The ideas that iron can be carburized simply by inserting it into a charcoal fire, and that it becomes carburized through repeated heating for forging, are untenable. To carburize iron, it must be enclosed with a source of carbon so as to exclude all or nearly all of the products of combustion. Temperature of over 740°C is necessary for carburization to occur, and a temperature of over 900°C is required to produce a useful thickness of carburized skin. Immersion of iron in a bath of molten cast iron held just above its melting point will carburize iron rapidly.