Platinotype has always been somewhat stubborn in producing deep rich blacks.The PVA and quick warm forced air drying immediately after coating seem to help. The best papers I have seen in this regard are double coated Platine and double coated Mars Vellum or Clearprint Vellum. I have gotten Dmax's of 1.5 with both of these papers. Add an ivory black gum-over layer for a brief exposure and you can do even better. I've never busted 1.35 with platinotype. But as Dick Arentz says, it's enough to convince most viewers that don't have scientific measuring devices in hand.