Fossil fuels are of great importance because they can be burned (oxidized to carbon dioxide and water), producing significant amounts of energy per unit mass. Furnaces are built of several kinds of high temperature (refractory) materials to hold the process material and hold in the heat without breaking down. Choices for heat source include arc, combustion, electrical or resistance, indirect or contact or conduction, induction, infrared or radiant, natural gas, propane, oil, other fuel, RF or microwave or dielectric, and steam. Common applications for industrial ovens include aging, annealing, baking, brazing or soldering, burn-off, curing, drying, firing or sintering, foundry or melting, heat treating, hot pressing, laboratory, preheating, and quenching.