DURABILITY OF OPTICAL GLASS*

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Abstract
Samples of polished optical glass were exposed under varying conditions, and the optical systems of instruments in use and in storage were studied. Two general types of change in the polished surface were noted in the cases where weathering had taken place. Glass that remained in a humid atmosphere for a long period of time became covered with a hazy film of soluble alkaline salts. All glasses are subject to such dimming in contact with water vapor, but the rate and degree of dimming vary with different glasses. The second type of weathering is the formation of transparent silica‐rich surface films because of the loss of high refractive index oxides such as lead or barium from the surface of the glass by solution in water. A filmed glass is usually not harmed in any way, but interference effects make the surface appear colored so that the user considers it to be stained.A study of durability tests has led to the adoption of a stain test similar to that of Berger. A dimming test has been developed in which glass samples are exposed to a saturated humidity atmosphere for 28 days at 50°C. and then compared with standard ground‐glass samples.

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