The next step is the preparation of the broken parts for welding. Most boys know how the doctor has to put splints on a broken arm so that it will knit properly. It is something like that with a thermit weld. The broken parts are supported in exact alignment by heavy blocks of concrete, and the fractured ends sliced off clean by the oxygen-gas torch. This leaves a space of from one inch to two and a half inches between the fractured ends, just according to the size of the piece to be welded. After the parts are all thoroughly cleaned the workmen are ready to take the next step. This is the preparation of the mould for the weld. First, a pattern of the weld, as it will appear when completed, is put on the fracture with beeswax. The space between the broken ends is filled in and a thick "collar" of wax is packed around the parts, so that when this is done the pattern looks like a swelling on the frame. The mould is then built around this wax pattern. The inventor of the thermit process had to make a number of experiments before he found a material refractory enough to stand the terrific heat to which the mould had to be exposed. Finally he decided upon an equal mixture of fire brick, fire clay, and fire sand. With this material, then, the workmen go about making the mould. It is solid, with the exception of three apertures or tunnels, which are left by inserting in the moulding clay, wooden models of the size and shape desired. These are a gate, or place into which the molten welding material is to be poured, a "riser" or larger hole into which the surplus material can run for the overflow, and a heating aperture. The gate runs from the top of the mould down to the lowest point of the wax pattern, while the "riser" extends from the top of the wax pattern to the top of the mould. Thus we really have a small inlet and large outlet, although it is always arranged so that the surplus metal remains in the riser, and as little as possible runs over. The heating aperture is a small hole in the side of the mould extending to the bottom of the wax pattern. With the mould complete the wooden models of the gate, riser, and heating aperture are pulled out and the first step in the process of welding is taken. The long pipe of a specially constructed gasoline compressed-air torch is inserted in the heating aperture and the process called preheating started. The gasoline torch, of course, quickly melts the beeswax, and leaves the space occupied by the pattern clear for the molten metal that is to be introduced to make the weld. The blast from the torch is continued through this heating aperture until the parts to be welded have reached a red heat, because if this were not done the cold steel would so chill the molten thermit steel that the weld could not be accomplished. The length of time taken by this preheating is governed, of course, by the size of the parts to be welded. Sometimes it is many hours.