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So, I've wound up in jail again (not my fault - I didn't start that fire, honest!) and I need to get out of here before the mayor has a chance to check his safe. Unfortunately, I left my lockpicks in my other pants.

Luckily, I happen to be carrying a little vial of metal-be-gone. A few drops on the hinges, and in a matter of minutes it's eaten through the metal, and I can escape. But it gets me wondering... Just what is in that vial?

Question: Is there a liquid which can be safely carried (probably in a glass container), and which will rapidly dissolve iron? Preferably something that does not release toxic fumes. If so, what is it, and how could someone (or some group) produce (ideally with a medieval level of technology) or obtain it?

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    $\begingroup$ If it were a (safe) liquid that can rapidly dissolve iron, and that can be produced with medieval level of technology, why would have we bothered to produce iron safes the last 3 centuries? $\endgroup$ – theGarz Aug 8 '18 at 10:29
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    $\begingroup$ @theGarz Because iron is cheap, and the point of a safe is to prevent the would-be-thief from simply carrying out the goods. Also, there's a major difference between destroying/weakening hinges and making a hole in a safe. $\endgroup$ – user10328 Aug 8 '18 at 10:49
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    $\begingroup$ @theGarz I have a lock on my door at home. It doesn't make it impossible to open - a burglar could just bash the door down - but it does make it more difficult to open. No security solution is 100% effective, it's always about making it difficult enough to open/steal/etc that people don't bother. $\endgroup$ – walrus Aug 8 '18 at 12:36
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    $\begingroup$ You're probably better off with an explosive than an acid. Dissolving iron is a slow process even with strong acid and an electrical power source. Mercury fulminate could be good. It's a friction-sensitive explosive, but storing it under water stabilizes it (mostly). Pour a vial of the suspended solid out on the hinges, wait for the water to dry, and then toss a rock or something at it. Bang! If you use enough I'm sure it could knock the hinges off a door. It's also easy to make. I won't tell you how, because it's dangerous to play with, but if you must know the internet can tell you. $\endgroup$ – realityChemist Aug 8 '18 at 15:16
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    $\begingroup$ @Alice Well besides being ungodly dangerous, I think you would have a hard time destroying iron with ClF3. My understanding is that you're going to end up binding fluorine atoms to the iron to create a passivated outer layer of the ceramic FeF3 (while giving off nice, comparatively safe chlorine gas) that stops further reaction, similar to what happens to aluminum in air. The article you linked actually mentions this: iron containers are one of the few ways to safely store ClF3 $\endgroup$ – realityChemist Aug 8 '18 at 16:41
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Gallium, otherwise: you don't.

Ok, you have to assume that whatever you are using this on is made out of Aluminium. But if we can make that assumption the answer is: Gallium.

Gallium attacks Aluminium in a quite spectacular fasion.

Here is an example of how to completely destroy a big Aluminium alloy padlock with Gallium.

Another example on how to defeat an Aluminium padlock with just a few drops of Gallium.

Gallium is fun because it is a metal that melts in your hand. This means your cute little vial can be disguised as a novelty toy/key ring. But since you need to little of it, you can just hide the vial in your belt, pants waistline or something like that.

EDIT: Assuming iron, assuming the Middle Ages and equivalent level of sofistication when it comes to chemistry.

Sorry, you simply do not do that.

The latter part of this video shows what happens when you put Iron in Hydrocloric Acid. In short: not a lot. Iron is simply too noble a metal to be easily attacked by acids. Sure, Aqua Regia was described in the 1300's... but 1) it is not fast acting 2) you need quite a bit of it and you need to keep it in contact with the iron for a long time; you cannot just pour some on and 3) that a common criminal that ends up in jail a lot would have access to this stuff is just not plausible.

