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Pyrex metallic mirror plugs
The process used to give the plugs a metallic front is called "fuming". In a flame that is very hot glassblowers vaporize metal and the metal is carried in the flame in vapor form until it condenses onto a cooler surface, in this case the front of the plug. If everything is just right the metal stays bonded to the surface of the glass, leaving a very thin layer of metal on the glass.
Cleaning of the plugs should be with warm soapy water and a soft cotton cloth. Much like cleaning a pair of glasses with an anti-reflective coating.