The conversion of a carbon-containing substance to carbon or a carbon residue as the destructive distillation of coal by heat in the absence of air, yielding a solid residue with a higher percentage of carbon than the original coal; carried on for the production of coke and of fuel gas.

1. In the coalification process, the accumulation of residual carbon by changes in organic material and their decomposition products.

2. Deposition of a thin film of carbon by slow decay of organic matter underwater.

3. A process of converting a carbonaceous material to carbon by removal of other components.