A mirror may appear to be made of reflective glass, but in reality, a mirror is made of plain glass with a silver coating behind it. The silver makes the mirror reflective. Over time, the silver may become damaged, leaving your mirror with dark or discoloured spots. If the mirror is an antique or otherwise valuable, such as if it has elaborately bevelled edges, you may want to resilver it instead of replacing it. The process uses several caustic chemicals best handled by professionals.
When you remove your mirror from its frame and backing, you'll see the back side of the mirror was coated with paint to protect the silver. You can use any commercial paint stripper to remove the paint and access the layer of silver.
Nitric acid dissolves the layer of damaged silver on the back of your mirror. It will also dissolve the copper applied over the silver. Nitric acid is a highly caustic chemical. Be sure to wear protective gear, such as gloves and goggles, if you work with it.
Tin chloride is used as a bonding agent when resilvering mirrors. When tin chloride is mixed with distilled water, it separates into its two components, tin and chlorine. The change in tin chloride's ionic structure is what allows the silver to bond with the surface of the glass.
Silver nitrate is a combination of silver in a salt solution. When it comes in contact with separated tin chloride, it takes an electron from the tin and precipitates, meaning it drops out of the mixed solution. The silver deposits itself on the glass, making the reflective surface that is a mirror.
Dual Nozzle Spray Gun
The easiest way to apply the mixture of distilled water, tin chloride and silver nitrate to the back of the mirror is to use a dual nozzle spray gun. This way you can coat the mirror with both chemicals at once, while they form the reaction that allows the silver to adhere to the mirror surface.
Once the silver has dried on the back of the mirror, you'll apply two layers of paint over the silver to protect it. First you use copper paint, followed by a coat of grey protective paint. These ensure your silvering won't be gouged or damaged.