The working principle of quartz glass heating tube

The quartz tubes support the electric filaments. Long electric filaments are too flexible to be strung over the lengths in such heaters. Encasing the filaments in tubes keeps them from breaking under their own weight or touching other parts of the heater (which could cause sparks or other damage).

Why choose quartz? First, it tolerates the heat. Second, it is an insulator. Third, it allows the infrared heat to pass through.