Sublimation apparatuses are pieces of laboratory-grade glassware explicitly meant to contain the transformation of solids to gases (or gases to solids), without an intermediate gas stage. This is commonly used to purify compounds. A solid is placed in an apparatus and heated under vacuum. The solid vaporizes, condenses on a cold surface (like a cold trap), and leaves impurities behind. Once removed from the vacuum, the purified compound may be collected. This can only be done with solids that have appreciable vapor pressure below their melting point.