We have a kiln and are considering making a 8" ceramic heat riser out of porcelain clay, it will be fired to 2381 degrees and be in two pieces. Should we use a smooth glaze finish on it our leave it rough? We will probably make it 3" thick and be wrapped in insulated fiberglass. Do you have any suggestions and do you think it will work?
Porcelain is not likely to survive the thermal shock of a RMH heat riser, especially at 3" thick. A riser that thick made of porcelain would be more conductive mass than desirable anyway; you really want a thin rigid liner with excellent insulation around it. Raku clay in a thin cylinder might work for the job. The varieties I have used are good to at least cone 10. I would make it in interlocking sections of a half-cylinder, probably as tall as wide per section. I don't see any point to glazing them.
Fiberglass may have too low a melting point for the job, but rock wool (Roxul or equivalent) may be good enough. I have seen autopsies showing even that partially melting around the lower, hottest parts of a (disintegrating) metal riser lining.
I used to make porcelain from the individual ingredients fir a major supplier in my youth, it will crack, porcelain is very much near the glass side of the pottery spectrum, glass is a poor heat conductor. Because of this the heat will expand one part of the tube before the other and the material will not be flexible enough to take it.
There is too much A-400 in porcelain, look for a clay that has a majority of a clay called Tennessee Ten, it is much more what you are looking for.
Most glazes will be melted of or slumped by the heats in a rocket...