THERMIT, a name given by Goldschmitt to a mixture of fine aluminum filings or pow der and iron oxide. When this mixture is ignited by some suitable means the aluminum unites vigorously with the oxygen of the iron oxide, forming a very pure variety of steel and a slag consisting mostly of aluminum oxide. This union of finely divided aluminum with oxygen gives a very intense heat, about 3,000° C. Besides the ordinary thermit (iron oxide and aluminum filings) other mixtures may be prepared from aluminum and the oxides of nickel, cobalt, chromium, manganese, etc.
When these are ignited in a properly prepared crucible violent reaction takes place, the oxy gen of the oxide being taken up by the alumi num, leaving a very pure metal, nickel, cobalt, etc. This process is now much used to get metals from those oxides that heretofore have resisted all ordinary methods of reduction. When ordinary thermit is ignited the temper ature produced is so high that the iron and the slag are left in a molten and highly-heated condition. If this iron is allowed to flow on to another piece of iron or steel it will heat it enought to soften it and the whole will harden to a homogeneous mass. In this way it can be used to replace broken parts of machinery, to mend broken or cracked propeller shafts, to weld together railroad rails so as to form one continuous rail, etc. Some of the im
portant features in this process are its cheap ness, ease of execution and the fact that ma chinery, etc., can be repaired in position. The method is to surround the part to be repaired with an ordinary mold box ; a magnesia-lined crucible with a plug in the bottom is placed over the opening; the thermit is placed in the cru cible, ignited and as soon as the violent re action has subsided the plug is pulled and the white hot metal allowed to flow into the mold. Iron tubes can be welded together by placing the ends in a mold and allowing the thermit product to flow in in such a way that the liquid slag first comes in contact with the tubes. The slag forms a protective covering which prevents the hot iron from uniting with them, though it does allow the tube ends to become hot enough to unite as one piece. Railway rails can be joined where they lie on the track, and large pieces of broken machinery in mines or on shipboard can be readily repaired without removal.