PURPOSE: To prevent a window glass from cracking and reduce the transparency, by forming a sulfuric acid-proof film of specified composition on the surface of an Li2O-Al2O3-SiO2 type transparent glass used for the window of a combustion apparatus.

CONSTITUTION: A soln. of an organometallic compd., such as ethyl silicate, aluminum isopropoxide or titanium tetrabutoxide dissolved in an org. solvent, such as butyl alcohol or ethyl alcohol is coated on the surface of an Li2O- Al2O3-SiO2 type transparent crystalline glass used for the front window of various kinds of combustion apparatuses. The org. components are decomposed by heat treatment to give a transparent film consisting of SiO2, Al2O3, TiO2, and the like oxides, and having 1.55±0.30 refraction coefft. and ≤1.0μm thickness. This film is superior in resistance to sulfuric acid, heat, and heat shock, and prevents small cracks on the combustion side surface of the glass, and reduction of the transparency.