In the preheat flame, a fuel gas burns in oxygen, generating heat. The flame consists of an outer and an inner combustion zone. The main purpose of the preheat flame in most flame processes is to achieve rapid, local heating. This imposes requirements on the combustion properties of the gas that is used. In cutting, it is not the preheat flame in itself that performs the cutting operation, but rather an oxygen jet. This oxygen jet oxidises the metal, with the evolution of heat, and expels the combustion products (the slag) from the kerf. The purpose of the preheat flame is to heat the metal to the ignition temperature, sustain the cutting process and support the cutting oxygen jet.