After the chemical reaction is complete, time is allowed for the dense steel and lighter slag to separate and the crucible is tapped 20–60s after ignition. The molten steel flows into the mould and fusion takes place with the parts to be welded, Fig. 15.1.
Since the ordinary thermit charge has a high ignition point, it is necessary to place a low ignition point powder on top of the charge. The ignition powder in general use is composed of fine aluminium powder with a peroxide, chlorate or chromate. For most welds the end of the pieces to be welded will be preheated to a temperature of up to 1000°C. Preheating torches, which are inserted into the heating gate of the mould, are fired by propane or butane and air, or by propane and oxygen. The process can be operated in the open air or under cover. It is frequently used for welding lengths of railway line.