Vitrified clay pipes may be capable of withstanding high heat, but they are not going to be capable of absorbing fast temperature changes, or probably uneven heating, without cracking. The outer steel drum will not be exposed to the major heating, and perhaps due to the oxygen-poor atmosphere inside, are known to last a long time (decades) without deteriorating. The internal chimney, or heat riser, needs to have the lowest mass and best insulation you can get. Vitrified clay fails on both counts. You want a semi-porous material that will hold its shape and not deteriorate with extreme heating. Insulating firebrick is ideal but expensive and may be hard to get in remote areas. Ceramic fiber is ideal but may be fragile (or may be quite durable), also expensive and can be hard to get. Perlite or vermiculite mixed with just
enough fireclay to hold it together, and formed into a cylindrical tube, is cheap and fairly easy to get, and very effective. A metal stovepipe of the right diameter as the outer form for this tube will also last a long time, as it is protected from the corrosive interior atmosphere by the lining.
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