proving-box

from The Century Dictionary.

• noun In bread-making, a large sheet-metal box having sliding doors and sheet-metal shelves, used to prove the rolls and loaves of dough before baking. It is sometimes mounted on wheels for convenience in moving the load to the oven, and is usually fitted with an inlet for live steam. The steam fills the box and condenses on the rolls, causing them to bake with a smooth hard crust.