The cauldron is a symbol of transmutation, germination, and transformation. But above all is symbolizes the womb, and therefore, is a Goddess symbol as well. In Celtic lore the cauldron is the symbol of the Underworld.
In Greek and Roman mythology the cauldron was hidden in a cave. From this, the cauldron is a mystery symbol of Wicca/Witchcraft. The Cauldron Mysteries are an integral part of the Wiccan Mythos. Some of the most famous cauldrons are found in Celtic lore. For example, the Daghda cauldron that provided sufficient food for everyone, and the cauldron of Bran the Blessed that conferred rebirth. There also is the Gundestrup cauldron.
The belief that the cauldron symbolizes the womb of the Great Goddess arises from the concept that everything is born out of it and returns to it. The original cauldron symbols were gourds, wooden vessels, or large shells.
Eventually the symbolism of metal cauldrons became linked to the hearth and home because they were used to cook meals. This latter aspect merged the Great Goddess with the Great Mother, as the cauldron combined them into a single deity.
Cauldrons have held a magical significance in many cultures throughout the centuries. In ancient Ireland, it was believed, cauldrons were never been depleted of food during feasts. In ancient times they were use for human sacrifice, which was related to death and rebirth.
In Greek mythology the Witch goddess Medea restored people to youth in a magic cauldron. Some relate the cauldron to the Holy Grail (since the Grail is supposedly the chalice used by Christ at the Last Supper), and speculate this was why some Christians were not too eager to seek the Grail because of its association with the cauldron and the Goddess.
Also the caldron has alchemic associations to some of the vessels used by alchemists in their experiments. A.G.H.