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Overfiring can result when any glaze is taken past its maturation temperature, and lower-melting-point oxides within the glaze volatize. The effect is similar to water taken past its boiling point.
Correction: Firing the glaze one or two cones lower will bring it into its maturing range.
An excessively long firing in the glaze maturing range can cause volatilization of oxides, resulting in blisters. A longer time to temperature imparts additional heat work in the glaze even if it is taken to its correct maturating temperature.
An excessively long cooling cycle in the glaze kiln contributes more heat work when the glaze is in the molten state, causing oxides to boil in the liquid glaze. Similar results can occur in over-insulated kilns that allow the glaze to remain in its maturing range for extreme periods of time. Long cooling cycles are more prevalent in hydrocarbon-fueled kilns (natural gas, propane, wood, oil, sawdust), which tend to be better insulated and larger in size (having more thermal mass than electric kilns).
Down firing the kiln, or leaving burners or electric elements on after the glaze has reached maturity, exposes it to excessive heat work when molten.
Fast firing leaves blisters in the glaze that would have healed in a longer firing. Some glazes go through a heating period when they boil and blister on their way to maturity. If this interval is too short, blisters are “frozen” in place and do not heal. Fast firing can also trap mechanical and chemical water locked in the glaze materials and are not completely driven off until above 932ºF.
Firing the glaze below its maturation range can leave a dry, pale color or blistering in the glaze surface.
Fast firing of the bisque kiln can trap organic materials in the clay that can then volatize during the glaze firing. The gas exits through the stiff liquid glaze, causing a blister.
Non-oxidation bisque firing can trap organic material in the clay that exits at higher temperatures as a gas through the molten glaze as a blister. Large platters stacked together or tiles placed one top of one another do not allow for the combustion and removal of organic material due to their relatively large surface areas touching.
Direct flame impingement can result in an over-fired and/or over-reduced area on a glaze, causing a blister.
Early and/or too heavy reduction in the glaze kiln can trap organic material in the clay or add carbon through excessive fuel introduction. Carbon trapped in the clay body can release at higher temperatures as a gas through the molten glaze, causing a blister.
A loosely stacked glaze kiln reduces thermal mass and subsequent radiant heat in the transmission to pottery.
A kiln atmosphere with no movement, which is most prevalent in electric kilns, can allow a saturation of volatile glaze materials that result in blisters.
The next entry in this series will discuss the impact of clay body conditions in blister formation.