The Thermite ReactionThe thermite reaction. Thermite is the reaction of three parts red iron oxide and one part powdered aluminum by weight. The reaction is used to repair the original electric welds of rails used for high-speed trains. The reaction needs to be started with a sparkler (magnesium sparks) because pyro fuse is not hot enough. The reaction creates molten iron which will leave blobs of red-hot molten iron in the bottom of whatever you used to hold it, such as a ceramic cup bedded in sand. The blob left in the bottom of the cup. The white pieces are what's left of the cup. The iron from inside the blob. Use a hammer to knock the slag off. A magnet separates the iron from slag.