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I have a story set in a world where the knowledge of metalworking was lost, although some artifacts survived as precious treasures; people know what they do, but not how they were made.

I want a character to notice metal melting when the city is on fire, and use some molten metal to make a copy of a key from a clay impression she's taken. What metal might work? (ie one with a low melting point)? How could the character handle it without specialist gear? Could you make a key by pouring metal into a clay mould? In short, does this work?

TheSpidermonkey
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    Tin would be the best candidate, imho. But with its low melting point, people have discovered very early that tin is melting when thrown in a fire. – Alexander Jan 25 '21 at 19:35
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    If the city burning is not set in stone, you could also discover it on hot places like volcanoes and their lava streams. – Trioxidane Jan 25 '21 at 19:39
  • Thanks all! Tin works beautifully, as they have tin coins... I suppose it's unlikely that nobody would have noticed tin melting in the thousand years they've been without metalworking, but I can handwave it with a reference to people's money being the first thing they save from a fire... – TheSpidermonkey Jan 25 '21 at 19:46
  • If it is a modern key you are still going to need to do some file work to make it fit, modern keys have fairly tight tolerances. – John Jan 25 '21 at 20:02
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    "They have tin coins": Where do they get them from? Tin is very soft; it is softer than gold; it can be cut with a knife. Those coins will suffer very high wear and tear, and will have to be replaced periodically. – AlexP Jan 25 '21 at 22:50
  • Why would she make a key? Those are useless without locks, and locks are enormously complicated. – Mary Jan 26 '21 at 00:10
  • @Mary: Locks and keys are very very old technology. Of course, made of wood, since the question says they don't do metallurgy; but wooden locks and keys were used well all the way to the end of the antiquity. Here is a YT video explaining how they worked. – AlexP Jan 26 '21 at 00:24
  • you can cold work many metals, no knowledge of smelting needed. – John Jan 26 '21 at 15:57

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I'm going to say that a character who does not even know that metal melts is not suddenly going to come to such an advanced idea as a forming a clay mold, and making a precisely shaped key.

People were accidentally extracting metals from over-fired ceramics for nearly 19,000 years before someone thought to do anything useful with that weird melted garbage. And even then it took about 1600 more years for anyone to think to cast metal in clay.

Beyond that, it took even more centuries before people started to figure out what kinds of clay could be used to cast something as precise as a key. This is because precision casting requires either a very specific kind of clay or a process for purifying clay to the needed fineness. Either way, this kind of clay is not something that your civilization will already have a use for; so, he will not have what he needs on hand.

Lastly, melted metal does not "look like metal". So, even if they already know what metal is, they will not know that they are looking at metal. In its molten state, it will glow red or yellow or what not; so, they will literally not know what to make of it, and even after the city is done burning, any melted metal will be fire blackened; so, unless someone comes along and tries polishing some of these strange black rocks for no good reason, it will look like all their metal tools just burned up.

