Hi, just passing through.
What's the thinking behind horse manure as a refractory material? Charcoal has good heat insulation but would need regular replacement as it's eaten away by the fire. The charcoal would also need to be a fine powder to mix with the rest of the aggregate.
Iron oxide (rust) is an additional improvised material that might work. You can also make improvised lime from (roasted) shells of garden snails.
With cement (store bought, wood ash, snail shell lime etc) the bricks would need to go through multiple wet/dry cycles to cure. Say, 10 days maybe? After that, the bricks would need to be dried out in an oven/fire at a moderate heat to drive out any remaining moisture before subjecting them to the full torture of a furnace. Improvised refractory is possible, but you'd need to do a lot of work either searching how they did it long ago or through a lot of trial and error.
