If, however, they are subjected instead to pyrolysis a process of controlled burning in a low-oxygen atmosphere the result is charcoal, a substance that is mostly elemental carbon.
Dr. Kennett and colleagues have published evidence in the prestigious Proceedings of the National Academy of Sciences that a "black mat" from the sediment of a Mexican lake dates to 12, 900 years ago and shows a sudden peak of magnetic and carbon spherules, "nanodiamonds" of a kind known as lonsdaleite, and charcoal: all of it evidence of extreme heat.