The metal spade is a quest item used during the Miss Cheevers section of the Recruitment Drive quest. It is found in the room, and can be used on a bunsen burner to remove the handle. The head is then put into a hole in the door. Adding cupric sulphate and dihydrogen monoxide will heat the spade, expanding the metal and trapping it in the hole, at which point it can then be used as a handle to open the door.
Once it is used, it cannot be taken out of the door, however you can make another one. It cannot be taken out of the room, as with any other items acquired within the tests. It cannot be used for digging, as the normal spade can be.