Glass, the major mirror component, is a poor reflector. It reflects only about 4 percent of the light but possess the property of uniformity, particularly when polished. Glass is also considered a good material for mirrors due to its transparency, ease of fabrication, rigidity, hardness, ability to take a smooth finish, and can be molded into various shapes for specialty mirrors. Silica, which can be mined or refined from sand, is used to make glass.
For the production of high-quality scientific grade mirrors a few other types of glass are used. These types of glass usually contain some other chemical components to make the glass stronger or make it resistant to certain extreme environmental conditions. Pyrex, for example, is a glass composed of silica and boron that is used when mirrors must persist high temperatures.
To manufacture mirror glass needs to be coated. The most commonly used materials that are appropriate for this application are metal coatings such as silver, gold, and chrome. One hundred years ago silver was the most popular metal coating for, leading to the invention of the term ''silvering''. Before 1940, mercury was the most commonly used metallic coating for mirrors because mercury is spread evenly over the surface of the glass and did not tarnish. This practice was eventually abandoned, because mercury seals in the toxic liquid. Today, mirror manufacturers use aluminum instead of mercury.
Scientific grade mirrors sometimes use coatings of other materials, like silicon oxides and silicon nitrides, in up to hundreds of layers of, each a 10.000th of an inch thick. These types of coating are used as reflectors, and as protective finishes on metallic coatings. In comparison with metal, they are more scratch resistant. Scientific mirrors are also coated with silver and sometimes with gold, to reflect light of a particular color of light more or less well.