The Tin-Mercury Mirror: Its Manufacturing Technique and Deterioration Processes

Abstract

Mirrors made of glass backed with a reflective coating of tin amalgam first came into general use in the sixteenth century. Production ceased around 1900. The mirrors were made by sliding glass over tin foil flooded with mercury. The mercury reacts with tin to form a layer of crystals containing about 19wt% of mercury alloyed with the tin. The voids between the crystals are filled with a fluid phase containing about 0.5% tin in mercury. This two-phase reflective coating changes with time: the crystals grow larger and cover a greater proportion of the glass surface, while mercury slowly evaporates. In time, tiny voids appear between glass and amalgam. The fluid phase also migrates to the bottom of the mirror. Corrosion of the amalgam produces tin dioxide and tin monoxide and releases liquid mercury from the solid phase. Amalgam mirrors contribute a few micrograms of mercury per cubic metre of air in a room. Measured values have all been far below the official toxic limit of 50μgm−3. The frames often contain drops of mercury and special precautions are necessary for safe transport, repair and disposal of the mirrors.

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