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I know aluminium was extremely hard to refine until the 20th century, which is one of the reasons why it wasn't used much. But what are the drawbacks to using it as armor or weapons in this type of setting? Would it be too brittle, or tough to smith into shape?

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    $\begingroup$ You sir, need to look at titanium and tungsten instead $\endgroup$ – Chris J Sep 1 '16 at 12:53
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    $\begingroup$ How is titanium any better? How is tungstan (heavy and brittle) useful at all? $\endgroup$ – JDługosz Sep 1 '16 at 14:11
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    $\begingroup$ I had read a time traveling tip that basically said to bring back a 12 pack of aluminum cans. The aluminum would make you exceedingly rich... then I guess you could build a castle? Does a castle count as medieval armor? $\endgroup$ – Culyx Sep 1 '16 at 14:14
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    $\begingroup$ Aluminum was more than just extremely hard to refine back then. It wasn't possible. Not until the 1800's, at least. Native aluminum (aluminum existing just as a lump of aluminum) does exist in nature, although it is exceptionally rare. Keep this in mind. In medieval times, they'd be more likely to find a lump of gold than a lump of pure aluminum. $\endgroup$ – Tophandour Sep 1 '16 at 14:48
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    $\begingroup$ Don't forget to watch out for people with mercury. en.wikipedia.org/wiki/Aluminium_amalgam $\endgroup$ – JAB Sep 1 '16 at 19:51

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Aluminium is too soft to build strong armor from it. On the Mohs scale aluminium just has a 2.75 while iron, for example has 4, copper has 3.

The Mohs scale is about how hard it is to scratch a given material and the higher the number the harder is the material. That means that while copper can scratch aluminium the opposite is not true.

Edit: While there are multiple properties to look at none of them alone shows what is good for armor and the kind of armor you want to craft.

Let's look at toughness:

In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing

So assume you want to craft rigid armor. If it is as soft as aluminium but has a high toughness any meaningful blow will dent it without fracturing. No fracturing is good but a big dent in the breastplate means you can't breathe freely anymore. That has happened a lot with iron/steel armor in medieval times but it would be much worse with aluminium. Because of that soft but tough materials are just good for flexible armor that doesn't keep the dent, or at least not over a large area.

Back to aluminium. Aluminium armor could be good for presentation purposes like for ceremonial guards. The armor looks good, needs little maintenance and is light. If those kinds of armor ever see combat it would be short skirmishes not prolonged battles. So the problem described above would not matter as much as for armies.

There are other kinds of armor that might be enhanced by addition of aluminium like brigandine or jack of plates where metal plates are added to cloth. In those the malleability isn't as much of a problem and the lesser weight could be beneficial.

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    $\begingroup$ When it comes to armor, you don't just have to look at the hardness but also at the toughness. Yes, that's a difference. A material which isn't that hard but is tough can still make a good armor, especially against blunt weapons. $\endgroup$ – Philipp Sep 1 '16 at 13:24
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    $\begingroup$ Yea, I don’t care about scratching it! I care about denting it and smashing through. $\endgroup$ – JDługosz Sep 1 '16 at 14:10
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    $\begingroup$ This is wrong. If that were the case, diamond armour would be the best, but it would be extremely brittle. $\endgroup$ – Davidmh Sep 1 '16 at 18:18
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    $\begingroup$ This answer shows a complete lack of understanding of the relationship between toughness, hardness, and overall fracture resistance of various materials. $\endgroup$ – enderland Sep 1 '16 at 19:14
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    $\begingroup$ Mohs scale is nonsense here. Good armour material is highly elastic, has high tensile strength, shows little tendency for strain hardening, doesn't fail under unidirectional compression. $\endgroup$ – Karl Sep 1 '16 at 20:24
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For a weapon aluminum is a poor choice since it can't hold an edge well and, due to the low density, can't concentrate force like iron (in a warhammer, for example). For PARTS of a weapon, it might be serviceable.

