Imagine the surprise of the first cave person who ever glanced into a reflecting pond. The image probably wasn't too pleasing but the effect was surely intriguing. Vanity was instantly born and the search for better reflective materials was underway. The discovery of metals yielded polished sheets which served well until sometime in the 12th century when it was discovered that glass with a metal backing produced a near perfect image. Craftsmen adept at making mirrors guarded the secrets of their process jealously.
A discovery by the great German chemist Justus von Liebig in 1835 made mirrors widely available. Liebig found a way to coat glass with a thin layer of metallic silver by depositing the metal directly by means of a chemical reaction. He applied a solution of silver nitrate in ammonia to the glass and exposed this to vapours of formaldehyde. The silver nitrate was converted into a thin layer of metallic silver which adhered to the glass. Presto, a mirror was born!
Everyone was amazed at the ease with which high quality reflective surfaces could now be formed. Everyone that is, except the workers who had to deal with the chemicals. One of the byproducts of the reaction turned out to be ammonium nitrate which is explosive! In fact if a residue of this substance was left on the mirror, the mirror could crack at the slightest disturbance. Could this be the origin of the notion that an ugly face can crack a mirror?
Like silver anywhere else, the backing on a mirror can tarnish. Reaction of the silver with sulfur compounds in the air can result in the formation of dark, non-reflective silver sulfide. This is usually not a problem because the silver is deposited onto the glass in an airtight fashion. Sometimes, however, mirrors do develop an unsightly black edge. This happens when water seeps between the glass and the layer of silver. Since the presence of water speeds up the tarnishing reaction, the reflective surface suffers.
The preventive technique of course is to wipe off excess water from the edge immediately after cleaning a mirror. The best technique is to wet the cloth, not the mirror. Special care has to be taken with mirror tiles to prevent the seepage of water into the spaces between the tiles. Mirrors can also be made by applying an aluminum coating to glass. With modern technology aluminum powder can be evaporated in a vacuum chamber and deposited on glass with the metallic coating being protected with a backing of waterproof paint. The best mirrors, though, are still made of silvered glass, and if we take proper care, that mirror on the wall will be able to tell us who is the fairest of them all for a long, long time. All because of some very clever chemistry.
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