What Is the Process for Silvering a Mirror?
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The Oldest and Newest
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Until the late Middle Ages, people admired their reflections in pieces of polished metal. Silver would have been the choice of the wealthy, but bronze, tin, or even copper were also used. Ironically, the most technically advanced modern mirror, developed by NASA for space telescopes, is also a piece of polished metal---in this case, aluminum.
Glass Mirrors
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Most modern mirrors consist of a piece of glass that has a thin film of metal on one side. While silver is only one of a number of metals that can be used, the process of applying the metal film to glass is generally referred to as "silvering." In common household mirrors, the silvering is on the back side, or "second surface," where it is protected by a backing of shellac or special paint. Mirrors made for use in fine optics, such as telescopes, have the silvering on the front, or "first surface."
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Amalgam Mirrors
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The earliest mirrors made this way were "amalgam mirrors," which used an alloy of tin and mercury. (Any alloy of mercury is called an "amalgam.") To make an amalgam mirror, mercury is spread over a thin sheet of tin, the glass is pressed down into the mercury, and the excess mercury is drained off. The tin-mercury-glass sandwich is then allowed to stand under pressure for at least a day, while the mercury dissolves the tin to make the amalgam, which adheres to the glass.
Silver Mirrors
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J. von Liebig developed a process for making true silver mirrors in 1835. The actual procedure is quite complicated, but it involves a chemical reaction in which the nitrate ion in a silver nitrate solution becomes attached to another ion, leaving the silver, which is deposited on the glass.
Aluminum Mirrors
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Economical modern mirrors use aluminum as their reflective surface. While a hobbyist could produce an amalgam mirror or a silvered mirror, aluminized mirrors require industrial machinery. In one method, evaporation, an aluminum wire is heated to vaporization by an electric current in a partial vacuum, where it then deposits on the glass surface. An even more sophisticated method, sputtering, involves ionizing the aluminum vapor and spraying it on the glass.
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- Photo Credit Photo: janelle siegrist