As for armor, depends on what you want it for. You can make chain mail from aluminum and it is comparatively light. This may work to resist slashing attacks. However it doesn't have the strength to stop piercing attacks, like the thrust of a dagger or an arrow head. The rings will spread and burst. So you can double or even triple up on the rings (or make them very thick) but then the armor starts to weigh as much as iron or steel. Same with a breastplate. Ok for turning a slash but unable to stop a thrust (either punch through or deform and crush the soft tissue underneath). This can be easily demonstrated with aluminum cookware versus stainless steel. The rigidity and strength of steel versus the same thickness of aluminum is readily apparent. And the aluminum cookware is significantly lighter.

One benefit will be a reduction in rust, as well as lighter weight for the same thickness of metal and the ability to take on a high polish. So aluminum would probably be great for a child's suit of armor or a parade suit, made for flash, not efficacy. Or if you are facing Magneto :)

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Yes, when thick enough The Aluminum castings we use my factory are about about 8mm thick and quite tough. It's unlikely any sword and strong man will be able to do much other than bend it a bit. If these castings weren't so expensive, I'd take it out back and try to smash it for empiricism. I have broken (snapped) a 4mm aluminum cover by tightening bolts out of sequence, but there where factors such as torque and leverage at play which would not be involved in the physics of a sheer strike. In a related point, Plate Armor is designed to dent! chest plate fits over padding at a distance from the wearer's body. If the armor absorbs a strike, it has room to bend without hitting the occupant. A semi-soft metal may be a benefit to armor because of the ability to bend instead of break. Given sufficient thickness, Aluminum Armor is plausible.

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    $\begingroup$ good point actually about dissipating impact energy. $\endgroup$ – MolbOrg Sep 1 '16 at 22:30
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    $\begingroup$ The problem is, aluminium is quite brittle. It doesn't bend easily, it tends to break (as you observed), and it's not very ductile (so you need to cast instead of forging; and forging is a pretty good way to work with heterogenous material that is both tough and hard). In your example, this can be seen clearly - if you were working with forged iron/steel instead, you would most likely bend the cover instead of snapping it. $\endgroup$ – Luaan Sep 2 '16 at 11:16
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    $\begingroup$ I think the difference between pure aluminium and its various alloys should be mentioned. While aluminium itself is relatively brittle—which is bad thing for armour—some alloys are malleable and can take quite a lot of plastic deformation before breaking. So while pure aluminium wouldn't be a good material for armour, some of the alloys certainly would. $\endgroup$ – Jan Hudec Sep 2 '16 at 19:19
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One weakness of aluminum versus iron or steel is its lack of a real fatigue limit (aka Endurance Limit.) Fatigue is the accumulation and enlargement of small cracks and imperfections when the material is stressed repeatedly.

To cause fatigue in iron and steel you have exceed a certain threshold of stress. As long as the stress is below that point, the metal does not weaken, no matter how many times you repeat the stress.

The problem with aluminum is that it is subject to fatigue from much smaller stresses than steel. Even a small stress will cause some fatigue, and over time as the stress repeats, the fatigue increases and eventually the metal breaks.

Take a steel bar and an aluminum bar of equal strength, and repeatedly bend them both back and forth. The aluminum bar will weaken and break much sooner than the steel bar.

In the case of armor, repeated stresses both from impact and from normal stresses of marching, riding a horse, and especially the stress from repairs will cause fatigue, which could cause sudden, surprising failure.

Another problem with aluminum from a medieval perspective is that it's much more difficult to forge weld due to its lower melting point, more rapid oxidation, and the inability to use the color of the glow as a gauge of temperature. Aluminum is highly reactive with oxygen, and quickly forms a coating of oxide which will prevent a good weld.

One further small issue with aluminum is its lower mass. A more massive object has more momentum than a lighter one. It's simply harder to push around. Of course, lighter armor has plenty of benefits, easier to put on, less tiring, and so forth, so this is a minor disadvantage at worst.

