heat riser on a rocket mass heater

I'd like to know if it is safe to use a piece of sheet metal for the outside of the heat riser. I'll be using black stove pipe in the center then cob and sheet metal rolled in the shape of a barrel to hold the cob in place. I have a 55 gallon barrel for the combustion chamber.

Hi James;  The black pipe in the center will melt.  The cob may last and the outer skin will be fine. The outer skin would only fail if the cob fails.

Hello James,   To add to what Thomas said, cob is not an insulant. A much better choice would be perlite stabilized with clay slip, fire brick or ceramic fiber blanket. There have been many conversations on this forum about this but it would really help if you could obtain a copy of the Rocket Mass heater builders guide as it is often the go- to manual answering tonnes of common questions and gives many proven examples.