On the interior, Strickland placed the grand staircase that connects the three levels immediately behind the portico in the central core of the structure. The floor system is composed of fired-clay jack arches supported on steel I-beams, a common feature of warehouses and other long-span structures. On the second floor, many of the larger rooms, which were used for coining and melting, contain ceilings with beautiful high arches supported by the walls and freestanding piers. The smaller rectangular rooms on the second level (and the basement), such as the former superintendent's office, also contain these arched ceilings with a single groin vault. The basement formerly contained the boilers inside a brick cage, but now contain museum exhibits devoted to the minting processes as well as the Coin Vault at the Mint, a coin shop.