Natural aging of soda-lime-silicate glass in a semi-arid environment

Abstract

Although soda-lime-silicate glass appears to be the most durable low-cost candidate material for second surface solar mirrors in the near term, only limited quantitativve information is available on the behavior of this material after long-term natural weathering in an arid environment. In an effort to assess the magnitude and character of any changes that occur, samples of soda-lime-silicate glass were examined which had experienced more than forty years of exposure as south-facing vertical windows. These samples contained areas that were both fully exposed and partially protected from moisture, sunlight and blowing particulates. The interior and exterior surfaces of both exposed and unexposed regions of the glass were compared for structural and compositional changes. The samples were also examined for spectral transmittance, reflectance, optical scatter and surface index of refraction changes. The results of these limited studies indicate that, at least from the optical point of view, the material had undergone only minor changes. The changes that did occur would not have significantly affected the performance of solar mirrors used in even the most optically demanding heliostat designs.