Resistant Glasses for Modern Electric Discharge Lamps
JSGT 1935 V19 T266-T278
Glasses suitable for the inner envelopes of high pressure mercury type electric discharge lamps, and having a softening temperature of the order of 900° (=viscosity 108·5 poises), a minimum specific electrical resistance of 2 megohms at 600°, and low iron content are obtained by using compositions of approximately 50% silica, little or no alkalis, 20% of alumina and a high content (at least 16%) of lime or of calcium, zinc and barium oxides. The effect of other oxides has also been studied. The glasses were melted at 1450° to 1480° in special refractory pots, the types of which are described. The coefficients of thermal expansion, viscosity and thermal endurance data are given for a number of the glasses. Glasses for sodium lamps contain 0–25% silica, 10–30 alumina, 30–50 boric oxide, 0–30 barium oxide, 10 calcium oxide, and other oxides in smaller amounts. The iron oxide content must be as low as possible (0·02%). The chief difficulty in the investigation was to find suitable refractory materials, because the glasses are fluid and extremely corrosive.
J. H. Partridge
