treating said coated substrate to harden said solution into a porous silicon oxide film, wherein said nonionic surfactant is first purified to remove alkali metal impurities; and wherein said ionic additive is a tetramethylammonium salt chosen from a group of compounds consisting of tetramethylammonium formate, tetramethylammonium nitrate, tetramethylammonium oxalate, tetramethylammonium acetate, tetramethylammonium hydroxide and other salt exhibiting complete decomposition to volatile species upon heating to {TAIL ~ a temperature} between 300 and 450{HEAD ~ ° C} 