The celadon colour is classically produced by firing a glaze containing a little iron oxide at a high temperature in a reducing kiln. The materials must be refined, as other chemicals can alter the color completely. Too little iron oxide causes a blue colour (sometimes a desired effect), and too much gives olive and finally black; the right amount is between 0.75% and 2.5%. The presence of other chemicals may have effects; titanium dioxide gives a yellowish tinge. Pieces made with a celadon glaze are themselves often referred to as "celadons".