A cremator is an industrial furnace that is able to generate temperatures of to ensure the disintegration of the corpse. Modern cremator fuels include oil, natural gas, propane, and, in Hong Kong, coal gas. Coal and coke were used until the early 1960s. Modern cremators automatically monitor their interior to tell when the cremation process is complete and have a spyhole so that an operator can see inside. The time required for cremation varies from body to body, with the average being 90 minutes for an adult body.