Doug Femrite, age 11, of Eugene, Oregon, for his question:
How do they make mirrors?
Reflections never fail to fascinate. No doubt our caveman ancestors were bewitched by mirrored images in quiet ponds. In time, they got used to reflections and learned to use them. But nobody could explain them until scientists figured out the nature of light. When we look directly at the scenery, images are carried to our eyes by beams of light. The surface of a mirror bounces off the light rays and we see reflected images.
The average mirror is a sheet of glass with a polished surface and a backing of material that stops the light from passing through. The backing is usually a thin layer of silver or aluminum, protected with coats of copper and lacquer. The process sounds so simple that you would think anybody could make one. However, the work must be done so precisely that mirror making requires expert skills.
The slightest irregularity in a mirror bends a few light rays at odd angles, causing distortions in the reflection. In making a quality mirror, the first requirement is a trained eye for selecting a perfectly flat sheet of flawless plate glass with a highly polished surface. The layers of backing must be so thin and even that they cannot be spread with brushes or spray cans. Chemical reactions are used to apply them, atom by atom. The first layer is a shiny metal that bounces the reflection straight back through the glass. Aluminum may be used, but silver gives the brightest image, though it is less durable.
In the best mirrors, a silver backing is applied with the help of a chemical reaction. First a thin layer of tin chloride may be spread on the glass and then rinsed with clean water. Next a solution of silver salts is applied. In a few minutes, the tin compound Arabs silver from the silver compound and sticks it to the glass. After careful rinsing, the back of the mirror should have a very thin, perfectly level layer of silver.
This is checked and verified for flaws. If the morror shows a bright, clear reflection with no distortions, the fragile layer of silver is coated with protective materials. Usually the first coat is copper and it too must be applied with precision. Since silver reacts to electricity, this can be done by electroplating. The mirror is dunked into an electrically charged solution which contains copper.
When the chemical bath is charged with electric current, the silver backing attracts copper from the solution, atom by atom, coating itself with a protective layer. For added protection, the two layers of metal pet an extra coat of durable lacquer.
Nobody knows who invented glass mirrors, but in the 16th century the best ones were made in Venice and the skills were kept secret. Anybody caught divulging the secrets of Venitian mirror making could be executed. And if the blabbermouth left town, his remaining relatives were punished by imprisonment. Nowadays, of course, anybody who has a mind to can practice or publish the skills of mirror making.