Why do glaze defects occur? There are seven main reasons. 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. 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. 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. You may also consider reducing crystal-forming materials in your glaze, especially zinc and titanium, and to a lesser degree silica and alumina. 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.
