I got some pre-mixed fireclay, as well as some IFB and a castable refractory called "lightweight", made by AP Green, from a supplier in Mobile, AL. I cast a solid-body forge out of the stuff, and used the fireclay and a few of the IFB to finish it. It's powered by 2 Reil EZ burners with Tweco welding tips as gas jets. When i saw that it was developing a few hairline cracks, I wrapped the whole thing in 2" Kaowool, and wrapped the whole thing with a piece of rolled 1/16" steel sheet(like a metal sheath covering the outside of the forge). I simply MIG welded the seam on the steel where it overlapped. Painted the thing flat black, and put flames on it in Grabber Green and Hemi Orange (had the paint and couldn't resist!). I mention this because if the fire place mortar is anything like the fire clay I have, it doesn't seem to work real well for plastering. It works well for mortaring bricks to each other, or to refractory though. The cracks in the mortar will definitely lose heat. You may need to use a different kind of fire brick - IFB for instance, and mortar the body up with fireclay since it probably has a higher temperature rating than commercial fireplace mortar.
