It is usual for the crucible support to be built in the form of a bridge spanning the mould box. Tubular steel scaffolding is often used for this purpose for reasons of economy and for ease of transport, storage, etc. The structure must be of sufficient strength to withstand the weight of the crucible and thermit charge. It will be subjected to vibration caused by turbulence in the crucible during the thermit reaction and to heat arising from waste gas issuing from vents in the mould during the preheating operation. These factors must be taken into consideration during construction. The structure can be shielded to some extent by the use of suitable deflector plates above the waste gas vents in the mould. All joints in the structure, partic- ularly if made of tubular scaffolding, must be examined at intervals during the preheating operation and tightened if necessary.