You can stack blocks in a cauldron to save space on your island. Up to 10,000 blocks can be placed in each cauldrin.
You can only stack the blocks that increase your island worth.
To toggle stacking on/off do the command:
Placing a stackable item on the ground will place it in a cauldron if there is not already a cauldron stack within 3 blocks of the same type. If there is a cauldron stack within 3 blocks the block will be added to the stack.
Shift-Placing blocks on the ground will add all the items in the stack to a cauldron within 3 blocks or will create a new cauldron with all the blocks in the stack.
Sneaking and Right Clicking on a cauldron with empty hand will open a windows where you can bulk add items to the cauldron. You can only add the item type that is already in the cauldron.
Right Clicking on the cauldron with empty hand will remove 1 item.
Breaking the cauldron will have entire contents fall on ground.