Abstract: Provided is a method for the efficient preparation of fine hollow spherical glassy bodies having a high strength and excellent whiteness from a volcanic vitreous deposit as the starting material. The fine hollow spherical glassy bodies as desired can be prepared by dispersing 100 parts by weight of a powder of a volcanic vitreous deposit in an aqueous solution containing 1 to 10 parts by weight of aluminum sulfate, adding an aqueous alkaline solution to this liquid suspension to cause deposition of an alumina hydrate onto the surface of the particles of said powder and washing and drying of the solid material followed by a heat treatment for 1 second to 1 minute at a temperature of 900.degree. to 1100.degree. C.

Abstract: Proposed is an efficient method for the preparation of vitreous hollow microspheres from particles of a vitreous volcanic deposit by a heat treatment to effect expansion of the particles by foaming. The inventive method comprises a step, prior to the heat treatment for expansion of the particles, in which the starting particles are dispersed in an aqueous medium containing aluminum sulfate and urea each in a specified concentration followed by a heating treatment of the dispersion so as to deposit a coating layer of aluminum hydroxide on the particle surface so that the efficiency of foaming can be greatly improved without the disadvantage of particle agglomeration.