Light bulbs, or "glass envelopes" as they are called, are of course shaped automatically by machine. In many parts of the world it is traditional for manufacturers to offer customers a choice between clear bulbs and bulbs with a matt finish, usually known as "pearl" or "frosted". In this "frosting" process, hydrofluoric acid is used together with ammonium bifluoride and once again it is the unique ability of hydrofluoric acid to etch the internal surface of the bulb which makes it possible to achieve the desired matt effect.