|The chemistry of mirroring is simple but to get a reflective and tenacious mirror on glass takes some care and timing is very important.|
Cleaning is primarily a mechanical process - pushing, scraping or wiping off contaminants. The same wetting agents used to wet the surface later on, however, are usually necessary to encourage the residue to leave the surface. If you have just made the surface, for example a blown glass sculpture, then it is already clean. If it is soft like plastic then abrasives will damage the surface so you will need a chemical cleaner. If it has been sitting around your shop for a while, it will have accumulated dust, grease and atmospheric dirt and cleaning will be needed. Mirrors are applied in a water-based process and the surface must be wet so that water will "sheet" off and not make spots or dry areas.
The surface must be chemically prepared to accept the metal. Much research has been done on how this works. It is thought that the sensitizer leaves a very thin layer of metallic tin on the surface, essentially tin-plating it. By thin, we mean about one atom thick so you will not see it but it is there. While the metal will deposit to some extent without sensitizing, it will be thin and patchy and will lift off easily. The amount of tin deposited is critical - too much will make the metal deposit too fast and black or gray spots will result.
The Actual Mirror
What we are trying to do here is to coat a surface with silver, copper, gold or galena. We do this by dissolving the metal (lead in the case of galena) in a strong acid. We add a complexer to keep it from depositing too fast and an activator to set the mixture to just the right alkalinity. When you are ready to pour it on the surface, you add a reducing agent which brings out the metal. It does this as very fine particles which are attracted to the sensitized surface. If done at the right rate, a shiny metallic covering results. If done too fast, the particles clump together and form a black powder which easily falls off of the surface. If done too slowly then a thin layer results and your chemicals wash off.
Sometimes you can repeat the process and make a second mirror on top of the first to make it thicker. There is a limit, however, and 2-3 passes is usually about it. In terms of numbers, each layer is about 50 nm thick. That's nanometers - a millionth of a millimeter. By comparison, gold leaf is about one five-thousandth of a millimeter - about 200 times thicker. While this may not seem like much, the typical bathroom mirror is made industrially by this process and is usually double-coated.
Why Not Do It Yourself?
While you can buy your own chemicals to make mirrors, we have spent a lot of time developing formulas to save you the research on how much of each to add, how to get it to deposit at the right rate and how to avoid common problems such as unevenness, spots, pinholes and blotches. We have also tested in our laboratories various combinations of sensitizers, activators, reducers, mirroring chemicals and backing paints so they don't work against each other.
One of the other things we continue to investigate is other surfaces to mirror and different ways to get the chemicals onto those surfaces. While we started with the basic flat sheet of glass and silvering chemicals to make a classic "looking glass" mirror, we have developed other metals such as copper, gold and galena which produce pink, yellow and gray mirrors. We also provide tools and techniques for other shapes of surface. Unlike mirrors made with silver or gold leaf, our process can mirror inside objects like boxes and bowls and over 3-dimensional shapes like jewelry.
Unlike glass, plastic surfaces do not readily mirror because they do not "wet" easily. Our sensitizers contain a wetting agent to facilitate this but different plastics require different degrees of wetting. If you will be mirroring the front surface of an object, you may want to paint that surface first with a material that will wet easily.
The earliest mirrors were made of polished sheet metal which had to be re-polished frequently to remove oxidation and finger marks. A later development was called tin-amalgam which consists of metallic tin dissolved in mercury and impressed onto the surface. The modern water-based mirror was invented by the German chemist Baron Justus von Liebig in 1835.