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H.7.2 Molten Material
[/FONT]It has been reported in the FEMA report (McAllister 2002) as well as in the media that what appeared to be molten metal was observed pouring from the north face near the northeast corner. This is the area where the sustained fires were seen. Video records and photographs indicate that the material first Interim Report on Evolution of WTC Fires Based on Image Analysis appeared at 9:51:52 a.m. and continued to pour intermittently from the building until the time of collapse. Some of the material can be seen falling in Fig. H–21. Close-up video and photographs of the area where the material is pouring from have been examined and show that it is falling from near the top of window 80-256. The most likely explanation for this observation is that the material had originally pooled on the floor above, that is, floor 81, and that it was allowed to pour out of the building when this floor either pulled away from the outer spandrel or sank down to the point where the window was exposed. The fact that the material appears intermittently over a several minute period suggests that the floor was giving way bit by bit.
The composition of the flowing material can only be the subject of speculation, but its behavior is consistent with it being molten aluminum. Visual evidence suggests that significant wreckage from the plane passed through the building and came to rest in the northeast corner of the tower in the vicinity of the location where the material is observed. Much of the structure of the Boeing 767 is formed from two aluminum alloys that have been identified as 2024 and 7075 and closely related alloys. These alloys do not melt at a single temperature, but melt over a temperature range from the lower end of the range to the upper as the fraction of liquid increases. The Aluminum Association handbook (Aluminum Association 2003) lists the melting point ranges for the alloys as roughly 500°C to 638°C and 475°C to 635°C for alloys 2024 and 7075, respectively. These temperatures are well below those characteristic of fully developed fires (ca. 1,000°C), and any aluminum present is likely to be at least partially melted by the intense fires in the area.