Chemistry
Carbon itself is solid at case-hardening temperatures and so is immobile. Transport to the surface of the steel was as gaseous carbon monoxide, generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon, but unworkably slowly. Although oxygen is required for this process it's re-circulated through the CO cycle and so can be carried out inside a sealed box. The sealing is necessary to stop the CO either leaking out, or being oxidised to CO2 by excess outside air.
Adding an easily decomposed carbonate "energiser" such as barium carbonate breaks down to BaO + CO2 and this encourages the reaction
- C (from the donor) + CO2 <—> 2 CO
increasing the overall abundance of CO and the activity of the carburising compound. It's 'common knowledge' that case-hardening was done with bone, but this is misleading. Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates, but is mainly calcium phosphate (as hydroxylapatite). This doesn't have the beneficial effect on encouraging CO production and it can also supply phosphorus as an impurity into the steel alloy.
Read more about this topic: Case Hardening
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