Materials for construction of vacuum tubes are mostly chosen and processed with essentially different points of view in mind than for other technological purposes. For general technological purposes, the mechanical and electrical properties, ease of working, corrosion, and similar data concerning the materials involved are most important. For vacuum tube materials, other properties may be much more significant: ease of outgassing, small vapor pressure, sufficient strength at high temperature, proper coefficients of expansion, suitable emissivity and transparency, high or low heat conductivity or electrical conductivity, small leakage at high temperatures, smallest or largest possible electron emission, chemical neutrality or affinity to other tube materials, etc. Besides this, the purity of the materials employed is often decisive since minute amounts of impurities within them which evaporate during operation may poison the cathode or the gas filling of the vacuum tube and change its characteristics and life considerably. Ease of working and costs of tube materials can only be considered after such necessary as factors mentioned above.