Calcination constitutes the first step in the factory treatment of ochres. It naturally transforms yellow or insufficiently red ochres by heating them to 1472-1652°F (800-900°C) to dehydrate the goethite (hydrated oxide), which becomes hematite. Traditionally carried out in wood-fired ovens, calcination now takes place in 49.2 ft (15 m) long and 3.2 ft (1 m) in diameter rotary ovens heated by gas.