This is Cutting Torch Slag?
The interiors of some of the jumbled support columns that littered Ground Zero contained molten metal (slag), as shown by this FEMA photo No. 4210. The slag is newer than the solidified surface of the cuts made in the column members at manufacture, evidenced by its darker, bluer color. The older cuts are oxidized to a light brown.
In places, this slag was apparently forced out of the joints between the column sections. Since the direction of the flow of the slag is from the center to the periphery in various directions, it appears that the slag is not the result of cutting efforts during the cleanup, but rather that the molten metal flowed while the columns were upright. FEMA photo 5709 also shows slag.
Since no official investigation questioned the origin of this slag, its determination remains an action item for independent efforts.