Mirrors started out as highly polished brass, gold, or other metal. Eventually, people found a way to make mirrors out of glass with a metallic back acting as the reflective material. Starting towards the end of the 12th century, metallic backing on mirrors became widespread by the 16th century. However, the backing of early mirrors was a mixture of mercury and tin, so it was a dangerous task due to the fumes and mercury poisoning. Because of the risk and time involved in making mirrors during this time period, mirrors were very expensive. Only the rich could afford a mirror, which would equal around $40,000 in today's money.
During the 19th century, a new process was invented for making mirrors, although the origins have been debated. This new method coated the glass with silver instead of the dangerous mercury mixture. This led to the modern techniques used now in which a thin layer of silver or aluminum is sprayed onto the back of the glass. Today, this is done in a vacuum to prevent contaminates from getting into the backing.
The process of silvering the back of the mirror is completed in an assembly line fashion in basically eight steps.
1. The glass is loaded into the vacuum unit.
2. The glass is cleaned and dried.
3. The backside of the glass is sprayed with sliver nitrate.
4. The silver layer is coated with copper or coppersulphate to protect the silver.
5. The mirror is cleaned, dried, and hardened.
6. A first coat of paint is applied to further protect the silver.
7. After the first coat is dried, a second coat is applied.
8. Finally, the mirror is cooled and cleaned.
If the manufacturer chooses an environmentally friendlier method, they will not use coppersulphate, lead based paints, or ammonia salts.
Protecting the silver backing of the mirror is very important since it provides the reflective surface of the mirror. If it is damaged or begins to deteriorate, the surface may appear dull, dirty, or streaked. You may need to buy a new mirror or have it resilvered if this type of deterioration occurs. Because of the advancements in the technology involved in making mirrors, everyone can afford some type. If you have more expensive mirrors, resilvering them will be just as cost effective as replacing them when the silver is damaged.