Also with tenser's disks you can cary 600lb of water using one which is 10 sq feet, so if you used 6, thats 60 cubic feet in 10 containers of water (If the containers balance, as the disk only holds 2 lb)
There are a few problems with this: First, your math is off; 600 lbs of water is roughly 75 gallons, which is roughly 10 cubic ft. (Helpful mnemonic rhyme: "a pint's a pound, the whole world 'round".) Second, there's the minor detail that Tenser's Floating Disk always follows the caster around, so you'd have to get the sorcerer to fly out right over the dragon.
If you're going with a water-cooling tactic, you'd be better off using dust of dryness, which holds 100 gallons in a convenient bead, which can be thrown. However, this brings me to my final concern: I rather doubt that water alone (even 60 cubic ft of water, let alone 75-100 gallons) would be enough to cool the magma to the point of solidification.
However, thinking in that vein of carrying loads of coolants reminded me of a ploy I used in a campaign once where we bought up enough alchemist's fire to fill an entire Heward's Handy Haversack and had my character fly over ye olde generic fantasy baddie army's encampment dropping it all over while invisible. A similar ploy might be effective using the Deepfreeze item Pyros123 mentioned. Assuming it costs and weighs the same as alchemist's fire, that's 120 flasks to fill a Haversack, which should run 2400 gp, definitely well within the means of an ECL 10 party (as a matter of fact, my group happened to be level 10 when we did the firebombing, and my DM for that campaign definitely kept us on the shallow end of the wealth-by-level curve). Just have someone fly out there over the dragon and dump the whole bag on it. That ought to solidify a decent section of magma to trap the dragon, and deal a good amount of damage to boot. (Be sure to remind your DM that damage reduction doesn't apply to energy damage like that produced by alchemicals!)
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TKDB It isn't exactly a semantic issue, they're two different words entirely. The gas trying to escape underneath a supercooled crust of magma would cause such great pressure as to create miniature volcanic events or geysers of magma rushing up through the surface and spraying everywhere, creating an even more deadly environment than the one they had previously been in.
That is probably one of the worst ideas the party could try, but fantastic as a defense mechanism for the dragon who can somehow survive in the environment anyway.
You make a fair point about the gas pressure; however, it hinges on several assumptions:
First, that the solidification is sufficient to seal off the entire top layer of the magma in the upper chamber. This would almost certainly not be the case with the Flash-Freeze spell or the "rain of Deepfreeze" strategy I just postulated, and isn't necessarily a guarantee for the Control Temperature strategy either, depending on the size of the chamber.
Second, that there are no other vents or channels with openings below the surface of the magma pool for the pressure to be redirected to.
Third, that the DM would even consider this possibility in the first place; I get the impression that you're something of an expert on this topic, and not everyone will necessarily be quite so knowledgeable
And fourth, that even if all of the above assumptions are true, the DM will actually treat it that way. If the DM is using the SRD rules for handling molten rock (which anyone with even a cursory knowledge of the subject outside of movies and games will agree are laughable), odds are he's not going for a super-realistic portrayal of the dangers of volcanic environments.
