body they cover. Problems occur when a glaze is either too large or too small for the pot it is covering.

    During firing when the glaze is molten, the glaze and clay body fit perfectly with each other. That can change as the kiln and pots inside are cooling.

    Clay and glazes can have very different coefficients of expansion, the measurement of how much something expands when heated and contracts when cooled. Clay does not tend to contract as much as glazes do when cooling in the kiln. The glaze can become too small for the pot and may craze. Sometimes, however, the glaze does not contract enough. In this case, the glaze becomes too large for the pot and shivering can occur.

  • 03 of 07

    Application Problems

    Poor application of the raw glaze to the bisqueware can lead to various glaze defects.

    Applying glaze too thinly can result in rough glazes and can ​affect the glaze's color. Applying glaze too thickly can cause the glaze to run off the pot, weld lids to pots and pots to kiln shelves, and can result in blistering. Applying glaze unevenly may result in splotches and streaking in both color and texture.

  • 04 of 07

    Lack of Adhesion

    Poor application procedures can also lead to interference with the raw glaze's ability to adhere to the bisqueware. Bisqueware must be clean and dry before glazes are applied. If a second coating of glaze is used, the first coat cannot be too dry or the first coat will loosen away from the bisqued pot. Adherence problems often cause a glaze to crawl.

    Continue to 5 of 7 below.
  • 05 of 07


    Occasionally clear glazes can become slightly milky in appearance. Check the fired glaze with a magnifying lens to see if there are tiny crystals in the glaze. If so, and this effect is not desired, be certain to allow the kiln to cool down a bit faster but not overly fast or you will break your pots.

    You may also consider reducing crystal-forming materials in your glaze, especially zinc and titanium, and to a lesser degree silica and alumina.

  • 06 of 07


    Blebs are air pockets caught in the clay body. Although this is more of a clay body defect, it can also affect glazes, causing blistering, pinholing, and pitting. To avoid blebbing, make certain there are no air pockets in your clay. If it is a serious problem with a particular clay body, consider adding some grog to your clay or change clay bodies.

  • 07 of 07

    Kiln Accidents

    No matter how carefully made a pot is, how carefully a glaze is applied, accidents in the kiln can still occur.

    Kiln accidents such as kiln shelves breaking, pieces leaning and sticking together, and power interruptions, are all possibilities. Volatile colors, especially copper and chrome, may jump from one pot and stain the glaze of an adjoining pot. Glazes can drip from one pot to another.

    Pottery demands patience and the fortitude to keep working even though you realize that sometimes good work will be lost. Even so, the fulfillment of working with clay makes up for it all.