So Why Does Pottery Explode?

Many potters have had the disappointing experience at least one time of their pottery exploding in the kiln.  The beauty of pottery is that you can get some very unexpectedly wonderful results.  However, a nerve-wracking array of things can go wrong with pottery once you hand it over to the kiln of fate.  So, exactly why does pottery explode in the kiln?

The main reason that pottery explodes in the kiln is residual moisture left in the clay body even when it appears bone dry.  Once the kiln reaches 100°C, the moisture starts to turn into steam.  The steam expands very rapidly into any small air pockets in the clay and shatters the pottery.

If a piece of pottery is going to explode, it normally does so in the bisque fire.  The reason for this is that prior to the bisque fire, the greenware still has some water content.  After the bisque fire, all this water will have gone.  So, the main reason for the pottery to explode will have basically evaporated.

However, there are other factors that can cause pottery to explode at various points during firing. It is commonly said that air bubbles in clay cause pottery to explode.  However, this is only partially true. Air itself does not expand enough when heated to cause the pottery to explode. However, when free water starts to evaporate, it looks for a way to exit the clay.  It migrates toward the surface of the clay. If there are air bubbles in the clay the moisture will evaporate into the bubble cavity.  Then as it expands within the bubble cavity, it will cause the pottery to explode in the kiln.

There are also factors other than non-dry clay, air bubbles and air pockets that can also play a role. Clay thickness can be an issue. It is a general opinion that the thinner the clay the better the chance of firing successfully. Anything greater than 25mm thick can be problematic and will require specialised firing.

Another important factor is the firing schedule.  If the piece is being fired too quickly, it puts a lot of strain on the pottery and explosions are more likely.  All these factors need to be taken into consideration to avoid the big bang!