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    $\begingroup$ The OP specifies it is iron, and preferably obtainable with medieval technology. $\endgroup$ – L.Dutch - Reinstate Monica Aug 8 '18 at 10:40
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    $\begingroup$ @Benubird Well, if you are going for high or low fantasy as your setting, then this is realistic. Only problem is your readers may not think that because few are aware that there exists metals of this kind. Gallium epitomises the trope of the Aluminium Christmas Tree You will have to do exposition to explain this "metal rot", or have footnotes as a sort of Author's Commentary. $\endgroup$ – MichaelK Aug 8 '18 at 16:49
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    $\begingroup$ I admire the creativity, but even if aluminum were technologically possible in a medieval setting (it was not), who would ever think to build a jail cell with it? It is weak and comparatively easy to bend or break. $\endgroup$ – plasticinsect Aug 8 '18 at 17:16
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    $\begingroup$ @plasticinsect Actually.,.. the videos I posted do show Aluminium alloy padlocks, and my bike chain is an Aluminium alloy. Sure, pure Aluminium is nothing like Iron, but mixed with other metals it is quite capable. And in any case it is up to OP to decide what they think about it; see comment above. $\endgroup$ – MichaelK Aug 8 '18 at 17:22
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    $\begingroup$ @Benubird: Even in our world, aluminum is more common than iron. However, its smelting temperature is much higher (smelting being the use of heat to extract a metal from its ore), such that it was essentially unattainable as a metal until the development of electrolysis (which enabled ways to extract it at temperatures comparable to those for iron smelting). $\endgroup$ – ruakh Aug 8 '18 at 21:09
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For fun I would go with Aqua Regia, or Royals Water/Kings Water based on the Wikipedia article. It can dissolve almost all metals including gold, silver and platinum.

As for how to get it and whether its safe in a glass container? I have no idea. But its fun to know you have a vial of liquid that is capable of chewing through almost any metal.

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    $\begingroup$ Aqua Regia is hardly "safe" though. $\endgroup$ – Skyler Aug 8 '18 at 13:41
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    $\begingroup$ Two vials (one of nitric and one of hydrochloric acid), or a vial of HNO3 (nitric acid) plus ammonium chloride in powder form (Al-Hayyan's original method according to wikipedia) and mix in situ -- this is good because it fumes and loses its potency. This would also make it easier to disguise $\endgroup$ – Chris H Aug 8 '18 at 15:32
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    $\begingroup$ It's safe to carry in glass or plastic. However, I wouldn't want to get it on me, and people might wonder why you're carrying a vial of "acid" around with you. I'm not sure how you plan to sneak that into jail with you. $\endgroup$ – pojo-guy Aug 8 '18 at 16:40
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    $\begingroup$ Due to the decomposition reaction of aqua regia, it cannot be stored in a vial; its components have to be mixed immediately before use. Furthermore, the large amounts of NO and NO2 that are released when aqua regia reacts with iron make it not really safe for this application outside of a fume hood. $\endgroup$ – Loong Aug 8 '18 at 19:35
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    $\begingroup$ -1 While Aqua Regia will react with iron it doesn't do so nearly fast enough to be useful. It's also as was pointed out not particularly safe to do this. $\endgroup$ – DRF Aug 9 '18 at 7:21
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If you can’t dissolve ‘em, freeze ‘em.

Your vial consists of two powders, Ammonium thiocyanate and barium hydroxide octahydrate, separated by a thin membrane. When combined, these substances become quite cold, as you can see in many scientific demonstrations (instant ice packs work the same way). Once the hinges are cooled to around -13F, you shatter them with a rock.

If you can’t freeze ‘em, melt ‘em.

In your vial is aluminum powder and a wee bit of magnesium powder, and you are fortunate in that the hinges are very rusty. You also happen to be wearing spectacles that your jailer was kind enough to leave you. You pack the hinges with the aluminum powder, use your spectacles as a magnifying glass, and light the whole thing up. The thermite reaction you get should slag iron.

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  • $\begingroup$ Note: both of these are a wee bit optimistic, but the principle is solid. $\endgroup$ – Daniel B Aug 8 '18 at 16:32
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    $\begingroup$ -13F is not enough to make iron/steel brittle. Otherwise most metal structures wouln't survive a winter. $\endgroup$ – Alexander Aug 8 '18 at 17:26
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    $\begingroup$ popsci.com/diy/article/2012-08/… “When cooled to –13°F... even very tough locks become brittle enough to smash open with a hammer.” And I can vouch for this effect. I’ve broken numerous locks using this strategy, though my cooling method was compressed air, not a chemical reaction. $\endgroup$ – Daniel B Aug 8 '18 at 18:30
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    $\begingroup$ The thermite reaction is the best of all the proposed solutions, but you'd have to use a non-aluminium-based thermite reactant mixture: aluminium is not really viable to have in usable amounts for chemistry in a medieval setting (practically, it requires massive amounts of electrical power to smelt). $\endgroup$ – Michael MacAskill Aug 9 '18 at 5:58
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    $\begingroup$ @Alexander, at that temperature, is not actually the metal bolt that becomes brittle enough to break, but the springs and pins inside the lock that do, hence why you can smash the lock, but the bolt itself would still not be brittle enough to break without a few decent tools $\endgroup$ – Blade Wraith Aug 9 '18 at 8:36
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If you renounce to not releasing toxic fumes, you can go with hydrochloric acid. (Actually, hydrogen is not toxic but "just" hazardous).

