I have some old files, how do you measure the carbon content?

You really can't, when I buy files or rasps I heat treat them and quench them. I put them in a vice and hit them with a hammer if they break they will make a good knife. If the bend, they go in the trash. Some do a spark test but I'd rather be safe than sorry.

I normally just do the tail of the rasps and files, that way I won't take too much off the stock I'm going to use. I heat them in a gas forge to test them.

Mike that's exactly how I do it, except I use my gas forge. My wife won't let me have a torch.

I couldn't survive without my torch.