Given these facts, it is very likely that the flow was due to light-alloy aluminum but also to the lead of the batteries, combined with other materials originating from the contents of the 81st floor. This combination of materials, after the failure of the floor, poured onto the 80th floor and from there flowed out from the building face. In my opinion, the weight of the batteries and the weakening caused by the fire on the 80th floor (which compromised the structural capacity of the overlying floor trusses) had an important role in the failure of the 81st floor.
Terrific job of research and analysis by Paolo's buddy Henry62! Highly, highly recommended!
Hat Tip: Ron (Pomeroo) Wieck.