The process by which carbon atoms are made to diffuse in from the surface of a piece of iron. Under reducing conditions carbon monoxide gas is produced which on decomposition forms nascent carbon atoms which are absorbed at the surface of iron. The process is slow and sufficiently reducing conditions (a minimum temperature of 900̊C) have to be maintained throughout: even carburisation is difficult to achieve in a smithy and would only have been possible with quite small pieces of iron. The heat and carburising medium may sometimes have been separated, for instance, by packing of pieces of iron and charcoal (or plain iron and cast iron) in sealed containers that were placed in the hearth. This is a basic description of how crucible steel was made in Iran, India and parts of the surrounding region during much of the 1st and 2nd millennia AD. As far as we know, carburisation methods for making steel were not used in the West until the post-medieval introduction of methods such as pack-carburising. It is not known to what extent the process was used in earlier times although it is mentioned in the 12th century by Theophilus, who was probably referring to case-carburising which only affects a thin surface layer. In more recent times this process was exploited in the process of case-hardening, occasionally referred to as carbonisation.

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