Glass Manufacture for Electric Lamps. The Development of Special Glasses for Mercury Discharge Lamps.

JSGT 1939 V23 T268-T280

Glasses containing 5% boric oxide plus phosphoric oxide, 20-35% alumina, and 15% of the basic oxides, lime, baryta, magnesia, and zinc oxide, the remainder being silica, have been studied with a view to finding suitable hard glasses for the inner tubes of high-pressure mercury discharge lamps. Glasses of this type containing more than approximately 27.5% alumina or more than approximately 7.5% lime, bubble too much during bench working in an oxy-coal gas flame. On the other hand, if the lime and alumina contents are too low, the glasses are difficult to found and work at the furnace. A glass of theoretical composition, SiO₂56, Al₂O₃ 24, CaO 7.5, BaO 7.5, B₂0₃ 5% has been melted on a semi-commercial scale in a small tank furnace constructed from china clayball clay blocks. Satisfactory lamps may be made using bulbs blown from this glass, and in addition to having excellent lumen maintenance can be run horizontally without magnetic control.

J. E. Stanworth, M.Sc.Tech.