In the bare-metal state, the surfaces of aluminium and its alloys have a very high reflectance for light and heat rays. On reflection the incident and the reflected ray form the same angle with the surface, in contrast to scattering where the incident rays are reflected in different directions and the surfaces thus appear matt. The smoother the surface, the better is the reflection and the shinier is the surface. Surface treatments are used to produce smooth surfaces on aluminium.
Foil or strip is usually subjected to final rolling and/or polishing using rolls with a high gloss finish. Acid or lye baths (with or without applied electrical currents) are used to remove any remaining extremely fine unevenness and a highly polished surface results. This would, however, quickly lose its sheen by reacting with the air to form a natural oxide layer (become dull). It is therefore covered with a permanent protective layer by anodising or, in some cases, by applying a clear varnish coating. Anodising does, however, reduce the reflectance for certain heat rays.
A second process is vapour deposition of aluminium in a high vacuum which leads to the precipitation of thin, uniform layers, only a few ten-thousandths of a millimetre thick, on other materials such as metals, plastics and paper.
One achieves the highest sheen on refined aluminium and on special alloys for mirrors and reflectors in floodlights, car headlights and torches. Unalloyed aluminium also results in a good sheen on household equipment and decorative trim.