Body bloating (larger bubbles) and blistering (smaller ones) occurs after a clay body matures to the point that the surface seals due to glass development but before generation of gases from decomposition of organic, carbonate or sulfate materials has completed. The internal pressures bubble the clay (since it has softened to the point of being flexible).
In many clay bodies (e.g. bodies many from air-floated industrial minerals, bodies high in silt or naturally clean materials), bloating never occurs, even if fired to zero porosity or beyond (where melting begins). In others having particles that generate gases (e.g. carbonates, lignite, or any kind of carbon material), bloating can begin to occur even at porosities above 1 or 2 percent. Bodies having added manganese granular are often very sensitive to this problem (the manganese is used to create visual speckling effects); these will almost certainly generate small bloats if the body is fired anywhere near the point at which the surface begins to seal. The problem also common in terra cotta type bodies (or those containing low melting red clays) that are being fired high enough to vitrify them. Clay bodies made from native materials that have not been ground to 200 mesh (and contain gas producing impurity particles) are more likely to bloat at some stage in their melting process.
Bloating can also occur in bodies that, by themselves do not bloat, but when glazed do. This is most likely to happen when the glaze is high enough in boron that it begins to melt and seal the surface at a fairly low temperature (all bodies have some LOI and therefore need to expel some gases of decomposition). Even thin walled vessels can bloating because of this. A complicating factor can be the presence of laminations parallel to the surface, perhaps caused by the forming or pugging process (of a plastic clay). Adjusting the glaze recipe to begin melting later in the firing and correcting issues in the forming process can help to alleviate this problem.
Modern production of most ware is done by fast firing, firing right to the vitrification point of the clay. The process is normally very finely tuned, all parameters are closely monitored. High quality clays and body materials of very fine particle size are needed (contamination of carbon particulates cannot happen), glazes must be made from consistent materials, they must melt late and smooth out quickly, firing must be absolutely even and follow the curve precisely, forming equipment must be kept in excellent condition. Because the process is so finely tuned, those in charge of monitoring it must be exceptionally wary of changes or irregularities. If not, bloating and blistering is just one of the problems that can occur.
Bloating in an over fired middle temperature high iron raw clay (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But between cone 6 and 7 (top bar) it is already bloating badly. Such clays must be fired at low enough temperatures to avoid this volatility (if accidentally over fired). This clay only reaches a minimum of 1% porosity (between cone 4 and 5), it is not possible to fire it to zero porosity. This is because of the particulate gas-producing particles (it is ground to 42 mesh only).
This one can take more temperature than most. It looks OK at cone 5 (bottom bar). But at cone 6 bloating (bubbles) begin to occur. This body, while smooth to the touch, contains some iron and sulphate particulates that generate gases during firing, these are the catalyst for the bloating (the clay matrix becomes dense enough that it can no longer vent the gases of decomposition through it).
Cone 6 iron bodies that fire non-vitreous and burn tan or brown in oxidation can easily go dark or vitreous chocolate brown (or even melting and bloated in reduction). On the right is Plainsman M350, a body that fires light tan in oxidation, notice how it burns deep brown in reduction at the same temperature. This occurs because the iron converts to a flux and the glass development that occurs brings out the dark color. On the left is Plainsman M2, a raw high iron clay that is quite vitreous in oxidation, but in reduction it is bloating badly. When reduction bodies are this vitreous there is a much great danger of black coring.
Bloating. These teapots have been refired to cone 6.
A cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). It is normally stable to cone 8, with the manganese it begins to bloat at cone 7. The particles of manganese generate gases as they decompose and melt, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.
This cone 6 brown functional stoneware has been fired across a range of temperatures. Cone 4 is too porous. From cone 7 it is expanding and density is not improving, it will likely warp or bloat. Cone 7 is losing the red color, there is no room for over-firing (by accident). The porosity at cone 6 is so much better than cone 5 and color is still stable. Therefore, cone 6 is the one we want.
This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.