The effectiveness of typical overcoating materials (SiO2, MgF2, and Al2O3) in preventing tarnish formation on silvered mirrors was examined in detail, as were side effects in the ir caused by the presence of these protective films. After comparing the reflectances of freshly prepared silver mirrors with and without a protective overcoat, the mirrors were aged in room air, dry nitrogen, and moist 10% H2S in air and their reflectances remeasured periodically over extended periods in the 1–20-μ wavelength range. These measurements lead to the following conclusions. The reflectance of unprotected silver is not seriously affected at wavelengths longer than 2.5 μ by even a fairly thick tarnish film. In many cases the ir reflectance is decreased by the protective layers, resulting in a significant increase in the ir absorption of the mirror. Protective films do not give absolute protection against tarnish formation. Mirrors exposed to room air for 30 days typically show a decrease in reflectance of several tenths of a percent for both protected and unprotected mirrors. This decrease does not occur if the mirrors are stored in dry nitrogen. Overcoating layers do inhibit extensive tarnishing and improve the mechanical properties of the mirror.
© 1973 Optical Society of AmericaFull Article | PDF Article
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