That brings back memories! Early in my career, I worked on evaluating transformers, anything from small door-bell (Class 2) transformers to large distribution transformers. Ignition transformers (for oil furnaces) were typically potted in tar at that time.
While I don't recall those melting down during testing, I vividly remember testing a wall mounted power transformer. It was installed in a steel enclosure and potted with a sand and epoxy mix. Part of the test program was a 30 minute Overload test at 200% rated current. At 29:30, the windings melted down filling the lab with the most disgusting, toxic cloud of smoke. We didn't make a lot of friends that day!
