FOAMALBLE FIBERS OF ALKALI METAL SILICATE GLASS ARE PREPARED BY DISPERSING FLOWING FINE STREAMS OF MOLTEN GLASS TO CENTRIFUGATION OR TO A HIGH VELOCITY BLAST OF GAS THEREBY FORMING FIBERS OF RANDOM LENGTH AND AVERAGE DIAMETERS IN THE RANGE 0.1 TO 1.0 MIL, AND HYDRATING THE RESULTING STAPLE FIBERS TO A MOISTURE CONTENT OF 5 TO 40% BY TREATMENT WITH STEAM OF A WATER SPRAY. IF DESIRED, FIBERIZING MAY BE EFFECTED BY CENTRIFUGALLY DISPERSING THE MOLTEN STREAMS IN A FIRST STEP AND THEN SUBJECTING THE DISPERSED STREAMS TO THE HIGH VELOCITY GAS BLAST. THE GAS IS PREFERABLY SUPERHEATED STEAM, HOT COMPRESSED AIR OR A FUEL GAS FLAME. THE THUS-HYDRATED PRODUCT MAY BE FOAMED BY CONVENTIONAL FOAMING TECHNIQUES.