Why Do Edges of Mirrors Turn Black?
A Silver Nitrate solution is applied to the cleaned surface of clear glass which forms a silver film reflective surface on the glass. This transforms glass into mirror. The silver film applied to glass is extremely thin. It can be scratched, damaged by chemicals, or attacked by particles in the air. During manufacturing the back surface of the film receives a coating of copper sulfate to protect the reflective surface from damage and contaminates. The copper sulfate then receives a coat of protective paint. During installation of the mirrors it is not unusual for the seal of the protective coating to be damaged particularly on the edges of the mirror where the seal is extremely thin. By its very nature the silver nitrate and the copper sulfate react to contaminates in the air, as well as moisture and corrosive cleaning agents, and then oxidizes, forming Black Edge.
