The hardest part of any homemade refractory is keeping it in one piece when serious heat lands on it. The maker attacked that problem directly, experimenting with alumina fiber mixed into the batch to avoid cracking, and also weighing grooves or corrugation as a way of letting the material relieve stress. Going after the cracking risk from two angles seems like a smart approach for a castable mix.
The build itself is pleasingly simple. The tile was cast from slaked lime, perlite and a triple phosphate fertilizer, all basic hardware store stock. Powdered pearlite went into the mix to raise the insulating value. The result is a flat, rectangular tile aimed at being a durable insulating refractory for a furnace, with the possible payoff of removing the need for fiber insulation altogether.
The test is what stands out for me. The maker held the tile in a bare palm while directing a MAP//Pro gas torch flame straight onto the top surface for roughly 50 seconds. The surface climbed past 1,000 degrees Celsius, yet the back side barely warmed, which shows very low thermal conductivity. There was no spalling after repeated thermal cycles either, and the tile handled the high temperatures perfectly fine.
Two things stay open. The maker will not commit to a definitive statement on spalling until a full furnace lining has been made, which feels like the right level of caution. There is also the observation that the perlite may soften, which could give the material plasticity, and how that plays out long term is still unknown.
Project by (Mis)Adventures in Materials. Watch it on their YouTube channel: