ALL-UNION CONFERENCE ON 'BORON-FREE, ALKALI-FREE AND LOW-ALKALINE VITREOUS SYSTEMS AND TYPES OF TECHNICAL GLASS BASED ON THESE SYSTEMS' (VSESOIUZNOE SOVESHCHANIE 'BEZBORNYE, BESSHCHELOCHNYE I MALOSHCHELOCHNYE STEKLOOBRAZNYE SISTEMY I TEKHNICHESKIE STEKLA NA IKH OSNOVE').