Refractory: castable, ceramic blanket, firebrick and the rigidizer step people skip

Ceramic fiber blanket (Kaowool, Inswool) is the easy forge lining: light, insulates well, cheap. But the loose fibers are a respirable hazard, so it must be coated with rigidizer (colloidal silica) and then a thin coat of refractory cement (Satanite, Plistix) or castable. Soft firebrick (the white kind, K-26) is also easy to shape and great for a forge floor, but it erodes under flux. Hard firebrick is for the floor where you will drag steel. Castable refractory (Mizzou, Kast-O-Lite) is for furnaces and anywhere you need durability.

Dry any refractory slowly over several short fires before going full heat or the trapped water will blow it apart.