Diagnosing Glaze Blisters, Part 2: Kiln Firing Conditions

January 31, 2008
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This next installment details firing conditions that can cause these defects, as well as some of the corrective measures that potters can take to avoid them.

The first installment of this series focused on diagnosing possible causes of glaze blisters. This next part will detail firing conditions that can cause these pesky defects, as well as some of the corrective measures that potters can take to avoid them.

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.
Correction: Shorten the firing cycle while still firing the glaze to its maturing range.

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).
Correction: After the kiln reaches temperature, pull the damper out and unblock the secondary burner ports for a short time to cool the kiln faster.

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.
Correction: In most instances, it is not necessary to down fire a kiln to achieve a stable glaze. However, if a particular glaze requires down firing, progressively shorten the down firing interval to decrease its time in the maturing range.

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.
Correction: Extend the length of time to reach the end point temperature.

Firing the glaze below its maturation range can leave a dry, pale color or blistering in the glaze surface.
Correction: Fire the glaze to its correct maturing range.

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.
Correction: A longer bisque firing cycle will enable organic material to escape.

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.
Correction: In hydrocarbon-fueled kilns, always use more air than fuel to create an oxidation atmosphere. In electric kilns, the use of an active venting system removes organic matter from the kiln atmosphere.

Direct flame impingement can result in an over-fired and/or over-reduced area on a glaze, causing a blister.
Correction: Move pottery away from the heat source.

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.
Correction: Use an excess of air-to-fuel ratio in the burners until 1860ºF, as it will remove organic matter from the clay body. Then use a slightly reducing atmosphere until the end point temperature is reached.

A loosely stacked glaze kiln reduces thermal mass and subsequent radiant heat in the transmission to pottery.
Correction: A densely stacked kiln can produce slower increases and decreases of temperature while radiating more heat between pottery, kiln shelves and posts. A densely packed kiln is a factor that can apply more heat work to the glaze, which liberates gases trapped in the glaze.

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.
Correction: In electric kilns, an active venting system can circulate the kiln atmosphere. In hydrocarbon fueled kilns, changing damper settings and primary and secondary air intake at the burner ports will increase the kiln atmosphere movement.

The next entry in this series will discuss the impact of clay body conditions in blister formation.

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