The cauldron is a typically a black, cast-iron, three-legged pot used for brewing potions, scrying, or, as in the Rede, burning. It is a feminine symbol representing the divine womb of the Goddess, and is sometimes used in conjunction with masculine symbols like the sword, wand, or athame. Like the broom, it can also be associated with witchcraft as a symbol of home, hearth, and feminine power.

The Greek Fates -- Clotho (The Spinner), Lachesis (The Drawer of Lots) and Atropos (The Inevitable) are often depicted as gathered around a cauldron or spinning wheel. This same imagery was used by William Shakespeare when he wrote the most famous (infamous?) piece of literature concerning a witch’s cauldron:

MACBETH
Act IV
Scene I.

A cavern. In the middle, a boiling caldron.

[Thunder. Enter the THREE WITCHES.]
1. WITCH. Thrice the brindle cat hath mewed.
2. WITCH. Thrice and once the hedgepig whined.
3. WITCH. Round about the caldron go.
In the poisoned entrails throw.
Toad, that under cold stone
Days and nights has thirty-one
Sweltered venom sleeping got,
Boil thou first i' the charmed pot.
ALL. Double, double toil and trouble,
Fire burn and caldron bubble.
2. WITCH. Fillet of a fenny snake,
In the caldron boil and bake.
Eye of newt and toe of frog,
Wool of bat and tongue of dog,
Adder's fork and blindworm's sting,
Lizard's leg and howlet's wing,
For a charm of powerful trouble,
Like a Hell broth boil and bubble.
ALL. Double, double toil and trouble,
Fire burn and caldron bubble.
3. WITCH. Scale of dragon, tooth of wolf,
Witches' mummy, maw and gulf
Of the ravined salt-sea shark,
Root of hemlock digged i' the dark,
Liver of blaspheming Jew,
Gall of goat and slips of yew
Silvered in the moon's eclipse,
Nose of Turk and Tartar's lips,
Fingers of birth-strangled babe
Ditch-delivered by a drab,
Make the gruel thick and slab.
Add thereto a tiger's chaudron,
For the ingredients of our caldron.
ALL. Double, double toil and trouble,
Fire burn and caldron bubble.
2. WITCH Cool it with a baboon's blood,
Then the charm is firm and good.

[Enter HECATE to the other THREE WITHCES.]

HEC. Oh, well done! I commend your pains,
And everyone shall share i' the gains.
And now about the caldron sing
Like elves and fairies in a ring,
Enchanting all that you put in.

[Music and a song: "Black Spirits," etc. HECATE retires.]

2. WITCH. By the pricking of my thumbs,
Something wicked this way comes.
Open, locks,
Whoever knocks!
[Enter MACBETH.]