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  • $\begingroup$ > One further small issue with aluminum is its lower mass. A more massive object has more momentum than a lighter one. It's simply harder to push around. $\endgroup$ – Daerdemandt Sep 2 '16 at 15:54
  • $\begingroup$ That would put Al at advantage then. Also, whereas Al would be hard to forge and weld, it would be way easier to cast. I'm also not sure if marching will actually cause some significant fatigue. And speaking about dents causing fatigue - that can be solved by the same blacksmith who fixes those dents, heat up the piece to let grid imperfections recombine or scatter or exit to surface. $\endgroup$ – Daerdemandt Sep 2 '16 at 16:05
  • $\begingroup$ Depend on what you mean by advantage. If the goal is to make it easier for the wearer to move quickly and easily, then of course yes, it's an advantage, but I did specifically mention that, as have several other answers. The point is that light weight is not always beneficial. Higher mass means lower movement on impact, and higher mass also means more energy is carried during movement, such as in a lance charge. In jousting, for example, where freedom of movement was less important than having lots of momentum, armor became a lot heavier. As I said, a minor point, but still legitimate. $\endgroup$ – barbecue Sep 4 '16 at 18:53
  • $\begingroup$ WRT repairs, of course they can be made, but aluminum is harder to repair using the same techniques used for iron or steel for reasons already stated. And the key issue with aluminum fatigue is that it occurs even with minor stresses, which does not happen with iron or steel. $\endgroup$ – barbecue Sep 4 '16 at 18:57
  • $\begingroup$ I'm not convinced by your argument. This failure mode of aluminium is a huge problem in aircraft, because the material is regularly exposed to big transient stresses (albeit not so big that they would cross the fatigue limit of steel). And yet aluminium is the dominant material there. OTOH, armour is normally only exposed to pretty small stresses, and only in battle suddenly to blows with impact strength many orders of magnitude stronger. Precisely those blows, where even the fatigue limit of iron is exceeded, are the benchmark for good armour! $\endgroup$ – leftaroundabout Sep 17 '16 at 17:36
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From Wikipedia:

Aluminium is a relatively soft, durable, lightweight, ductile, and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness.

Emphasis mine. Medieval armor design was based on hardness, stiffness of steel. Weapon needs hard edge, too.

Aluminium has about one-third the density and stiffness of steel.

So to have the same stiffness, you basically need the same mass, and more thickness - only drawbacks.

Of course, modern aluminium alloys are better than that. At the same time, modern steel varieties are better, too.

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    $\begingroup$ Same mistake the other answer made - whether it's hard/soft is largely irrelevant. $\endgroup$ – Stop Harming Monica Sep 1 '16 at 18:03
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    $\begingroup$ Hardness is not the only factor to consider, but it's hardly irrelevant. The cuir boulli technique was specifically for the purpose of hardening leather for use in armor. $\endgroup$ – barbecue Sep 2 '16 at 1:05
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    $\begingroup$ @barbecue Once you get past a certain point, it's irrelevant. Unhardened leather is somewhat resistant to slashing, but not much good against piercing or blunt force. Even aluminium is harder than hardened leather. $\endgroup$ – Adeptus Sep 2 '16 at 1:24
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    $\begingroup$ Hardness means resistance to perforation or penetration. Steel is harder to pierce than aluminum. That's relevant to armor if you're worried about being stabbed. It's not the ONLY factor, but it's relevant. $\endgroup$ – barbecue Sep 2 '16 at 1:39
  • $\begingroup$ @barbecue Historically, steel armour was rarely hardened bfore firearms came around (bullets being extremely soft and rather light, but concentrating a lot of force in a small spot). Even then, the hardening was only done on the surface - you don't want to make the armour brittle, obviously. You can make hard "coating" in many ways, but aluminium isn't a good fit for either - it's not hard, and it's not tough. $\endgroup$ – Luaan Sep 2 '16 at 11:13
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Aluminium is used (wikipedia) in modern armour for amoured fighting vehicles such as the M2 Bradley (wikipedia). This demonstrates that the idea isn't ridculous.

In the case of the Bradley, the aluminium (presumably an alloy, most references to aluminium actually refer to Al-based alloys) is laminated. Wikipedia doesn't say what with, but in your case even a thin skin of steel over an aluminium plate could give a hard surface over a tough backing. This would be like ironclads, in which much of the strength came from wood, hence clad. Such an approach would be lighter than steel alone. Aluminium frames, even hollow tubes bent into shape could form a strong structure to hold its shape against impact and support a shell of steel over aluminium.

