The carbonization is carried out by means of known methods, which can be summarized as follows: the pre-shaped body is subjected in an inert gas atmosphere (N2, Ar) or, by a vacuum in a process comprising one or several steps, to a temperature treatment up to at least 1000??? C, preferably, however, at 1600??? C, or in particular cases to at least 2500??? C (for example 3000??? C).