If you have oxy, run the flame on acetylene only to leave a coating of soot on the panel, turn the oxygen back on, heat panel from the other side, the soot will burn off when you get to the right temperature. Allow to cool and ally will be workable. Have also heard dad talk about placing soap marks and when they go, it is hot enough. Never seen the soap trick but have seen metal shaping vids where the soot method was used, Ron Covell was the craftsman who shows the soot trick.

