Alexander the Great has often been depicted, this head is Hellenistic, from the first half of the 2nd century BC (from Pergamon). It is of a type used in the fourth century BC by Lysippus in Athens. The style however seems to be typical of the Pergamon school, the Dying Gaul in Rome and the friezes of the Zeus altar in Berlin being other examples.

With a digital camera I could get some better exposure, compare the picture before this.