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 that seals the surface preventing gas escape. Clay bodies containing manganese granular particles to produce fired speckle will almost certainly bloat if over fired.
What happens when you fired a terra cotta at cone 5-8?
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).
Some bodies cannot be fired to even near zero porosity
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).
Iron oxide goes crazy in reduction
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.
Some iron clays bloat before reaching zero porosity, others do not
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.
Bloating can happen suddenly
Example of a buff stoneware clay bloating at cone 10 oxidation (whereas it appears very stable at cone 8).
Raw clays can be volatile in firing
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.
What on earth is happening with this fired clay bar?
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
Decomposing manganese granular particles in a buff stoneware causing it to bloat
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.
What happens when a terra cotta body is fired to cone 6
It may not melt, but will certainly warp and blister/bloat. If there is inadequate kiln wash it will stick to the kiln shelf.
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