Digitalfire Ceramic Glossary

•The secret to cool bodies and glazes is alot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at
•The place to get the knowledge is

Sign-up at today.


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.

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 with touchy bodies. Better yet, fire the clay body to a temperature well short of an range where it might bloat. Finer grinding of the clay containing the offending particles will also help alot, enabling vitrifying the it more without fear of bloating (although warping will be an issue).


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

Out Bound Links

In Bound Links

  • (Glossary) Warping

    Normally refers to a body firing problem where ves...

  • (Glossary) Stoneware

    Most often the term stoneware refers to a high fir...

  • (Troubles) Body Bloating
    Bloating in clay bodies occurs when the firing goe...
  • (Glossary) Porosity

    In ceramic testing this term generally refers to t...

  • (Glossary) Laminations

    Laminations are planes of weakly connected materia...

By Tony Hansen

Feedback, Suggestions

Your email address


Your Name


Copyright 2003, 2008, 2015 https://, All Rights Reserved