Molten metal

Just before 9:52 a.m. a bright spot appeared at a window on the 80th floor of the South Tower and a glowing liquid poured out for about four seconds; there were many such flows from near the same spot in the seven minutes before the collapse, and none from anywhere else. NIST concludes it was molten aluminum from the aircraft. [1]

Critics say molten aluminum looks silvery and that orange-yellow means hotter iron. NIST agrees pure aluminum would look silvery, but says this metal was very likely mixed with hot, partly burned furniture, carpet and computers, which “can display an orange glow, much like logs burning in a fireplace.” [1]

FEMA’s metallurgists examined two steel members with unusual erosion, one apparently from WTC 7 and one from a tower. They found “a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting,” with temperatures approaching 1,000°C. “No clear explanation for the source of the sulfur has been identified,” they wrote; it might be the result of long heating in the pile, or have started before the collapse. They called for a detailed study. [1]

NIST says investigators who inspected the steel found no evidence of steel melting in the fires before collapse, and that the condition of steel in the rubble was “irrelevant” because it tells nothing about the buildings while they stood. [2] Demolition proponents argue sulfidation is a signature of thermate, a thermite variant containing sulfur; NIST notes the gypsum wallboard throughout the buildings contained sulfur.