Blisters can be caused by a number of conditions that exist within the clay body itself.
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.
Correction: Substituting coarser for finer particle clays will open the clay body and help release trapped organic material.
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
Correction: Less surface area will be exposed when using a finer mesh grog. The grog can also be pushed down into the clay during the trimming process.
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.
Correction: The amount of water used in mixing an engobe can be reduced by the addition of small percentages of a deflocculant. Also, slowing down the rate of heat increase until 1112ºF is reached will allow mechanical and chemical water to escape through the 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
Correction: The use of gums such as C.M.C. (carbonxymethylcellulose), Vee Gum CER or other binders (1/8 to 2% added to the dry weight of the glaze) can slow down the drying rate of the glaze, preventing fast absorption.
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.
Correction: Slowing down the rate of heat increase in the 572 to 1292ºF range can safely release volatile organic materials and moisture from the clay body.3
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.
Correction: The addition of barium carbonate (1/4 to 2% based on the dry weight of the clay body) can neutralize soluble salt migration.
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.
Correction: Less water used in the glaze batch, and waiting until the first glaze layer dries before applying another, will prevent blisters.
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
Correction: Decreasing the rate of heat increase in the 1112 to 1976ºF temperature range can allow the slow release of oxygen through the glaze layer.
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.
Correction: Increasing the bisque firing by one or two cones will decrease the absorbency of the pottery. Also, gums such as C.M.C. (carbonxymethylcellulose), Vee Gum CER or other binders added to the glaze (1/8 to 2% based on the dry weight of the glaze) can slow down the drying rate of the glaze.
Warm glaze on a cold bisque pot can trap air in the glaze layer, causing a blister during firing.
Correction: Cool the liquid glaze before applying to the pottery. 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
Correction: Use a clean source of water, tools and sponges.
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.
Correction: Covering the wedging board with canvas will prevent chips from entering the clay. Mixing plaster with the correct ratio of water will ensure maximum set strength. Discarding molds that show signs of wear can prevent plaster contamination in the moist clay.
The next entry in this series will discuss glaze conditions and how they can affect blisters.