PROBLEM TO BE SOLVED: To improve strength and whiteness degree and to add a photocatalytic function by precipitating titanium oxide hydrate on the surface of volcanic vitreous deposit powder and heat treating the powder at a specified temp. for specified time.
SOLUTION: In order to precipitate titanium oxide hydrate on the surface of volcanic vitreous deposit powder, for example, the volcanic vitreous deposit powder is dispersed in an hydrochloric aq. soln. containing titanium chloride, in which an alkaline aq. soln. is dropped. Thereby, the surfaces of powder particles are coated with titanium hydrate to maintain inner water. Then the suspension is subjected to solid liquid separation and the obtd. solid component is sufficiently washed and dried. The dried powder is heat treated to foam at 900 to 1100°C for 1 to 60 sec. By this heat treatment, the surface of foamed particles is coated with titanium oxide, which effectively prevents melt sticking among the particles. Thus, fine hollow glass spheres having a photocatalytic function and having ≤50 μm particle size and ≤1 g/m3 particle density can be obtd. in a high yield.