. . . "It is possible to use oxides of elements of main groups III to V of the periodic table and salts, preferably Na--, K--, Ca--, Zn--or Al--salts, of oxygen acids of an element of main group III, IV, or V, provided they have a melting point of 300??? C. to 1400??? C., preferably of 350??? C. to 1200??? C., in particular from 400??? C. to 1000??? C., and of compounds, for example hydrates or carbonate" . .