`MBTA Thermite Rail Welding at Rosendale`

Tom's 30-years-ago site visit. Used to illustrate thermite welding as an exchange reaction: aluminum + iron oxide → iron + aluminum oxide + heat.

I went down to Rosendale once about 30 years ago to watch this. The MBTA was welding two train tracks together. It's a very simple process. You take this mixture of aluminum and iron oxide, put it in a steel bucket lined with firebrick. It's got a hole in the bottom, but the hole is plugged with a little piece of aluminum foil, actually maybe a 16th of an inch of aluminum sheet. You set up ceramic molds contoured to fit the sides of the railroad rails around the two tracks. You set the bucket on top, put the mixture in, set it off with a blasting cap, and stand back, because you've got the Fourth of July inside of there — sparks flying 10 feet in every direction. Hopefully you don't set fire to someone's lawn in Rosendale. Within 20 or 30 seconds you've built up enough iron that it melts through that aluminum plug, because aluminum melts at a fairly low temperature, and all this molten iron drops down and makes a little casting. We call it a thermite weld.