In kerosene stoves, some metals are exposed to the fire and yet they last long. But use those metals in rocket stove and they'll soon burn out. So I'll like to know the operating temperature of a typical kerosene stove and a rocket stove.
Some rockets, as it seems, can reach 1200C° at some peak during the burn.
I for sure can tell you that i've seen my batch rocket back wall and heat riser bottom glow bright orange. That's more than 940C° already if i follow a firebrick glow chart. So the gases in contact must be at a higher temp!
Red clay and sand. for the inner layer i'd say, then followed by your sawdust and red clay mix. Have you seen Matthewwalker's casting techniques? There's few links in recent posts to his videos.