M2 O.xSiO2 in which M is an alkali metal or a nitrogen-containing basic group such as N(CH3 OH)4, N(C2 H4 OH)4, N(C2 H4 OH))3 or C(NH3)2 NH, and x is a positive number not less than 0.5, and for example, there are alkali metal silicate consisting of silicic acid and alkali metal belonging to the group 1A of the periodic table, tertiary or quaternary ammonium silicate consisting of silicic acid and