Key mould is obtained during the Elemental Workshop III quest. It is made by using a soft clay with a ragged book. It is used to create a ragged elemental key. It can also be used to create ragged gold and silver keys, though these are simply red herrings and are entirely useless.
Players may not have more than one mould at a time. Should they try to make a second, they will get a message stating "You already have a key mould from the book in your inventory/bank."