It is a mixture of CaSO4 with alum – i.e. plaster of Paris soaked with alum solution, then burnt again and finely ground. After setting, it obtains high mechanical strength and hardness. According to Leixner, alum-gypsum, also called marble-gypsum, has been primarily used up to the 1960s. Wittenburg (1999); Leixner (1993). [J. Cassar, R. de Angelis]