Barran

For a start, hello to all! RegionalChaos, sounds to me like you carbonized the mild-steel. I'm presuming that you burnt the end in a coal forge and not a gas forge, remember that coal is a form of carbon and it can migrate to the steel / iron. If you divide your pile of burning coal in to three horizontal levels, the bottom layer ( closest to the air grate ) is the Oxidizing Area. the middle is Neutral and the top layer is the Carbonizing Area! It would take a while to carbonize the bar all the way through but you could quite have easely have hardened the outside enough to notice it on the bandsaw. Note that modern case hardening powders have replaced this very old form of carbonizing steel/iron!!