[jimmontg]

Before going to the trouble of the perlite AO mix just go to Ace hardware they sell refractory cement, Rutlands Refractory castable cement and and Rutlands Troweling cement for Rutlands fire bricks and there are six 9x 4.5x 1.25 bricks per $20 package. I have no idea how long the plaster of Paris would last or how it holds up, I wouldn't have thought you could use it until I saw that video some time ago.

I have made huge burners for asphalt plants to heat the tar and gravel-sand mix and we used the refractory cement to which we added some perlite to stretch it out, but for a small forge I wouldn't bother. We used rebar wire also as the cement was thick about 5". Wouldn't need it for a small forge. We let it dry for over a week and then used a brush burner to warm it up and finish drying overnight. I doubt you would need to do the burn cure, but wait at least a week for it to dry. Oh and use a cardboard tube for the inside mold as a metal tube may be hard to get out.