As Al alloys are quite easy to work, the steel skin might even be chainmail.

Inspired by your corundum answer: You could even replace the steel (if casting the aluminium) by a suitably hard stone embedded in the matrix. Of course individual chunks would get knocked out and broken, but so do ceramic trauma plates in modern armour (wikpiedia again) and they're widely used.

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    $\begingroup$ Well, modern ceramic trauma plates are used despite their troubles because we have no better alternative. The same thing wasn't the case in medieval armour. If you were rich enough to afford a full set of metal armour, you were likely to survive lots of damage to the armour, and repair it afterwards. You're not going to throw it away just because it's dented. Ceramic trauma plates aren't exactly cheap, but they're still much cheaper than the steel armour was in medieval times, and their cost is usually measured against the huge investment of time and money that any modern soldier represents. $\endgroup$ – Luaan Sep 2 '16 at 11:27
  • $\begingroup$ If my idea worked at all (and I'm prepared to accept that it might not) it would be repairable. Worst case by recasting. (@Luaan) $\endgroup$ – Chris H Sep 2 '16 at 11:56
  • $\begingroup$ Yeah, recasting would work. But that would just make it all the more expensive :) Why would you choose a suit of aluminium armour instead of the much cheaper and more practical steel armour? $\endgroup$ – Luaan Sep 2 '16 at 11:57
  • $\begingroup$ @Luaan my preferred approach was steel over aluminium. But that "why would you...?" question of applies more to the question than an individual answer. $\endgroup$ – Chris H Sep 2 '16 at 12:01
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TL;DR: Aluminum armor is plausible in the form of corundum.

As many others have mentioned, pure aluminum foil/sheets is a poor (though light) armor choice. However, if someone really wants aluminum armor, we have another option. I understand it's not exactly what the question was answering, but let's explore it anyway for the benefit of others.

Crystallized aluminum oxide, better known as corundum, is a surprisingly feasible armor choice. It's extremely strong, the definition of a 9/10 on the Mohs scale. Normal steel has a hardness of 4-4.5 and hardened steel has a hardness of 7.5-8. What this means is that a steel blade, even a tempered one, wouldn't scratch the armor. In fact, the armor would be more likely to scratch it!

However, I understand hardness isn't the only consideration for armor material choice. We also must consider its weight. The specific gravity (A highly specific version of density) for corundum is around 4.1 The specific gravity of steel varies slightly, but is normally 7.7-7.8. This means that corundum armor would be nearly half as heavy as steel armor, weighing in at only around 26 kg!

We should also consider the availability of the material and manufacturing costs. This is where steel has a clear upper hand. Corundum is found only rarely (It's literally sapphires and rubies) and only in small sizes (rarely over 4-5 grams). This means that you couldn't have an armor made entirely of corundum (except if you allow magic, which I'm doubting based on the question), but you could have armor made entirely of small corundum gemstones inset in steel links.

Finally, if we're literally going to have an armor of small, glimmering gemstones, we might as well make it look like this:From the Residenz treasury in Munich

1 I tried converting this to density and then multiplying by the volume of a suit of plate armor, but then I realized two things: 1. You can just compare the SGs, and 2: The volume of plate armor is really difficult to find.

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    $\begingroup$ It’s probably worth noting that corundum (not “conundrum,” which is a word for a puzzle or mystery) is a specific crystalline form of aluminum oxide. It has to grow as that crystal to have the properties you describe. You could not fashion the armor from pure aluminum and then attempt to oxidize that—you would get aluminum oxide, but not corundum, and it would be even softer than pure aluminum in that form. $\endgroup$ – KRyan Sep 1 '16 at 18:02
  • $\begingroup$ Yes, which is why I suggest that the most feasible solution is to use the already-crystallized conundrum (gemstones), not oxidizing aluminum or gathering random scraps of aluminum oxide. $\endgroup$ – Ethan Chapman Sep 1 '16 at 19:05
  • $\begingroup$ tensile strength of sapphire is around 400MPa, as good mono crystals - not something superior. Nice picture - I like it. $\endgroup$ – MolbOrg Sep 1 '16 at 22:27
  • $\begingroup$ I wonder what the effectiveness would be of a "concrete" made with a mixture of corundum bits, cotton fibers, and a binding compound. The cotton fibers would tend to ensure that the armor holds together even if parts of it crack, while the corundum would protect the cotton. $\endgroup$ – supercat Sep 1 '16 at 23:25
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    $\begingroup$ Steel might not scratch corundum, but if you bang them together, which bends/cracks first? That's a much more important consideration for armour. $\endgroup$ – Adeptus Sep 2 '16 at 1:16
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The simplest problem is that it would have been cost a king's ransom to make. As you say it was hard to refine, and as a result of that, incredibly expensive.

