Q&A Diagnosing Glaze Blisters
A glaze blister is one of the most vexing defects, and it is one of the most complicated to diagnose as there are so many possible causes. Often the potter thinks they are correcting the fault only to discover it continues to surface in subsequent firings. Glaze blisters appear as pronounced sharp edged burst bubbles that look like craters on the fired glaze surface, often revealing the underlying clay body. Glaze blistering can really tax potters’ investigative abilities. Any exploration into this common defect will require an analysis of kiln firing, clay body, and glaze conditions. The priority is to accurately diagnose the problem, then determine what incident or series of events caused it. Only then will it be possible to enact the appropriate correction.
POSSIBLE CAUSES OF GLAZE BLISTERS
KILN FIRING CONDITIONS
Over-firing 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.
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 which allow the glaze to remain in its maturing range for extreme periods of time. Correction: 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. Upon reaching temperature, pulling the damper out and unblocking the secondary burner ports for a short time will 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.
