The Mould was created to help in mass crafting, so that the user can shift click into a crafting grid without having to rearrange items.
The Mould is made by placing a block of clay on a turntable and letting it rotate for 10 turns. You will get 3 Clay Balls back in the process, so each Mould costs 1 Clay Ball total. The Crucible, Planter, Vase and Urn are all made before it.
It can be placed in its unfired form, however it does not have a use like this.
You can exit the Anvil interface without the moulds being dropped, so that you don't need to place them each time you enter the interface.