# Could I use molten lava as a permanent design feature--like a moat? **URL:** \ **Category:** Factual Questions\ **Created:** [March 19, 2009, 1:36am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972 "2009-03-19T01:36:32Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [March 19, 2009, 5:39pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/21 "2009-03-19T17:39:01Z")
> [@Chronos](#): > > I still think molten iron is his best bet. […] it’s relatively cheap Figure that you want a moat at least a foot deep and 10 feet wide around a square structure that’s 60 feet on each side. So you’ve got 1300 cubic feet of iron. It looks like iron is currently selling for about [10 cents a pound](http://www.metalprices.com/FreeSite/metals/fe/fe.asp), if I’m reading that right. And it seems that it is about [450 lbs per cubic foot](http://www.reade.com/Particle_Briefings/spec_gra2.html). So we would have 585,000 lbs @ 10 cents/lbs, which gives us a total cost of $58,500. I suppose that’s not actually too bad. I’m surprised it’s so cheap. I guess most of it is recycled, so they don’t need to go out and find more of it? ---
**Author:** ![Fish](https://avatars.discourse-cdn.com/v4/letter/f/bc8723/32.png) [@Fish](https://boards.straightdope.com/u/Fish)\ **Post date:** [March 19, 2009, 5:50pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/22 "2009-03-19T17:50:41Z")
Even a volcano, which has the immense heat of the Earth’s core beneath it, can’t sustain a permanent lava flow. Think of it: you’ve got a tongue of tremendously hot stuff flowing out[ul][li]over cold ground andbeneath cold air.[/ul] The lava is going to lose heat to the ground and to the air and crust over swiftly. To make a lava moat practical you’re going to need to keep that heat going beneath the moat, which will probably mean your Evil Castle is sitting on ground that is much too hot to be practical.[/li] Besides, lead melts at 327C, and mercury boils at 356C. If you kept a nice mixture of mercury and lead about 340C you’d get the best of both worlds. Of course, not to be forgotten is the very important matter of the kind of puns you can make when you’re throwing people into the moat. “Lead-mercury” just doesn’t lend itself to puns, unless you’re speaking of the _Leeds Mercury_ newspaper, and puns for magma just don’t leap to mind.\* So let’s consider molten iron. You could say, “We shall strike… when the _iron is hot_.” And then your minions would laugh. \*Except this one: Adventurer: I noticed there’s lava in the moat. Evil Overlord: A’a. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 19, 2009, 5:56pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/23 "2009-03-19T17:56:26Z")
> [@Shamozzle](#): > > Man, that would be one hell of an electrical bill, keeping all that lava molten. You might as well spring for a nuclear power plant as well. > > Maybe some sciencey-types could calculate the radiation rate in watts, based on your average sized and shaped moat full of standard-issue Lava of Evil. You rang? Radiative cooling alone is going to cost you. According to the [Stefan-Boltzmann law](http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law), a body at 1000 F (about 810 K) will emit about 24 kW of energy (mostly as infrared radiation, with a smaller fraction of visible light) _per square meter of surface area_. A reasonable-sized moat, say 5 metres wide around a 500-meter perimeter, is going to lose about 60 megawatts of energy to radiative cooling alone. Add in convective and conductive losses to the air and I would guess that you’re looking at something on the order of hundreds of megawatts just to keep the fool thing molten. On preview: I see that **Sage Rat** is estimating a moat about half as long and two-thirds as wide as I am, so if that’s what you want you can divide the above numbers by three. I think “hundreds of megawatts” is still an accurate order-of-magnitude estimate, though. ---
**Author:** ![Great\_Dave](https://avatars.discourse-cdn.com/v4/letter/g/46a35a/32.png) [@Great\_Dave](https://boards.straightdope.com/u/Great_Dave)\ **Post date:** [March 19, 2009, 7:06pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/24 "2009-03-19T19:06:13Z")
> [@Fish](#): > > \*Except this one: > > Adventurer: I noticed there’s lava in the moat. > Evil Overlord: A’a. All my punny ideas are crumbling. It’s hard to rope in a good pahoehoe joke. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [March 19, 2009, 9:14pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/25 "2009-03-19T21:14:01Z")
> [@Sage\_Rat](#): > > I’m surprised it’s so cheap. I guess most of it is recycled, so they don’t need to go out and find more of it? Consider also that you’re just buying a raw material. You probably still have to shape it, and you may want to create some kind of alloy. As with most products, the cost of capital expenditures and labor can make the final result more expensive than you’d expect from the base ingredients. Very few people these days are buying iron so they can dump it in a moat. 🙂 ---
**Author:** ![Kobal2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kobal2/32/20_2.png) [@Kobal2](https://boards.straightdope.com/u/Kobal2)\ **Post date:** [March 19, 2009, 9:15pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/26 "2009-03-19T21:15:30Z")
> [@Grumman](#): > > The problem with a moat of magma or molten lead is that everyone _knows_ it’s a moat of magma or molten lead. So while it may look awesome, it denies you the entertainment that comes from people who don’t know that your moat is boiling _finding out_ that your moat is boiling. Two words, times two : Pit trap. Conveyor belt. Or your average gun-toting henchmen, if you’re more overlordy, less mad scientist-ish. But do be careful about their critical weakness to judo chops. (Great, now I’ve got a hankering to reinstall Evil Genius. Those wonderful, wonderful trap systems…) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 19, 2009, 9:31pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/27 "2009-03-19T21:31:23Z")
