I need to cut away a section of the 4" vertical stack pipe in my 1917 house to add a 2" PVC line from a new shower. Trouble is that I've never seen this kind of pipe before. Most of the house is cast iron but this 6' or so section of the stack is a dark gray nubby rubberized material. Sounds like metal underneath when I bang on it but I'm not sure. Anyone know what this is? Can I replace part of it with PVC and tie it together with 2 Ferncos?