What Makes Pottery Fire & Water Resistant?

Pottery dishes.

When pottery is first formed, the clay is not water resistant. A few chemical changes occur while the clay piece is fired that make the pottery fire and water resistant.

Clay

Clay has water within its structure in two ways: unbound and bound. Unbound water is what makes the clay wet. It's like adding water to dirt to make mud. Bound water is actually part of the chemical makeup of the clay. Because clay is made of mainly inorganic materials, it is naturally somewhat fire resistant. After being formed into pottery, the clay must be fired to be useful. Firing is baking the clay in a kiln at very high temperatures. A piece of pottery must go through two firings: a bisque firing and then a glaze firing.

Air-drying

Clay must be air-dried before it can be fired. During air-drying, much of the water in the clay evaporates, leaving the clay dry to the touch. After air-drying, the piece is not water resistant. It can still be reconstituted, or put in water so that the clay becomes wet again, and can be reshaped.

Bisque Firing

Bisque firing causes the unbound water still trapped within the clay body to turn to steam and escape the clay. At this point, there is no unbound water left in the clay. When the bisque firing is complete, the piece is water resistant but not waterproof. The clay is still porous and will leak water.

Dehydration

Dehydration is a chemical change that happens during the bisque firing. During the dehydration process, the water that is bound within the chemical makeup of the clay is released. Now the clay cannot be reconstituted.

Burn Off

The high temperatures of the bisque firing cause the small pieces of organic matter, such as small sticks and even insects, to burn. This process is called burn off. At the end of this process, there is no more inorganic material in the clay, and the pottery is completely fire resistant.

Glaze Firing

The glaze firing is done at temperatures even higher than the bisque firing. During this firing, the clay is heated to the point where some components of the clay melt and partially turn to glass. This process is called "vitrification." After the glaze firing, pottery is completely waterproof and can hold water for any amount of time without leaking.

References

• Lakeside Pottery: The Firing Process