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The blisters trace their origins to the generation of gases as particles in the body and glaze itself decompose during firing (losing H2O, CO, CO2, SO2, etc). If the glaze melt has sufficiently high surface tension these bubbles can grow quite large (even if the melt is quite fluid). Many mineral particles gas when heated, each has its own thermal history (temperatures at which it releases gases). If a glaze has already begun melting while gases are still being generated, bubbles can grow within (this glaze is an example, it is high in Gerstley Borate). Bubble populations and distributions of sizes depend on properties of the glaze melt (e.g. viscosity, surface tension, thickness of application). As populations and sizes of bubbles increase they break at the surface. Bodies often generate gases during early stages of firing (to expel water and carbon). This tapers off but can begin again at higher temperatures if any mineral particles present are late gassers.
Strategies to deal with the problem often involve minimizing gas expulsion by adjusting body and glaze recipes to favour materials of lower LOI or earlier gassing. Lowering the surface tension of the melt is another option. Employing glaze fluxes that melt later in the firing can be very effective since pretty well all gases of decomposition will have been expelled. Glaze thickness can be reduced. Firing curves can be adjusted to slow the rate of rise as the top of the curve approaches. Holding at temperatures on the up-ramp can give gases more time to escape. Holding at top temperature can help if the surface tension is low enough. If not, the bubbles will just stay, and grow. In these cases, drop-and-hold firing schedules will often remove the blisters (giving them a chance to heal because the increased viscosity of the glaze melt can overcome the surface tension holding the bubbles in place). Testing is required to determine how much the drop should be (e.g. 100F).
Troubles |
Glaze Blisters
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution |
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