I'd say it was a little bit of carbon burning out of the surface due to the oxidising atmosphere .. probably hardened fine under that .. if you use it again, get a small bottle of CO2 or Argoshield .. and squirt some in before closing the door to blast the oxygen out and replace the atmosphere with an inert gas .... it will reduce the scaling and oxidising on the surface ..... other than that. sounds like a very controllable way of HTing