On my last session summary report, in the comments, there is some discussion about fireballs vs. undead, melting temperatures, and so on.
Basically, I mentioned that one reason the PCs haven't disposed of the 33 draugr is concern about the best method to kill them also being a good way to reduce the value of the loot. Flame-broil the whole bunch and you're flame-broiling their gold jewelry, their armor, their weapons, their shields, etc. It's not a question of temperature, but of damage. Let me explain.
How hot is a Fireball spell?
I'm not sure. It's not defined. It doesn't really need to be defined, either. It's hot enough to inflict 1-6 burning damage for 1 energy, in one second, reduced normally by DR.
Ordinary flame inflicts 1d-3 or 1d-1, depending on how long you stay in the same hex with it. Being partly on fire and having all of your clothing on fire merely match those, respectively.
We know what that will set fire to, because GURPS Basic Set conveniently lists the damage needed under Making Things Burn (p. B433).
We know how much damage it takes to punch big holes in objects (p. B558) and how many DR and HP most objects have (various sources, such as LTC2, Basic Set, etc.)
We know how much damage armor can stop before the things behind it take damage.
So even DR that would normally be fully protective from ordinary flame (say, DR 5, enough to ignore maximum damage from standing in a hex of flame for 1 full second) might allow damage through from a Fireball spell. Even fully sealed armor from heavily fire-resistant materials (steel, for example) won't necessarily stop a Fireball from inflicting burning damage on the person beyond.
All of that is looking at a 1d Fireball. They can go all the way up to 18d in a Dungeon Fantasy campaign. An 18d fireball on the low end will set even Resistant objects on fire and will, on average damage (63 HP) set flesh or green wood on fire.
Even a 3d Fireball is inflicting the same damage as contact with molten metal for one second.
Will that melt gold? Is it hot enough?
Maybe. Probably, even. But it doesn't matter. It's damaging enough that most gold objects aren't going to have the DR and HP to stay as intact objects from the hit. That spell can easily do enough damage to melt a hole in a 1/8" steel wall or punch a hole in a 3" thick brick wall.
And that's what matters. Damage, not temperature.
Because mainly, really, this isn't science. It's gaming. None one at my table is going to want to stop the game to determine how hot 28 points of fire damage from a Fireball is vs. 40 points or 60 points or whatever, and the smoke/ignition/melting temperatures of everything. It's enough to say, yeah, it's unlikely a gold necklace will be intact after 30 fire damage in one second, even if it could sit all day in a fire doing 1d-1 per second . . . so it won't be. It melts, it's warped, it's time to start thinking "value of weight of gold" and not "value as a well-crafted piece of jewelry." That feels right, it feels consistent with Fireballing down barriers, golems, and punching through armor, and it's easy enough at the table.
Defeat a bunch of undead with massive amounts of fire, expect melted stuff. It's just how it's going to work at my table.