Iron doesn't dissolve readily in water, although it will definitely rust more rapidly (as you've probably noticed from experience). Hydrochloric acid, however, can dissolve iron, and a more concentrated solution will dissolve it more rapidly. [...] First of all, it releases highly flammable hydrogen gas, so it should be performed under a fume hood. Moreover, hydrochloric acid is also a hazardous chemical if misused; it's especially important to avoid spilling it on skin or eyes. With these cautions in mind, you can dissolve iron using hydrochloric acid.

If you manage to avoid any sparks during the process, you will be free and alive. Else there will be nice fireworks involving you, and you will be, again, free but very likely dead.

Production of HCl with medieval technology should be possible, since

Hydrochloric acid has been an important and frequently used chemical from early history and was discovered by the alchemist Jabir ibn Hayyan around the year 800 AD. Free hydrochloric acid was first formally described in the 16th century by Libavius, who prepared it by heating salt in clay crucibles.

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    $\begingroup$ I can't imagine that dissolving a hinge or two with the bottle of hydrochloric acid in your pocket (funny how the intake officer missed that, BTW) would release that much hydrogen. $\endgroup$ – Spencer Aug 8 '18 at 10:41
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    $\begingroup$ HCl does not work fast. Worse, it doesn't even react strongly with iron. If the hinges were zinc, you MIGHT have some chance of dissolving them, but iron? You might rust through them in a week. $\endgroup$ – piojo Aug 8 '18 at 13:42
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    $\begingroup$ @piojo Spot on. However, if the hinges are badly made there’s a chance of quickly weakening a fold enough to break it with blunt force (in post-industrial times I’d suggest looking for welds; in medieval times, some other weak point should do). This does involve (quite a bit of) chance but maybe it’s not too implausible. $\endgroup$ – Konrad Rudolph Aug 8 '18 at 15:19
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A solution of acetic acid, salt, and oil.

You're not actually "dissolving the metal", you're de-rusting and lubricating the hinge pins so that you can pull them out effortlessly - as a bonus, you can then slot the pins back to hold the hinges together again when you're out of the cell, and leave everyone bamboozled as to how you made your escape.

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    $\begingroup$ Jail cell doors are not routinely built with prisoner-removable hinge pins. $\endgroup$ – Sneftel Aug 8 '18 at 15:02
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    $\begingroup$ @Sneftel We're talking about a gaol that will apparently let repeat offenders take in lockpicks or mysterious glass bottles, then leave them entirely unguarded. Hardly the epitome of "secure" as it is... $\endgroup$ – Chronocidal Aug 9 '18 at 7:37
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Fluoroantimonic acid (H2FSbF6)

If you want something very strong, which can dissolve also almost anything, I suggest you to transport a small vial of Fluoroantimonic acid (H2FSbF6). It may be currently the strongest acid that we know as it can dissolve metals but also glass!

The reaction to produce it involves hydrofluoric acid and an Antimony derivative. Antimony has been firstly described in 1540 which matches with your medieval setting and it was even used before that in ancient Egypt.

SbF5 + 2 HF → SbF6- + H2F+

The only problem is that you have to transport it in a vial made of Polytetrafluoroethylene (PTFE) as it will dissolve any other recipient! PTFE is more wildly known as Teflon and has been discovered in 1938. But it has been synthetized by mistake and with a really simple process... So simple that you could imagine a very advanced alchemist sect which would discovered it during the middle age:

Just store some compressed Tetrafluoroethylene (TFE) in a steel bottle in some ice for a night and there you go, Teflon in the morning (simplified version of course). Again, TFE is not a natural molecule so you have to imagine your alchemist sect to be way ahead of his time...

By the way it surely release a ton of toxic fumes as Antimony is known to be very toxic !

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