Karen Binns, age 12, of Florence, S.C., for her question:
HOW DO THEY MAKE MIRRORS?
A mirror works because its smooth shiny surface bounces back the light. All sorts of shiny surfaces can perform this trick. For example, you can look down and see your face reflected from a quiet pool of water. However, the best kind of mirror is a sheet of glass, backed with several layers of dark material to stop the rays of light from passing through.
A good mirror is made from the very best glass and the dark material on the back is applied by experts. Actually, the basic job of mirror making sounds simple and quite easy. However, the work must be done with perfect precision, for every detail must be smoothly in place.
A mirror maker starts out with a thick sheet of sturdy plate glass. He checks to see that it is free from flaws and then polishes the surface to shiny perfection. Usually the backing is a thin layer of silver, and this is applied in a roundabout way. First, the glass is coated with a thin layer of tin chloride, then rinsed to remove any loose, flaky fragments. Next, the surface is coated with a solution of silver salts ¬and a chemical miracle occurs. Molecules in the first layer of tin chloride pull out silver atoms from the salts and stick them securely onto the glass. This miracle takes just a few minutes.
Then the glass is rinsed very carefully to remove all the surplus particles. The back of the mirror now has a well stuck coating of silver, which is thinner than thin. The glass is now a mirror, able to reflect a clear image. But the thin layer of silver needs some protection.
Usually the protective layer is copper and the job is done by electroplating. Silver happens to be one of the metals that reacts to electricity. So the mirror is placed inverted in an electrically charged bath which contains particles of copper. The silver attracts the copper in the solution. Atom by atom, the copper is drawn and deposited on top of the silver layer.
The back of the mirror now has a thin coating of silver to reflect back the light and a protective coating of copper. As a rule, the copper gets an extra coating of protective, long lasting lacquer.
There are dozens of ways to make a mirror, and nowadays anybody can try to make one. But in Venice, Italy, during the 16th century, only trained experts were allowed to make mirrors. What's more, if a mirror maker told his secrets to an outsider, he could be executed.