When
most people use the term “mirror,” they are referring to what is known
as a plane mirror. This object takes the light that
hits it and reflects it back. Those used for common consumer purposes
are of this sort.
A
mirror is essentially a highly reflective surface. The type that people
see on walls or in bathrooms are known as back-silvered mirrors. This
means that the reflective surface — in most modern ones, this
is aluminum — is viewed through a thin layer of glass. The
glass protects the aluminum from scratching and bubbling, but also
distorts the image somewhat.
Early
mirrors were created by simply polishing a suitable substance until it
became highly reflective. Examples from the Neolithic era have been
discovered, made by grinding down obsidian rocks and polishing them to
an incredible sheen. They have remarkable properties, allowing even
subtle details to be clearly seen in their reflections.
To
make such a device, a person first needs to find the right stone:
obsidian, ideally. A rougher stone is used to grind the base stone down
to a flat surface on one side. After this is achieved, a finer grinding
stone and clay slip can be used to polish the stone to a fully
reflective sheen. Extremely fine abrasives, such as ash, can help
further. After a substantial amount of time and effort, the person will
have created a very primitive form of mirror.
Modern
ones are made using an entirely different process. Allowing liquid
metals to condense on a sheet of glass can provide a surface far more
reflective than anything achieved by polish. This can be done at home
with only a few supplies easily acquired at a local chemistry
shop.
With
pure silver nitrate, distilled water, and ammonia, a person can make a
mirror virtually indistinguishable from those purchased at a store. The
process (in abbreviated form) involves dissolving a small amount of
silver nitrate in distilled water, then adding diluted ammonia until the
mixture goes through distinct chemical changes. A second mixture is
made using silver nitrate and Rochelle salts. This mixture must be
boiled and filtered. By pouring these mixtures on to a very clean piece
of glass sufficiently heated to the proper temperature, the silver will
precipitate and form an even coating on the glass. After drying, the
back of the silver can be coated with a solid paint to help prevent
degradation of the silver.
Commercial
mirrors are manufactured in more or less this same manner, though
materials such as aluminum might be used instead of silver. Ones made
for specialized purposes, such as those to be used in lasers or
telescopes, are manufactured using much more exacting techniques to
achieve much greater precision, but the general principles remain the
same.