Chemical vapour transport results in very pure materials. The reaction typically occurs in a sealed ampoule. A transporting agent, added to the sealed ampoule, produces a volatile intermediate species from the solid reactant. For metal oxides, the transporting agent is usually Cl or HCl. The ampoule has a temperature gradient, and, as the gaseous reactant travels along the gradient, it eventually deposits as a crystal. An example of an industrially-used chemical vapor transport reaction is the Mond process. The Mond process involves heating impure nickel in a stream of carbon monoxide to produce pure nickel.