Originally Posted by Sunstealer
Good catch on this. I've often suggested that the material in question was the anti-corrosive coating applied to the steel structure during construction. NIST describes the anti corrosion coating applied to the steel beams (also used to do the infamous 600 C temperature test) in 1-3C
appendix D (check around page 433). Not shockingly, the pigments listed have iron, zinc, silicon and a proprietary pigment known as Tnemec. What's further, if you compare figure D4 to the ones shown in Harrit et al, you can't even begin to think it's anything else.