What happens when you “fire” a wet clay pot in a kiln?
Before the clay is placed in the kiln, it is usually dried in the air for at least several days. This first step has already removed the majority of the water, but there will still be some trapped inside the clay. As it is heated in the kiln, the remaining water will turn to steam as it evaporates from the clay. If it is heated too fast, it may turn to steam while still trapped in the clay and cause the pot to explode! As the pot continues to heat some of the organic materials in the clay will burn off, which is necessary for the clay to form a strong final structure.
The next stage is an interesting one, and to understand it we need to consider the chemical composition of clay. Clay consists of a unit of alumina (Al2O3) and two units of silica (SiO2) complexed with two molecules of water. So even after all of the “excess” water has evaporated away, there is still a significant quantity of water that remains chemically bonded within the clay (at this point water accounts for about 14% of the mass of the clay). As the temperature continues to increase those remaining water molecules begin to be released, and they too evaporate away. This is another step where the heating must be done slowly, otherwise the water can create steam pockets within the clay that will expand and eventually explode.
Other changes occur as well, such as changes in the crystalline structure of the silica that will occur multiple times as the pot is heated. Eventually the glass-making oxides within the clay melt, and the clay will fuse into a ceramic material. The materials that melt relatively easily will tend to fill in remaining empty spaces, strengthening the final product. One final note is that changes in the crystalline structure of the silica will also occur upon cooling, and one must take care to cool the pot sufficiently slowly so that these changes don’t cause cracks to develop during cooling.