Phosphorus Cauldron

When demons left Magic School, they left behind a cauldron that was capible of holding all magic. When the Charmed Ones where worried about this, Leo assured them that the cauldron was neutral and could be used by them without expectation of trouble.

Unnatural Resources
Paige used the cauldron to collect all the good magic she could. Besides that of her sisters, Coop, and herself, Paige notably got some magic from a mermaid, a nymph, a leprechaun, a fairy, a unicorn, and Billie. Most likely she got some from everyone hiding in Magic School as well. They used the contents of the cauldron to create a potion.