How to heat treat a file?
""Hi Shaggy, The best method to establish the temperature to reharden at is by using a magnet. The steel will become non magnetic once the bottom end of the critical temperature zone is reached. If you go above this by around 200 degrees you will obtain the full hardness without the guess work""
Reminds me of being at the Farmers Museum in Cooperstown NY ~30 years ago. Watching a blacksmith working on an old forge. I asked him how he knew when the metal was hot enough. Showed me using an old Cow Magnet. But then said: 'or you can do it the way the 'old timers' did': ...and ran the piece of hot orange bar across his palm!!! Said ' just about right'. I said 'What' and he did it again. Asked if it didn't hurt?? He said: 'No, you get use to it!!'
Still one of the darnedest thing I've seen!