Reply #5: Here is a good discussion ... look at the temperatures they measured.

[hack89]

Combustion products continued to be emitted from the debris pile in the ensuing months. Dust was "no longer part of the plume per se after about day three or four because the rains came and washed some of the dust and smoke away," Lioy said. What was left were smoldering fires.

The fires, which began at over 1,000 °C, gradually cooled, at least on the surface, during September and October 2001. USGS's AVIRIS also measured temperatures when it flew over ground zero on Sept. 16 and 23. On Sept. 16, it picked up more than three dozen hot spots of varying size and temperature, roughly between 500 and 700 °C. By Sept. 23, only two or three of the hot spots remained, and those were sharply reduced in intensity, Clark said.

However, Clark doesn't know how deep into the pile AVIRIS could see. The infrared data certainly revealed surface temperatures, yet the smoldering piles below the surface may have remained at much higher temperatures. "In mid-October, in the evening," said Thomas A. Cahill, a retired professor of physics and atmospheric science at the University of California, Davis, "when they would pull out a steel beam, the lower part would be glowing dull red, which indicates a temperature on the order of 500 to 600 °C. And we know that people were turning over pieces of concrete in December that would flash into fire--which requires about 300 °C. So the surface of the pile cooled rather rapidly, but the bulk of the pile stayed hot all the way to December."

http://pubs.acs.org/cen/NCW/8142aerosols.html