Locomotive boilers were originally made of wrought iron but a steel boiler was first used in the UK by George Tosh of the Maryport and Carlisle Railway in 1862. A locomotive with a steel boiler had appeared in Canada two years before in 1860 and it gradually became general around the world from that time although wrought iron was still being used in the UK on some railways 30 years later. The steel used in boiler construction has to be of good quality. The stresses required of even a small boiler are considerable and the results of a weakness in design or construction could be fatal if a boiler explosion occurred. Leaks, although less serious, would cause poor steaming, with possible delays to trains and would cost money in repairs and lost availability. It is essential that a boiler be manufactured to a precise circular form in section. This ensures an even distribution of stresses. The boiler plates are first bent by passing them through rollers and are then riveted together to form circular sections. Barrels up to 12 feet long are usually made of two sections, longer ones being of three sections. The rolled plate of each section is butt jointed to give the circular form required and special 'butt strips' added above and below the joint. The whole seam is then riveted up so that each section or 'ring' has one longitudinal joint. It is important that a true circle is formed by each boiler ring so that, when steamed, the boiler rivets are not put under excessive stress. Boiler rings were also secured to each other by riveting lap joints. In the later years of locomotive development, some parts of and even complete boilers were welded. An all-welded boiler was fitted in the US to a Delaware & Hudson 2-8-0 locomotive in 1934 but it had to undergo several years of regular use before it was accepted as safe by the Interstate Commerce Commission. In Britain, the Southern Railway 4-6-2 locomotives designed by O.V. Bullied and introduced in 1941 had welded boiler seams and parts of the firebox were also welded. Adding the firebox was a complicated operation. The 'throat plate', which joined the lower part of the firebox to the boiler barrel was a particularly awkward shape requiring a hydraulic press for its manufacture. The remaining parts of the firebox have also to be specially formed and then assembled to provide the inner and outer fireboxes. The assembly involves screwing into position a large number of staybolts or stays, which hold the inner firebox in the correct position inside the outer firebox. Later locomotive designs could have over a thousand stays in the firebox. Before inserting the tubes, the riveted joints are caulked to ensure that the boiler is steam tight. The tubes are expanded into position in the tube plates.