Early use of special glasses in the German electric lamp industry

Abstract

This paper continues the author's publication "History of glass for the incandescent electric lamp" [1] and covers the development in Germany during the two decades 1920 to 1940. The replacement of the earlier carbon filaments in incandescent lamps by metal filaments and coils ahowed the usage of higher electric power and consequently higher light output. Because of that, the operating temperature of the lamps increased significantly making it necessary to use glasses with higher thermal shock resistance and higher softening temperatures. The requirements became even more stringent for the new high-pressure mercury-discharge lamps since they needed glasses with still higher softening temperatures and high resistance of the glass body to internal gas pressures. For the incandescent lamp chemical corrosion of the glass body was negligible. Resistance to chemical attack, however, became of concern with the introduction of the high-pressure mercury-vapor lamp and was paramount for the development of the sodium-vapor lamp. The durability of normal soda-lime-silica glasses with or without lead proved totally insufficient and new types of glass resistant to sodium vapor had to be developed. The glasses used to solve these problems were: hard glasses of the borosilicate and aluminosilicate type, silica glass (quartz glass) and sodium-vapor resistant glasses.