Vegetable cooking spray worked very well as a mold release with the castable refractory fire bricks I made for my Glenwood #6. Does not affect the cure, or strength of the refractory.
Sprayed it on and then brushed it out to make sure all surfaces were evenly coated. Needs to be recoated for each casting.
Also, ridged molds usually have removable sides to make cast part extraction easier and less risk of damaging the cast part, or the mold, by trying to pry it out.
Or, as is often used commercially, a flexible mold like mold-making polyurethane within a box to support it - which is what I did. The mold can "give" to allow extraction without damage to box, mold, or cast parts. Works like latex molds but the polyurethane last far longer- especially working with concrete (refractory).