Certain substances may have special properties that cause them to emit light at different temperatures but ALL THINGS INCLUDING BLACK HOLES -yes, black holes- emit electromagnetic radiation http://en.wikipedia.org/wiki/Black_body so technically everything at 798K or 525C or 976F should start glowing red. ... in TPT metal starts glowing at around 300C. So it's not accurate anyways... but fo' sho' Iron should glow too.