The enclosure on a locomotive where steam is produced. The boiler must be filled with water almost to the top. When the water boils, the steam it generates forms in the space between the top of the water and the top of the boiler. When enough steam collects, the pressure begins to build up until it reaches a useful working level. It will continue to build up until the maximum pressure is reached. This can be anything between 150 pounds per square inch (psi) and 300 psi, depending on the age and type of locomotive. To get a locomotive boiler up to working pressure from cold takes several hours. The water in the boiler is heated by fire. The fire is placed in the firebox at the rear end of the boiler and the hot gases generated pass through hollow tubes (made of brass, steel or copper) running the length of the boiler. "Tube plates", provided with a large number of holes to take the tubes, were fitted at the ends to seal off the boiler and to provide mounting plates for the tubes. At the front end of the boiler the hot gases escaped from the tubes into the "smokebox" and then upward to atmosphere through a "chimney" or "stack" as the Americans call it. At the rear, the tube plate formed the front of the inner firebox and is, in UK practice, made of copper, instead of the steel used for the front tube plate. The traditional form of boiler was the same diameter throughout its length and was known as the parallel boiler. A later type which became popular was the tapered boiler, which was narrower at the front than the rear. This allowed more of the water to be at the rear where the greater heat from the firebox was available. To reduce heat loss boilers are insulated and then covered with a thin steel sheathing. For many years, the insulating material was wood and then asbestos. In recent years various forms of natural or man-made insulating materials have been used.