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Nosajimiki
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  • "People were accidentally extracting metals from over-fired ceramics for nearly 20,000 years before someone thought to do anything useful with that weird melted garbage" [citation needed] – N. Virgo Jan 26 '21 at 08:26
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    Careful about analogies with stone age => the world is very much different in a post-apocalyptic setting! In a post-apocalyptic people are already used to tools and crafts, which changes a lot of things. For example, if the character is already molding melted plastic, then seeing melted metal and having the idea to mold it would not be a huge leap at all. It will very much depends on what the state of tooling/crafting is. – Matthieu M. Jan 26 '21 at 09:33
  • @Nathaniel, Anyone who's done ceramics before knows how easy it is to fire to the wrong temperature and cause your clay to melt and separate, but primitive kilns get hot spots and clays have very different firing temperatures based on their contents. The kinds of clays most often seen used in the stone age fire ~1750-2000F, but many clays include large ratios of non-iron metallic oxides that fire at as little as ~1200F. So, if you put that clay in a >1750F kiln... – Nosajimiki Jan 26 '21 at 15:40
  • what you are left with is a puddle of metal like copper, tin, or zinc and a ruined clump of silicates. We know these accidents would have been unavoidable since they happen to people every day. As for dates, I dug a bit deeper and some things had to be adjusted a bit do to more obscure "oldest examples" . The first fired ceramics go back to ~24000BCE, the oldest metal tool goes back to ~5100BCE, and the oldest example of cast metal goes back to ~3500 BC. – Nosajimiki Jan 26 '21 at 15:40
  • @MatthieuM. The OP made no stipulation that plastic casting survived either. On the contrary, the question says that the protagonist has no tools or knowledge of metal working which means he logically can not know what casting is. If he already knows about casting, then the question is kind of invalidated. – Nosajimiki Jan 26 '21 at 15:46
  • @Nosajimiki: Your reading of my comment is too strict. I am not saying that the character has knowledge of plastic casting, I am using plastic casting as an example of something that may have survived that didn't exist in the stone age to illustrate why your argument that it took millenia in the stone age to get the idea of smelting/casting metal may just not apply. – Matthieu M. Jan 26 '21 at 16:00
  • For what it's worth, tin and lead have melting points low enough that they still look like metal when molten to human eyes. – notovny Jan 26 '21 at 22:16
  • @notovny, that is a good point. But there is also the problem with both lead and tin that they both much too soft to make a normal sized key with. With metals that soft, you are probably better off just making a key out of the clay itself. So they might better inform future experimentation that leads to a metallurgy renaissance, but will probably be useless in the moment. – Nosajimiki Jan 26 '21 at 22:58
  • Very grateful for all these thoughts. A few points just to clarify what game I am playing: it's not a post apocalyptic world, but an "egg" fired through space to a habitable planet with a load of frozen embryos and some "parent" robots to raise the first generations and teach them essential skills. For plot-related reasons, metalworking was not among the skills that got passed on before the robots vanished, and the civilisation left behind has metal objects the robots made (and a huge city they built), but is technologically behind. So, no plastic. – TheSpidermonkey Jan 27 '21 at 20:27
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What metal might work?

Most metals will not melt in "typical" fires, with tin (thanks, Alexander!) being an exception; tin melts at almost exactly the auto-ignition temperature of books.

Brass, bronze, copper, silver and gold all melt around 1,000°C (give or take 50-150°). Some alloys of iron melt around there, while others need higher temperatures. (This chart may interest you.) Lead also has a low melting point (though not as low as tin), but is of course nasty stuff. Metals with near-room-temperature melting points are probably not useful for your purposes, and at least mercury is also really nasty stuff.

Keep in mind, however, that not understanding that metals melt if you get them sufficiently hot implies a really bad grasp of what we'd consider "basic" science. After all, we see phase changes in ice all the time, and they'll almost certainly know about phase change in fats and waxes. A much more likely issue is going to be not being able to make a sufficiently hot fire. However, you should be able to get to around 1,100-1,200°C with just charcoal (in a suitably lined chimney). If they know the basics of a blast furnace, they can probably melt iron, too.

Could you make a key by pouring metal into a clay mold?

Well... that depends on what you mean by "clay", which can mean a lot of things. The sorts you might use for ceramics can probably take the heat, though. Actually, most minerals tend to have higher melting points than metals, and "fire clay" can be as high as ~1,700°C. For that matter, even today many metal casting molds are still made from clay/sand/rock.

The important thing is firing the mold first. If your character doesn't [know to] do that, the mold will explode when the water mixed with the clay flash-boils.

How could the character handle it without specialist gear?

Short answer: not possible.

More useful answer: the "specialist gear" necessary is not actually that complicated. You only need two or three things.

First, you need a crucible. This is just a bowl made out of something with a higher melting point. If you're using tin, this could even be iron, but it's most traditional to use rock. If you're sticking to something with a melting point around 1,000°C, granite will probably do in a pinch. Fired clay might work (I want to say it will, but might only survive a few uses).

