In Peter Jackson's Hobbit movies, there's a scene in which the heroes attempt to kill or entrap the dragon Smaug by covering him in molten gold.

An answer on SciFi.SE claims that this is physically implausible due to the density of gold:

...even if Smaug's body was solid lead (11 g·cm−3), he would easily float on liquid gold (19 g·cm−3)....

As noted in the answer, the scene does not actually show Smaug falling into the gold, but rather being covered in it as a giant gold statue melts, forms a "wave" of molten gold, and envelopes him:

...Smaug sits on the ground in front of the liquid-gold statue before it collapses.... the disintegrating dwarf statue would constitute a tsunami wave that would push stuff out of its way much more powerfully than a comparable water wave. It would not drown the dragon, but push it towards the opposite wall with incredible speed.

In a comment, the writer clarified that even though the movie doesn't incorrectly show Smaug sinking into the gold (at least, that's not the way it appears), even though the gold is in wave form, Smaug still shouldn't have been submerged:

...a wave of gold is to an animal like a water wave is to a piece of styrofoam or cork: that floats to the crest so easily that the wave doesn't have a chance to bury it underneath.

I'm not sure that this explanation makes sense or that the analogy applies. Granted, the movie scene probably isn't very accurate in terms of how liquid gold would behave (it does seem to crest and break much like a wave of water, and to move quite quickly), but I would expect that submersion would be a question of surface tension and pressure rather than density, and even though liquid gold does have a very high surface tension, a "wave" of molten gold doesn't seem like it should have enough surface tension to prevent Smaug from being submerged. I don't see how Smaug would be lifted by the wave unless it were coming up from below him somewhat. (Granted, the layout of the lair and the logistics of the scene don't seem to be entirely clear in the above video.)

So, the question is: how plausible is it that a very large (Smaug-sized) object or heat-resistant creature could be submerged by an even larger "wave" of molten gold?

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    $\begingroup$ Google for "man sitting on mercury" to find a classic photo that appeared in National Geographic many decades ago. It shows a man sitting on top of the liquid mercury in a large vat. $\endgroup$ – Solomon Slow Jun 14 '17 at 19:23
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    $\begingroup$ I think the answer has to do with the density of liquid gold, but also with its viscosity, which depends on temperature. In principle, after the wave submerges the dragon, it should cool and maybe solidify sufficiently to block the dragon underneath. Of course I want to stress that this is only speculation, at the moment I can't check what the viscosity, specific and latent heat of gold are to construct a valid hypothesis. $\endgroup$ – GRB Jun 14 '17 at 19:32
  • $\begingroup$ Don't remember any attempt to kill Smaug with molten gold in the book. As I recall, he was defeated by an arrow shot through a bare spot in his natural armor scale. $\endgroup$ – user93237 Jun 14 '17 at 19:58
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    $\begingroup$ @SamuelWeir The scene in question is from the movies, as I stated in the post (I even linked to the scene on YouTube). Spoiler (in case you care): the attempt to cover Smaug in gold doesn't work in the movie anyway, and I believe that he is indeed killed by an arrow in the last movie. (Since you apparently haven't seen the movies, let me recommend that you not bother; they are terrible.) $\endgroup$ – Kyle Strand Jun 14 '17 at 20:12
  • $\begingroup$ @KyleStrand - The movie's storyline sounds like a pretty big deviation from the story in Tolkien's book then. Takes a lot of chutzpah for someone to re-write Tolkien. I wouldn't have had the guts to do it. I'll take your recommendation and pass on the movies. $\endgroup$ – user93237 Jun 14 '17 at 22:35

I agree with the analysis on the other stack exchange site.

The force/pressure of a golden wave would be extremely powerful.

Imagine you are standing still and then a wave of water starts flowing at you. If the wave is tall enough to account for your buoyancy, it will push you away, but also lift you up while it takes you with it.

This same thing should happen with the dragon and the gold, except the wave has 19 times the force per unit volume; it's going to hit hard; but it's also going to lift you in the flow (ignoring the gold reacting with your surface and any effects of heat). You have no way to stay submerged, even temporarily. As soon as gold has a contact angle that can apply a force upwards you will begin to float; and due to the force the gold has, it will make sure it displaces you quite quickly to make room for itself.

  • $\begingroup$ Part of the issue is that the scene doesn't make it clear (to me, at least) what the contact angles are, but I don't really see why they'd be applying upward force until at least the dragon's feet are already covered. $\endgroup$ – Kyle Strand Jun 14 '17 at 20:14
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    $\begingroup$ @KyleStrand I just watched the scene, it doesn't really make any sense. Once he fell he would definitely be floating, and in this he clearly sinks. It would take a lot of force to push him down, and there's nothing that should keep him down that long. If that was a person in water I doubt they would have sank that fast or stayed submerged that long; let alone gold. When he fell he should have stayed above the surface. I can't speculate on his ability to withstand the initial gold stream because he's a mythical creature. $\endgroup$ – JMac Jun 14 '17 at 20:29

The force to be reckoned with here is not surface tension but rather density; the density of the dragon relative to the density of molten gold.

Whether the object maintained its composition or decomposed due to high temperature, either way it would have to have a density higher than gold to be submerged by the liquid gold. If the magical properties of dragons make them denser than gold then yes - they would be submerged.

  • $\begingroup$ So the question of submersion really is primarily based on the density comparison? Does surface tension enter into it at all? $\endgroup$ – Kyle Strand Jun 14 '17 at 19:18
  • $\begingroup$ I edited my answer to make that point clear. With regards to flotation of a solid object in liquid, both forces of surface tension and density (buoyancy) lead to flotation. The contribution of both forces depend on the mass of the object and it's size, particularly the contact line with the fluid. A one Yen (Japanese) coin, made from aluminum can be made to float on water! In the case of this coin, both forces are significant. For a battleship, buoyancy (displacement) forces far outweigh the forces of surface tension. $\endgroup$ – docscience Jun 14 '17 at 20:06
  • $\begingroup$ ... So to answer your question we would need to know more about the properties of the dragon. $\endgroup$ – docscience Jun 14 '17 at 20:06
  • $\begingroup$ How much does gold expand at higher temperatures? Would it matter? $\endgroup$ – Nemo Jul 17 '20 at 21:02

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