Robert; Most common brand of "fireclay " is lincoln 60, another is hawthorn. fireclay is a powdered clay sold in 50 bags with properties known for high heat resistance. Furnace cement is generally used to seal leaking conventional
wood stoves, and would not make a good additive to a core mixture. Refractory cement and refractory mortar are probably a very similar product, either
should work as an additive to your fireclay / perlite mix commonly used to cast a core (walker style). Adding sodium silicate (
water glass)or painting it on afterwards will also harden up your casting.No
concrete products can stand up to the heat of a
rmh core,only refractory products can take the high heat. My complaint with a cast core is the frailty of the
feed tube area, mine went from a 7.5" square to the size of a 5 gal pail by the end of one burning season. Cast cores burn super hot extra fast but... they are fragile and will crumble if you treat them roughly. I now build my cores from heavy fire brick , they take much longer to heat up but are very durable.