Second, you probably want need tongs. You aren't going to be touching that crucible with your bare hands. Metal tongs are traditional, but might be hard to find in your situation. This may be your biggest sticking point. A dense, hard wood with a high ignition temperature might work (especially for tin), if you don't mind that you might have to try several times.

Third, you probably want something to protect your hands/arms from spills. Good, thick leather gloves may be sufficient.

If you're sufficiently clever (to create a setup that minimizes the time you need to be in contact with the crucible) and/or don't mind possibly getting very serious burns, you might be able to get away with only one of the latter two items.

FWIW, I recommend watching some videos on backyard metal casting; this will give you a better idea of what's involved.

Matthew
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    I've seen many videos show that a stick or piece of bamboo cut in half then bent back on itself being used as tongs for handling blooms of iron. As it turns out, you can hold very hot things with wood, not because it won't burn, but because it takes so long to burn enough to fall apart. – Nosajimiki Jan 25 '21 at 20:18
  • @Nosajimiki, huh, I honestly would not have expected that... – Matthew Jan 26 '21 at 13:05
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Bear in mind that metalworking in the real world was invented/discovered by people working with no specialist knowledge or tools. However, it was a slow incremental build-up of techniques over centuries.

I am not a metalworker, and yet a year or so back I tried to cast some objects from tin. I made an impression into plaster-of-paris (it survives the heat long enough), and poured some molten tin onto it. At that point I discovered something: Molten tin has quite a high surface tension. This meant that the metal did not take the shape of the impression - it just sat on top. The metal kind-of blobbed to a out 4-5mm radius, so features smaller than that were lost.

So I'm not sure you could cast a key from tin using an impression very accurately.

Tin is also quite a soft material - you can bend it with your fingers. I doubt it would be strong enouh for a key.

Your person has a potter as a friend

A pottery kiln often reaches in excess of 1000 degrees. This is hot enough to melt copper, and allows your enterprising metallurgist to also make a crucible.

Other ways to make a key from a clay impression:

  • Casting resins? Even if knowledge of metals have been lost, maybe other techniques haven't been? You could potentially cast a key from epoxy resin.

  • Filing down other materials. Stone, wood, bone, found bits of metal etc. Go at it with a file. It'll take a couple hours and maybe a couple goes, but this method will definitely work. I believe it is how (modern pin-tumbler) keys were replicated before profile cutters.

  • Rawhide. Rawhide is soft when wet, but dries hard. Maybe you could smush it into the key impression?

There's something fun you could do at home.... Since your character has to do this with no skills, tools or experience, it sounds like a perfect candidate for you to try at home. Go grab some clay, make an impression of a key and have at it!

sdfgeoff
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    There must have been something wrong with your tin (maybe it was actually tin-lead / Sn-Pb solder?), or with the mold, or with the process. Tin has excellent castability. All those tin soldiers were made of tin for a reason. – AlexP Jan 25 '21 at 22:51
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    @AlexP Tin, like most metals, get WAY better detail when you use a technique like die-casting, or vacuum molding. Those tin soldiers were not just being poured, but a force was being applied to help it fully fill the mould. – Nosajimiki Jan 26 '21 at 14:34
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    your mold was likely too cold, it was not surface tension as much as the metals were cooling on contact. – John Jan 26 '21 at 16:00
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    Thanks for the tips on molding! Yup, the mold was cold, it probably wasn't pure tin and there was no pressure applied to force it into the mold. But as I am a guy with no metallurgical skill, I think it's still a fair enough approximation of a guy in a world with no metallurgical knowledge. – sdfgeoff Jan 27 '21 at 00:45
  • Thanks for all the thoughts! Pottery is reasonably advanced in this world, so a kiln is very much an option, and for plot-related reasons, if my protagonist can basically invent bronze, that would be very satisfying. – TheSpidermonkey Jan 27 '21 at 20:29