The process of depositing a reflective aluminium coating on a mirror. The mirror is placed in a vacuum chamber and the aluminium is vaporized by heating. With no air molecules to collide with, the aluminium atoms travel directly to the mirror, coating it with a thin, even layer (about 100 nm thick). Subsequent layers of other materials, such as silicon dioxide, may then be deposited in a similar way to protect the coating. Large telescopes have aluminizing chambers housed within the observatory building so that the mirror can be re-aluminized on site whenever necessary. The reflectivity of a fresh aluminium coating with overcoating is 89%, declining by a few per cent a year as a result of oxidation of the surface.