I recently picked up 3 large files at a flea market. This afternoon I put them in my fire pit and with the help of a leaf blower got them cherry red. My problem was that due to their size 12"-14", I couldn't get them cherry red all at once (in other words only one half at a time). I thought I got them to non magnetic, but once they cool a little do they return to magnetic? I put my magnet to them while in the fire and sometimes they wouldn't be magnetic, but I just checked them and they are completely magnetic again. Do you think I got them hot enough?