The protective gases used in furnace atmospheres are usually a mixture of hydrogen, nitrogen, carbon dioxide (CO2), carbon monoxide (CO), and trace amounts of water. The necessary gases are either injected directly into the furnace as a nitrogen and methanol (CH4) mixture or via an endothermic generator using propane or natural gas as the source for carbon. An endothermic atmosphere is produced by cracking methane into nitrogen, hydrogen, and carbon monoxide in a retort.