Rather than giving a specific recipe I have decided to make a video describing the key aspects of refractory fire bricks, what you should consider, how they work, and some possible suggestions to fulfil each requirement.
Videos from other creators have been used briefly throughout. Below are listed the origins of those videos in order of appearance:
E.ON Energy UK - The nature of insulation - https://www.youtube.com/watch?v=P6fEMxH_EpE
VogMan - How to easily Insulate Your furnace (foundry) using plaster - by VOGMAN - https://www.youtube.com/watch?v=T1Mxg4EVh98&t=63s
Supreme Perlite Company - What is Perlite? - https://www.youtube.com/watch?v=QUOnWMl_k3c&t=74s
MAN about TOOLS - Aircrete Garden Boxes PART 3.6 - Air Crete Lightweight Foam Concrete from Portland Cement - https://www.youtube.com/watch?v=N9EKXTTA7v0&t=569s
TKOR - How To Make The Mini Metal Foundry - https://www.youtube.com/watch?v=hHD10DjxM1g&t=73s
Milamotk - How to make Sodium Silicate (water glass) - DIY Refractory cement - PART 1 - https://www.youtube.com/watch?v=dLoedeB9GU8&t=139s
Robinson Foundry - Satanite Refractory Mortar│Ceramic Fiber Coating│Forge Making│Foundry Furnace Making - https://www.youtube.com/watch?v=vLHx186eGoE&t=612s
0:00 Intro
0:21 Basic Principle of Insulation
1:03 Why Most Methods Won't Work
1:34 5 Required Capabilities
2:11 Air Pockets
3:07 Cold Temperature Performance
3:27 Hot Temperature Performance
4:02 Fracture Resistance
4:19 Boric Acid Resistance
4:50 Outro