The paper deals with intumescent polypropylene (PP) undergoing extreme fire (burn-through test with heat flux higher than 100 kW/m2). The purpose of this unusual approach is to explore the possibility to design intumescent plastic (here PP) resisting to burn-through test. A combination of commercial intumescent flame retardants (ammonium polyphosphate-based compounds containing a char former; AP766 (AP) and FlameOff (FO) of the companies Clariant and FlameOff Inc) with zinc borate (supplied by US Borax, ZB) or Kemgard (combination of ZB and molybdate supplied by Huber, KZ) was incorporated in PP. Use of ZB and KZ as synergists in FO formulations increases dramatically the time of piercing (formation of hole through the plaque of polymer at 80 s without ZB or KZ vs. 280 s with KZ) at the burn-through test (heat flux ¼ 116 kW/m2, propane burner) while the combination with AP does not show any benefit. Analyses of the residues obtained at different times of combustion by solid state nuclear magnetic resonance (NMR) of 31P, 11B and 13C shows the formation of borophosphates creating a glass reinforcing the intumescent char: it acts as a ‘glue’ providing flexibility and cohesion to the char.