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.
Bubbling occurs in some clay bodies (especially those made from coarsely ground raw materials) if they are over fired; others just melt without bloating. Aggravating conditions that produce bloating include the presence of mineral particles (e.g. sulfates) that generate gases during the the firing stage at which the body densifies toward zero porosity, the presence of excessive carbon or carbon-containing particles not burned away by bisquit or oxidation firing, laminations in the clay matrix or the presence of an early melting glaze (or soluble salt) that seals the surface preventing gas escape. Clay bodies containing manganese granular particles to produce fired speckle will almost certainly bloat if over fired.
Many kilns do not have reliable shut-offs or have deteriorated temperature measurement sensors, thus overfiring can be quite common. It is best to confirm firing temperature using properly set cones to avoid bloating (especially for volatile bodies). Better yet, fire the clay body to a temperature well short of a range where it might bloat. Finer grinding of the clay containing the offending particles will also help alot, enabling vitrifying it more without fear of bloating (although warping during firing will be an issue).
It may not melt, but will certainly warp and blister/bloat. If there is inadequate kiln wash it will stick to the kiln shelf.
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.
The two clay bars were fired side-by-side at cone 01. The back bar is of a raw clay dug from a creek bed in Alberta, Canada. Notice how it puffs up inside and eventually splits open the outer layer (which has sealed in the gases of decomposition). The front bar is that same clay, but mixed 50:50 mix with Redart. It is stable and strong as a stoneware. You can see all the lab tests I did on this in my insight-live account at http://goo.gl/KiUoU0
Here is a good reason not to have single-temperature-tunnel-vision when evaluating or using a clay body or clay material. This high-iron clay looks great at cone 3 or 4 (the bottom bar is cone 5 and out-of-place). But by cone 5 the solubles (invisible at lower temperatures) begin to melt. Shortly after it rapidly descends into serious bloating and then melting by cone 8.
A very fine particled low fire red burning terra cotta clay (Plainsman Redearth) fired at cone 2,3 and 4 (top to bottom). Notice the cone 4 bar is beginning the melting process (signaled by the fact that it is expanding). Yet it is not bloating as this type of raw clay normally would. The cone 2 and three bars have reached zero porosity also. Other clays that fire to very similar color begin to bloat long before they reach zero porosity.
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 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).
We tested four different clays (brought in by customers). One is from BC and three from Alberta. These fired sample bars show rich color, low soluble salts and high density and strength at very low temperatures. L4233 (left): Cone 06 to 3 (bottom to top). Reaches stoneware-density at cone 02 (middle bar). Plasticity is very low (drying shrinkage is only 4.5%). But, it is stable even if over-fired. L4254 (center bottom): Cone 04,02,3,4 (bottom to top). Very fine particled but contains an organic that is gassing and bloating the middle two bars. L4243: Fires lighter and looks stable here (cone 02,01,1,2 shown) but melts suddenly less than a cone above the top bar (well before vitrification is reached). L4242 (right): Hyper-plastic, with 12% drying shrinkage! Already melting by cone 02 (third from top). Achieves almost zero porosity (porcelain density) at cone 04 (#2 bar). Even when mixed with 20% kaolin and 20% silica it still hits zero porosity by cone 1. What next? I'll mix L4233 (left) and L4242 (right), that should produce stoneware density at cone 02 (about 1% porosity).
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, but if you are aware of the limitations and take precautions they are workable.
Interpreting Orton Cones
Interpreting how high a kiln fired based on the look of the cones can be a much more complicated matter than it might first appear.
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
Laminations in pugged the pugged clay body matrix or in a formed piece can cause separations during drying and firing or produce a fired matrix prone to cracking and failure under stress.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.
Clay Body Porosity
In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
Warping happens during the firing of ceramic ware when there is a high degree of vitrification or a shape is unstable. But warping is expected in translucent ware, it is just a factor that must be compensated for.
Bloating in clay bodies occurs when the firing goes high enough to seal the surface and prevent the passage of gases releasing inside.