The Witch's Cauldron is a machine from Witchery. This machine is used to cast spells, create custom brews and potions, and create magic items. It requires a nearby Altar to provide it with power for most of its functions.
Recipes can be cast by placing water in the block, then, once the water is boiling, throwing the items into the cauldron. The water must be boiling, which can be achieved by placing fire underneath it. Lava will not boil the Cauldron's water. Only a single recipe can be cast per usage, so putting any more items than absolutely necessary for the recipe is wasteful.
For brewing Custom Brews, the same steps should be used. The difference is that instead of the item being automatically created, the potion must be scooped with an empty glass bottle. At the start, only a single potion will be made per brew. But, with the use of the Witches' Armor, a Toad Familiar, and some special potions, up to 15 potions can be made per brew.
Custom Rituals can also be cast using the Witch's Cauldron.