Small quantities of explosives, in total just a day's worth, were delivered continuously to the pouring and filling buildings. Ammonium nitrate, a white crystalline solid, was crushed to a fine powder and delivered to kettle rooms, where it was mixed with molten TNT. Heavy brick firewalls separated the kettle rooms from the other portions of the building. The mixture, known as amatol, was then sent to extruding machines, which workers operated to fill each shell. Heavy concrete walls separated the extruding machines from each other, and outward-swinging doors were placed directly opposite the workers' stations in case of explosion. Ventilating systems kept the air free from noxious fumes and gases. At intervals, a specially made split shell was loaded, then taken apart and examined so loading defects could be corrected before large quantities were produced.