The furnaces can also be found in the massive factories(?) that you can only enter the stairs on. There is one in the Saturn factory as well as Novy Bor. For reference the building is the one the sniper hides in for Novy Bor siege in this video: https://www.youtube.com/watch?v=82cUAELaHpY

Would someone happen to have an extra first melting metals book they would be willing to part with?

Sooo Where Can We find the TECHNOLOGY OF MELTING COMBINED ALLOYS ??

I would farm sabina and vlad.