Effect of atmospheric exposure on the infrared reflectance of silvered mirrors with and without protective coatings.

The effectiveness of typical overcoating materials (SiO(2), MgF(2), and Al(2)O(3)) 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% H(2)S in air and their reflectances remeasured periodically over extended periods in the 1-2 0-micro wavelength range. These measurements lead to the following conclusions. The reflectance of unprotected silver is not seriously affected at wavelengths longer than 2.5 micro 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.