The belief is often communicated in these ways:
“Poorly wedged clay containing air bubbles will explode.” or
“If you create an enclosed form, you need to poke a hole for the air to escape or it will explode.”
While it’s true that enclosed pockets of air in your clay can lead to explosions, the explosions don’t have anything to do with air being trapped and unable to escape. The explosions have everything to do with trapped water. If your pieces aren’t 100% dry inside and out, problems can arise during the firing.
What Happens When Water is Heated?
At room temperature, water naturally evaporates. Natural evaporation happens relatively slowly and is dependent on the humidity of the surrounding atmosphere. If the relative humidity of the surrounding air reaches 100%, evaporation stops until the moist air is replaced with drier air, and evaporation can continue.
As water is heated, the molecules start to move faster and evaporation speeds up. If we continue to heat to boiling temperature, water (a liquid) converts to steam (a gas). At sea level, this occurs at (100°C).
Three important facts about water turning to steam:
It expands greatly in size—over 1500 times
It can produce a high amount of pressure
This all happens at a very high speed
Consider boiling a kettle of water. As the water begins to boil, you hear a faint whistling sound. As more water converts to steam it expands, increasing the pressure in the kettle, which increases the speed at which the steam passes through the tone hole and the whistling sound gets louder.
When the kiln temperature starts to climb, any moisture left in the clay starts to evaporate faster. If that moisture reaches boiling temperature before it has a chance to evaporate, it will convert to steam.
The expansion, pressure, and high speed of water converting to steam within a clay body will cause the clay to explode, unlike the kettle that allows the steam to escape. This is why it’s so important for clay to be completely dry before it is fired.
Do Air Bubbles Cause Explosions?
When clay is wet, all the spaces in between the clay particles are filled with water. As the water evaporates, the clay becomes porous, meaning there are tiny pathways of void space (air) in between the solid clay particles.
Essentially, bone-dry clay is full of teeny tiny pockets of air. Air within the clay is not a problem. If we make a piece and we leave an air bubble in the clay, that air pocket does not directly pose a risk of explosion. Air is free to move through those tiny porous pathways in the dry clay.
Only water leads to explosions. While enclosed air doesn’t cause explosions, it can prevent thorough drying and trap moisture, which does cause explosions. When we enclose air bubbles within a clay form, that pocket of air provides water molecules with a nice humid place to hang out.
A thin piece of clay with an air bubble will dry and fire without issue. When the clay is thicker, it can feel very dry on the outside while the inside still contains moisture. When the kiln temperature rises, the air pockets fill with water vapor, which builds pressure as water turns to steam. The pressure of the steam causes the clay to explode from the inside.
Say you throw an enclosed form or you join two pinch pots together. Poking a hole will promote more thorough drying before the firing by allowing dry air to circulate so evaporation can occur from the inside as well as the outside. The bigger the hole, the faster the piece will dry. During the firing, the hole gives the steam a pathway to exit, preventing pressure from building up and hopefully, preventing an explosion.
But, is it possible to leave air bubbles in your clay and make enclosed forms without holes in them that don’t explode in the kiln? The answer is yes! If the clay is 100% dry inside and out at the time it reaches boiling temperature, there is zero risk of explosion. But on the flip side, explosions can occur even when air bubbles are not present if your clay isn’t thoroughly dry.