And if I may add: There were welding events after the collapse as well, in order to remove debris. Unless I'm missing something, post 9/11 events could also contribute to the formation of the spherules, correct?
Or were the samples confirmed to have been taken prior to major rescue operations?
The argument about salvage welding goes that, without starting at great height, a huge thermal plume, or the energetics of collapse, any iron microspheres created later are unlikely to have traveled from Ground Zero to their alleged recovery point. This is a tricky argument, but it does bias us away from post-collapse explanations.
However, as Dr. Greening correctly points out, there are numerous potential sources pre-collapse that have nothing to do with thermite. Dr. Jones is exerting both Assuming the Consequent and Special Pleading in his reasoning, as usual.