Well, from the Ice Age FAQ...
"The basic nature of Ice Cauldron is this: you can use it to store both a spell from your hand and the mana needed to pay for that spell. After the spell has been stored on the Cauldron, you can play the spell as if it were in your hand. This mea ns that you must follow all normal timing rules, targeting rules, and so on. Typically, you will tap the Cauldron for the mana you stored in it before playing the spell.
When you use the first ability of the Cauldron, you choose a spell in your hand and pay some X (which can be zero). When the effect resolves, you put that spell "on" the Cauldron, and store a charge counter of X on the Cauldron. The spell stored this way is not considered in your hand or in play; it is in a sort of limbo, not unlike a creature affected by Tawnos's Coffin. You can play this spell later, whenever appropriate, regardless of whether the Cauldron is tapped and regardless of whether you control the Cauldron. (The fact that you put the spell on the Cauldron, which makes you the person who can play the spell, does not change after the Cauldron is used, regardless of who controls the Cauldron).
If the "storing" effect of Ice Cauldron is countered (say by Brown Ouphe), then the mana is lost but the card is kept in your hand. You can only put the spell and charge counter on the Cauldron if there is not a charge counter on it already. Spells stor ed in the Cauldron are not actually considered to be in your hand, so they aren't counted for Ivory Tower or Black Vise, you can't be forced to discard them by Hymn to Tourach, etc.
The second ability of the Cauldron is to put the mana you stored in it back into your pool. This mana can only be used to play the card you chose when you put the charge counter on the Cauldron; if you put the mana in your mana pool and do not use it fo r that purpose, you'll suffer mana burn from it. Note that if you steal the Cauldron, you can remove the charge counter (and get mana burned) so that you can use the Cauldron later. Also, mana is not drawn from the Cauldron at the speed of an interrupt, so if you store a Counterspell or other interrupt in the Cauldron, you will typically want to find mana sources other than the charge counter to pay for that spell.
Changing the Cauldron's controller will not affect the spells stored on it; the player who stored a given spell on it can still play that spell. Removing the Cauldron from play causes any and all spells stored on it to be placed in the graveyard; this sh ould be treated as card errata.
Any number of spells can be stored on the Cauldron at any given time, as long as you remove each charge counter before storing another spell. You must still play these spells individually, not simultaneously."