> [@](#): > > Consider also that you’re just buying a raw material. You probably still have to shape it, and you may want to create some kind of alloy. As with most products, the cost of capital expenditures and labor can make the final result more expensive than you’d expect from the base ingredients. Even considering just the raw material alone, though, I expect that lead or mercury (as some were suggesting) would be prohibitively expensive. ---
**Author:** ![Yllaria](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yllaria/32/3452_2.png) [@Yllaria](https://boards.straightdope.com/u/Yllaria)\ **Post date:** [March 19, 2009, 11:09pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/28 "2009-03-19T23:09:28Z")
Wouldn’t heated mercury produce toxic gases? Not good if you’re living nearby. I’m voting for molten lead. It’s classic. It’s shiney. It’s much prettier, when melted, than iron: a bright silver. Less off-gassing. Less heat transmitted into the near environment. Yep, molten lead is the way to go. Although, now that I think of it, pewter has a similar melting point, and you can use that to mold miniatures for use while plotting evil. Hmmm. I think I’m going to have to switch to a moat of molten pewter. If I was going to go Evil Overlord, I’d want the minis. ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [March 19, 2009, 11:14pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/29 "2009-03-19T23:14:59Z")
> [@Ranchoth](#): > > Inspired by…countless movies, TV shows, comics, novels, etc., as well as the response to [this](http://boards.straightdope.com/sdmb/showpost.php?p=10929093&postcount=27), I’m finally going to ask this one. > > That is, like I said in the title, is there any way, with roughly current technology (or at least physically possible technology) to use molten lava as a design feature in my Fortress of Doom? Like for a moat, or a decorative lavafall, or something? > > I know I can’t just have little koi fountains of lava sitting around the lobby, for worker safety issues if nothing else, but can I do the bigger stuff without hopelessly gumming up the works within days, or quickly killing all of my minions? \ \ psst!..Skeletor? Is that you? ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [March 20, 2009, 12:24am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/30 "2009-03-20T00:24:31Z")
> [@Sage\_Rat](#): > > Mercury would look roughly the same and be liquid without being heated at all. But the intruders could just walk across your moat. Not much of a deterrent. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [March 20, 2009, 1:55am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/31 "2009-03-20T01:55:57Z")
Where’s **Skald the Rhymer** when you need him? he might be able to contribute, having been an evil overlord and all. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [March 20, 2009, 2:20am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/32 "2009-03-20T02:20:06Z")
Okay, here’s what you do. You build a large trough to contain the lava in the moat. It can be slightly sloped to maintain lateral flow; the lava wells up in a pool at one end and drops into a pit at the other. Pit and pool are next to each other; the lava is heated in the pit and pumped from there back up to the pool. The coating on the inside of the trough, which touches the lava, is of course capable of withstanding white heat without breaking down, but it doesn’t need to be structural or strong under tension, because it’s supported by the next layer. The next layer is thermal insulation: an improved and less fragile version of silica aerogel (those tiles they use on the space shuttle). Underneath this is the structure of the trough, brick or concrete which can become red-hot without problem. There’s a space between the lava trough and the surrounding ground. Walls extend upward from the base of the trough and support a platform lip above the edge of the trough; the space appears as a horizontal break in the wall of the moat, well above the surface of the lava. The space provides some thermal separation between the trough and the ground, and huge fans continually draw air into it, so that the fumes and heat above the lava are lessened to bearable level at the lip of the moat. The structure around the pit and pool is built in the same way, but the pump (an archimedes screw or wheel might be simple enough) must be able to withstand the loads and heat. Alternatively, with a lot more energy, you could heat the lava intensely in the pit and bubble gas through it to propel it up the tube to the pool; I think this is more like how an actual volcano works. In _The Incredibles_, I don’t think that backdrop in Syndrome’s conference room was real lava; it seemed too rigid as it moved down. I think it just _looked_ like lava, without most of the heat. Still doesn’t mean that it wouldn’t have squashed Mr Incredible if he’d been caught, but notice Mirage never got singed? ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [March 20, 2009, 3:13am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/33 "2009-03-20T03:13:56Z")
> [@Princhester](#): > > Wait, surely we need the dull glow of something red hot? A moat comprised merely of a liquid that is hot enough to kill someone (like hot lead or mercury) is dull, dull dull. Heck hot water could do that. Any evil bad guy who’d settle for a moat of hot water would have their licence revoked. Ever heard of red light bulbs? I mean, I hate to cheapen this whole exercise but if it is red glow you want… ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [March 20, 2009, 3:42am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/34 "2009-03-20T03:42:47Z")
