Fire
Even with how fast Cassiel was able to find out the design of a mana sauna, it took some time just to acquire enough steel for the construction.
Iron was a bit of a rare commodity in Heaven, with almost all of it requisitioned for the Heavenly Host. The better part of a month passed before they were able to acquire enough, mostly by searching the manor for unused equipment. Their final break on that front came when Iriel showed up at the forge one night.
She arrived right when Rainier was trying to melt together a floorplate out of different kinds of steel, and when he expressed a need for more steel, she showed him to a disused armory full of the rusted detritus of her past projects.
Iriel seemed to think Rainier was forging himself a set of plate armor, and so had commented "Not many angels appreciate good steel nowadays. That factory shit just can't compare," before giving him advice on how to fit a breastplate for wings.