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Feature: Mirror, mirror on the wall…

August 11, 2012


Mirror, mirror on the wall…” go the well-known lines from Snow White and the Seven Dwarfs. Not just kings and queens but all of us use mirrors in our everyday lives.

What is a mirror? The typical mirror is a sheet of glass that is coated on its back with aluminium or silver that produces images by reflection.

In fact, from time immemorial, people realised the value of mirrors. Evidence found at excavation sites suggest that early mirrors were created by simply polishing a suitable substance until it became highly reflective.

The Neolithic Age is traditionally considered as the last part of the Stone Age. The discovery of Neolithic mirrors enlightened us with fascinating information. These mirrors were made by grinding down obsidian rocks (volcanic glass) and polishing them to an incredible sheen, allowing even subtle details to be clearly seen in their reflections.

It was noted that a Neolithic mirror was made by first selecting an igneous rock (obsidian) and then using a rougher stone to grind the base stone down to a flat surface on one side. After a flat surface was achieved, a finer grinding stone and clay slip (a suspension in water of clay) was then used to polish the stone to a fully reflective sheen. Extremely fine abrasives, such as ash, were also used. After a substantial amount of time and effort, a very primitive form of mirror was thus created.

Much later, the mirrors used in Greco-Roman antiquity and throughout the European Middle Ages were simply slightly convex disks of metal, and either bronze, tin or silver, that reflected light off their highly polished surfaces.

Then, a method of backing a plate of flat glass with a thin sheet of reflecting metal came into widespread production in Venice during the 16th century; a combo of tin and mercury was the metal used.

The chemical process of coating a glass surface with metallic silver was discovered by Justus von Liebig, a German chemist, in 1835, and this advance initiated the modern techniques of mirror making.

Present-day mirrors are made by sputtering a thin layer of molten aluminium or silver onto the back of a plate of glass in a vacuum. For a mirror of high quality, it is essential to use top quality glass. Polished plate glass is used for the finest mirrors.

How is a mirror actually made? A solution containing silver nitrate is poured onto the glass, and a deposit of silver sticks to the glass. The excess solution is poured off and the glass is dried. The silvered surface (the back of the mirror) is then coated with a protective substance to keep the silver from being rubbed off or scratched.

History tells us that the earliest mirrors were hand mirrors. Mirrors, large enough to reflect the whole body did not appear until the first century AD. Today, mirrors are used in household items and are considered as objects of decoration. Ornate mirrors in homes look elegant and add a touch of class to any corner. But more importantly, their use is of great value to the various instruments which are employed in the field of medicine.