. Electric railway journal . bles are the only ones which satisfy these material is furnished in the form of magnesia tar. The tar acts as a binder for the magnesite and isburned out in the process of baking the lining. In the usual operation required for welding motorshells, a crucible lining will stand about twelve reactionsbefore it becomes so thin as to endanger the metal cas-ing. Ordinarily the point at which it is necessary tohave the crucible relined can be determined by watchingfor heating of the outside sheet iron. If a dull redspot should appear during the reaction, t
. Electric railway journal . bles are the only ones which satisfy these material is furnished in the form of magnesia tar. The tar acts as a binder for the magnesite and isburned out in the process of baking the lining. In the usual operation required for welding motorshells, a crucible lining will stand about twelve reactionsbefore it becomes so thin as to endanger the metal cas-ing. Ordinarily the point at which it is necessary tohave the crucible relined can be determined by watchingfor heating of the outside sheet iron. If a dull redspot should appear during the reaction, the crucibleshould not be used again until it is relined. Crucibles should be handled very carefully as thelining is liable to crack and fall out under rough treat-ment. It is also always important that they be stored ina dry place, as the lining, being porous, will absorbmoisture, and produce a violent thermit reaction. A certain amount of slag will be found adhering tothe inside of crucibles after they have been used. It is. SINGLE BURNER PREHEATING EQUIPMENT not necessary to clean out this slag, as it is a very re-fractory material itself and can do nothing but helppreserve the crucible if left on. At the bottom, how-ever, in the vicinity of the stone and thimble, the slagmust be removed so as to clear the opening of thethimble or permit the old thimble to be knocked outand a new one inserted. Very often it is possible toincrease the life of crucibles by patching the liningswith magnesia tar. This is particularly the case wherethey wear away in spots, or at the bottom. In thelatter case, care should be taken to patch the liningat the bottom with magnesia tar so as to cover the magnesia tar should be thoroughly baked beforethe crucible is used. In some cases fire clay has beenused for patching crucibles. This method is not recom-mended, however, and it will be found that if magnesiatar is used instead, it will stand up much better underthe heat of the thermit reacti
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