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 pott
