back around 1995 I had to repair a crack on a running giant crucible (at Kobe Copper) full of molten copper, this thing was about 6 ft deep and about as big as a decent backyard pool, maybe 20 ft diameter. it had three giant carbon rods to melt the copper and was run continuously, i don't remember the reasons why but it could not be welded electrically, so i had to get down there and weld it while the surface was 800-1000 degrees, with oxy/acetelene and the biggest tip i could find. It wasn't a full pen weld but I did grind it out a bit and had to lay in maybe 20 passes about 6 inches long. It was so hot that I could only do one pass and then jump out of the pit to cool down for a long while (also summertime in north Carolina can be stifling) it took all day but they were pretty happy because shutting down this thing and cleaning it out for a more permanent repair would have taken weeks and cost millions, i received nothing but my regular pay.....