To keep the top of the oven warm, masons may build walls around the oven, crib-like, to make space for a loose cover of perlite, pumice, or vermiculite. For a smaller profile and a rounder look, the oven may be wrapped in mineral wool blankets (similar to fiberglass but made from clay or rock and much more resistant to high temperatures and thermal cycling). Mineral wool blankets do need to be covered — either with a reinforced stucco shell, tile, or equivalent.