I suggest molten salt. It can be heated to an orange glow, retains heat well, and can use electricity directly as the heat source. It also flows well for the fountain. “Too much salt is bad for your health, Mr. Bond” ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [March 20, 2009, 6:18am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/35 "2009-03-20T06:18:20Z")
> [@Grumman](#): > > The problem with a moat of magma or molten lead is that everyone _knows_ it’s a moat of magma or molten lead. So while it may look awesome, it denies you the entertainment that comes from people who don’t know that your moat is boiling _finding out_ that your moat is boiling. When I visited Yellowstone Park a few years ago, we were warned to pay attention to the signs and stay on the designated walkways, because the area off the designated area may look safe, but is actually only a thin crust over very hot areas. Thin enough that if you walk on it, you will fall through, and be burned or scalded, quite possibly fatal. About a couple of dozen people have died from this over the years. So your evil overlord could have landscape the inside of his fortress with pretty hot springs or geysers, and have it surrounded by an invisible caldera moat, covered with a thin crust of dried mud. ---
**Author:** ![Incubus](https://avatars.discourse-cdn.com/v4/letter/i/c68b51/32.png) [@Incubus](https://boards.straightdope.com/u/Incubus)\ **Post date:** [March 20, 2009, 6:26am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/36 "2009-03-20T06:26:00Z")
I’m glad other people backed my idea of lead. On a lead vs mercury debate: Lead is a lot cheaper (if we’re talking about…per gallon of the stuff) than mercury, and I would imagine the benefit of having it molten at a hot (yet manageable) level of ~350 F means its going to be nasty if you touch it (vs mercury at room temperature, which I guess is toxic isn’t as immediately awful to fall waist deep into. Another odd quirk about mercury is that it is _repelled_ by most other substances; getting ‘splashed’ means getting beads of the stuff rolling off you. The Lead, however, is going to be cooling the second it touches manflesh, making it more prone to harden/stick to whatever it comes in contact with. Nastier. Plus it would be fun to have chutes/ducts full of moltel lead running everywhere, so with a pull of a lever you can have torrents of the stuff falling on unwary heroes. What about liquid sodium? Don’t they use that for reactor coolant? :eek: I know sodium in its pure state is incredibly reactive, but what if the moat had a ‘buffer’ of noble gas, one that was denser than air that would form a layer to keep the sodium from reacting to the ambient oxygen? A moat of liquid sodium would be pretty nasty, too, since it would react to just about anything that came in contact (can’t go wrong with a substance that _bursts into flame releasing lye_ when it comes in contact with water! 🆒 ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [March 20, 2009, 6:35am UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/37 "2009-03-20T06:35:35Z")
How about molten gold ? Sure, it’s expensive; but that’s what extortion schemes with death rays are for. Besides being stylish, having your gold supply molten would make it harder for a minion to pocket and carry off; and your victims would polish up prettily once they cool. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [March 20, 2009, 12:38pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/38 "2009-03-20T12:38:21Z")
that is too cool! ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [March 20, 2009, 1:28pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/39 "2009-03-20T13:28:27Z")
Lead fumes are a long-term health issue, especially for the growing mini-Ranchoth clones and/or young illegitimate R anchoth offspring, but to some degree for everyone. Really, lead’s toxicity is the worst of both worlds: won’t immediately incapacitate goody-two shoes Heroes who wander across it, but slowly builds up in henchpeople (which is maybe OK; we don’t need them too bright) and scientific assistants doing the details of various doomsday devices (which is very bad; we don’t want idiots designing fuses on our mega-bomb). Really, the only issues with molten iron are 1. the energy needed to keep it molten 2. I’d have gotten away with a two-item list if it weren’t for that meddling Opal! 3. cooling the fortress near the moat Sunspace’s ideas about reverse air-curtains makes sense; that will help with thermal management, especially as we use the hot air to do something useful. The moat lining is easy: commercially available firebrick should do it (though it will need to be rebuilt regularly) without needing a fancy R&D effort or complex heist of NASA material (though I’m all for including a complex heist as part of the plan, of course). ---
**Author:** ![Kobal2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kobal2/32/20_2.png) [@Kobal2](https://boards.straightdope.com/u/Kobal2)\ **Post date:** [March 20, 2009, 1:33pm UTC](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972/40 "2009-03-20T13:33:25Z")
> [@Der\_Trihs](#): > > How about molten gold ? Sure, it’s expensive; but that’s what extortion schemes with death rays are for. Besides being stylish, having your gold supply molten would make it harder for a minion to pocket and carry off; and your victims would polish up prettily once they cool. I don’t know, that’s a little nouveau riche, isn’t it ? On the other hand, maybe something both affordable and original would be a moat of liquid nitrogen (or similar). Brittle freezing death is more scary that hot molten death IMO. Plus, great brewski cooler for the minions. [Previous page](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972.md?page=1) [Next page](https://boards.straightdope.com/t/could-i-use-molten-lava-as-a-permanent-design-feature-like-a-moat/489972.md?page=3)