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Higher than normal levels of organic material not removed from the clay during bisque firing. Periodically, some clays (notably, fireclays) can contain abnormally high percentages of organic material. In such instances, a “normal” bisque firing cycle will not remove all the organic material from the clay. During the subsequent glaze firing, organic material carbonizes and releases as a gas through the clay body into the molten glaze, causing a blister.
Correction: A clean oxidization atmosphere in the bisque kiln, fired to the correct temperature in enough time, will release organic material from the clay.
Clay bodies containing high percentages of plastic clays, when raw glazed, can trap organic material. As the covering glaze vitrifies during firing, the resulting carbonaceous gas in the clay exits through the glaze, causing a blister.
Grog exposed in the clay body during the trimming process can cause glaze contraction around the particles, leaving air pockets and eventual blisters in the glaze.1
Clay slip (engobe) applied to once-fired or bisque pottery can release mechanical and chemical water that can turn into a gas exiting through the covering glaze layer.
Raw glazing an unfired clay body can drastically increase its absorbency. When glaze is applied, it can be drawn into the clay body too rapidly, causing bubbles and air pockets as the glaze dries. During firing, the bubbles migrate to the surface and cause a blister.2
Raw glazing can trap organic material and/or moisture in the clay body or engobe. At higher temperatures, the organics or moisture exit as a gas through the glaze layer.
Soluble salts in the clay body can migrate to the surface as the clay dries, leaving a disruptive layer of sulfates that release gas into the covering molten glaze.
Thin-walled pottery saturated by water during spraying, dipping or painting during glaze application. Trapped moisture on the clay surface can be released as a vapor during glaze firing, causing a blister.
Granular manganese added to the clay and naturally occurring nodules of manganese decompose at 1112ºF and liberate oxygen at 1976ºF, which can exit as a gas through the molten glaze and cause a blister.4
Low bisque firing can yield extremely absorbent ware that “sucks in” the wet glaze. If the glaze is highly viscous, air pockets formed in the application process can migrate to the surface, leaving blisters in the stiff glaze.
Warm glaze on a cold bisque pot can trap air in the glaze layer, causing a blister during firing.
Contamination of the bisque clay body from the breakdown and disintegration of organic materials, such as sponges used to clean the bisque before glazing.5
Contamination in the clay from a plaster molds or deteriorating wedging boards can impart plaster chips into the moist clay. Upon heating, the chips release gas and/or water vapor in the covering glaze layer.
The next entry in this series will discuss glaze conditions and how they can affect blisters.