Hygroscopicity of optical glasses as an indicator of serviceability

Several modifications of a powder-hygroscopicity method have been tried as a rapid means of determining the serviceability of optical glasses, i. e., their ability to maintain a clear polished surface under normal conditions of service. The method has been applied to a wide variety of glasses, and it shows the typical optical glasses in common use to be much less hygroscopic than the average commercial sheet and container glasses. In fact, many of the optical glasses even compare favorably with the better chemical laboratory wares, such as Pyrex, Tamworth, and Kimble N-51-a. This grouping of these glasses of such very dissimilar chemical durabilities serves well to emphasize that the;capacity of aglass to maintain a clear surface upon exposure to the atmosphere, and its chemical durability as conventionally determined are two inherently different properties. A comparison of the hygroscopicity valu es of the Corning 015 electrode glass with those of the optical and chemical ware glasses whose electrode capacity is very poor suggests strongly that the pH response of a glass is primarily a function of its hygroscopicity. Tests made on a limited number of glasses of the binary series Na20-Si02, K 20-Si02, Li20-Si02, and PbO-B20 a appear to reflect some of the critical compositions of the respective phase equilibrium diagrams. CONTENTS Page