(wherein M is a cation of valence n, and preferably potassium or a mixture of K+M1 in which M1 is an alkali metal or alkaline earth metal such as sodium, calcium, barium, or rubidium, provided that K2/n O/M1 2 O+K2 O) is at least 0.7) is heated to a temperature of from at least 75??? C. and preferably from 100??? C. to 250??? C., more preferably from 120??? C. to 225??? C., to form the desired cyl