Blisters are characterized by sharp-edged craters that are generally round and range from 1/8 to 1 in. in diameter.
Some glaze defects can be very difficult to diagnose. Potters often think they are correcting the fault, only to discover it again in subsequent firings. One such defect is glaze blistering or (descriptively called) boiling. Blistering appears as a pronounced, sharp-edged burst bubble and looks like a crater 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. In most instances, a number of possible causes have to be examined and eventually eliminated to arrive at the exact point of origin. The first 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.
Obviously, if the wrong correction is enacted -- aside from being ineffective -- the potter loses time and labor, so it is imperative to diagnose the defect correctly. It is essential to have a systemic approach to isolate the actual factor(s) causing blistering. Potter can ask several question to isolate the cause(s) of glaze blistering. The answers will offer guidelines to determine the appropriate correction. Does the blister heal when fired again?
One general rule can apply to any glaze defect: if the glaze can be re-fired successfully, it should have been fired longer during the first glaze firing. The second firing supplies more heat work to the glaze, which can bring it into a defect-free configuration. Are different glaze formulas in the same kiln blistered?
The problem probably originates in the firing procedures, glaze mixing errors or a common raw glaze material. Are the blisters only on one side of the pot?
Direct flame impingement might cause an over-fired area and/or an over-reduced area in hydrocarbon-fueled kilns. Are the blisters only on overlapping glaze surfaces?
When overlapped, incompatible glazes can have a eutectic effect with resulting over-fluxed areas and blisters. Are the blisters only on horizontal surfaces?
High-surface-tension glazes with high viscosity are stiff and do not move when molten. Gravity on the vertical molten glaze pulls down causing the formed blister to heal. Another possible cause occurs when flat pots are placed directly on the kiln shelf. If the glaze is not formulated or fired correctly, the radiant heat from the shelf upon cooling can cause it to remain in its maturity range longer, causing a blister. Are the blisters only on the edges or high areas of the pots?
Fast cooling of the kiln and/or pottery loosely stacked can “freeze” the glaze in its maturation process. Are blisters present only in one kiln and not in others?
This could be an indication of an error in kiln firing. Are blisters present in only one part of the kiln?
Check for direct heat source impingement or kiln atmosphere irregularities. Are blisters present on one clay body and not another?
Check the level of organic material in the clay body causing the blisters. Has the clay body been bisque fired long enough in an oxidation kiln atmosphere? If the clay body contains high levels of iron-bearing clays or iron oxide, it can be more reactive to extreme reduction in the glaze firing, which can cause glaze blistering. Are blisters present only on underglaze, engobe or overglaze areas?
Check levels of gums and percentages of metallic coloring oxides used in the underglaze, engobe or overglaze formulas. Does the glaze have high percentages of whiting?
Statistically, whiting -- calcium carbonate (CaCO3) -- is one of the leading causes of glaze blistering. Wollastonite -- calcium silicate (CaO.SiO2) -- dissolves more readily in the molten glaze and can be substituted with an adjustment to the silica content of the glaze. Are blisters present only on one color glaze and not on other color glazes that use the same base glaze formula?
Some glazes have an excessive percentage of metallic coloring oxides, and/or the kiln atmosphere might have been too heavily reduced. Are blisters present only after a new batch of glaze is used?
It is best to carefully weigh a new batch of glaze and note any new raw materials used in the problem batch. The remaining three installments of this four-part series will consider in more detail how conditions in kiln firing, clay bodies and glazes can contribute to glaze blistering.