Hi - I just started studying in preparation to build a rocket stove. I'm confused about what materials can be used for the inner pipe of the heat riser. My hope is to be able to use either a 6" cast iron pipe (old plumbing pipe) or sheet metal piping from Menard's that is typically used for duct work in a home. Should I have any concerns about using either of these two materials? I'm planning on using a sheet metal pipe for the outer layer to contain the insulation. Thanks in advance.