Electricity
Perhaps the number one heating source in use in today, at least in the United States, is electricity. Heat is produced by electrical current being sent through heating elements, in much the same way that a toaster works. The process is just on a much greater scale.
Electric kilns are becoming ever more popular. They are relatively inexpensive to buy, and with fuel becoming increasingly expensive, electric kilns are becoming more economical to fire.
The main drawback to electric kilns is that they fire pottery in a neutral or oxidation atmosphere. Reduction firing requires the burning of fuel, which is problematic in an electric kiln. The good news is that many interesting glazes have now been developed especially for oxidation firing.
Propane and Natural Gas
Gas, whether propane or natural gas, has become the fuel of choice for many studio potters. Natural gas is known for burning cleanly, and both types of gas are excellent at giving the potter control over the kiln's atmosphere during firing.
The gas comes to the kiln via pipelines and enters burners where it is mixed with air. The proportion of air to fuel determines whether the kiln's atmosphere is in oxidation of reduction.
Kiln's need to be in oxidation for the beginning of their firing cycle. The amount of fuel entering the kiln's burners must also be closely monitored at the beginning of a firing. Too much fuel will result in the temperature climbing too quickly. This in turn can result in cracked and shattered pots.
Wood
Wood has been used for thousands of years as a fuel for firing pottery. To fire a kiln using wood requires a very large amount of both wood and time. For example, a kiln that could be fired in eight hours if using natural gas could take thirty-six hours of constant tending if the fuel used was wood.
This is a hypothetical example, however. A kiln is constructed for the type of fuel it will consume. Firing with wood requires a large firebox where the wood is tossed in, as opposed to a much smaller firebox required for a gas burner.
Wood firing is an intense, hands-on process. Many wood-firing potters also value the effects of the wood ash. Ash naturally settles on the tops and shoulders of the pottery during firing, creating its own glaze.
Microwave Assisted Heating
Recently, new kiln technology has begun to merge microwave energy with that of conventional heating sources such as electricity and natural gas. The microwave energy gives the other heating source a boost. The most intriguing aspect of microwave-assisted firing is that there is a significant reduction in firing costs.
Coal
Although not often seen in the United States anymore, kilns have been fired using coal for centuries. For example, many early Industrial Age kilns were coal-fired.
Although you are not as likely to encounter this fuel as a kiln heating source, it is always possible.
Other Fuels
Through the centuries, people have used a number of different fuels to fire their pottery. Different cultures and various regions have all utilized the fuels that came to hand. These fuels include dried dung, peat, grasses, reeds, and brush.
Even today many potters still use some of these other fuels. Some use them because that is what is available to them. However, many studio potters employ these fuel types in their creative process, mainly in conjunction with pit firing or adding them inside saggers (containers to fire ware in) with pots.