emperor Napoleon III reserved a prized set of aluminum cutlery for special guests at banquets. (Less favored guests used gold knives and forks.)

This is quoted everywhere, unfortunately I can't find an original source, but this was already the 19thC. The earlier you go the less commonly available it is.

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  • $\begingroup$ I seem to recall seeing an old British crown that had aluminum as part of it on a visit to the royal jewels display at London Tower, but I could not find any source for it. I did however find Frederik VII's aluminum helmet kongernessamling.dk/en/rosenborg/object/frederik-viis-helmet, which is obviously a ceremonial, precious metal helmet. $\endgroup$ – user151841 Sep 1 '16 at 15:14
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    $\begingroup$ And don't forget that the Washington Monument in Washington, DC is capped with an aluminum pyramid. Back when the monument was built, that was one of the most expensive metals. $\endgroup$ – TMN Sep 1 '16 at 17:41
  • $\begingroup$ @TMN And more importantly, it was a great way to showcase the industrial power of the US, since the expense of aluminium is in the processing, not availability of the raw materials. Too bad it only took a few years after that to perfect a process that made aluminium relatively cheap :) $\endgroup$ – Luaan Sep 2 '16 at 11:18
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Armor

To disagree with the other stated ideas here, I think Aluminum could be a surprisingly good armor material.

Disclaimer: I don't have great sources on this and I am definitely not a materials engineer so please correct me if I'm wrong

From what I can tell aluminum is about a third as dense as steel, about a third as stiff as steel, and about half as hard as steel. That may sounds like a bad material, and it is if it is difficult to make in large quantities, but it also has some advantages.

  1. Because it is a third as heavy, you should in theory be able to make it about three times thicker than a steel alternative while being about as easy to wear and use. This more than makes up for the lack of hardness, and actually has a significant advantage just by being thicker. That advantage is that when slicing through it, you have to pass through three times as much material, which means three times as much friction and mass to displace, which means that you may actually have better piecing/slashing protection from aluminum armor than from steel armor by weight.
  2. Because aluminum is softer than steel, it is more likely to deform rather than totally break. This is particularly good when dealing with blunt impact because it absorbs a portion of the energy of the blow by bending, similar to crumple zones in a car
  3. Another advantage of being softer is that it could in theory be easier to shape for the smith, which would also potentially make it easier to repair when damaged. This may be contravened by needing to be thicker, but I don't actually know that much about smithing, so I'll leave that question to someone with expertise.
  4. If you need lighter armor at the expense of protection, aluminum would make for great chain mail. It would still do well enough for glancing/slashing blows, though it would do much worse than iron or steel at stopping a piecing weapon. The major concern (and I don't know how this would work in reality, but I would love to see a practical test), would be if the steel slashing weapon (lets say a knife) might just cut through the aluminum rings and get to you anyway.

Weapons

Also, just to make this answer complete, I agree with the other answers about using aluminum as a weapon material. That would not work well because being soft makes it not hold an edge, and bending on impact is bad for a weapon for the same reason it is good for a set of armor, it spreads out the force of the impact. Additionally, if you are trying to cut someone else wearing iron or steel armor, your aluminum weapon won't be able to penetrate it at all.

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    $\begingroup$ If you make it three times thicker, you'll have to pass through exactly the same mass of metal as with the three times "heavier" steel. And you're relying too much on things being linear, which is really quite an exceptional relationship, not typical. And while aluminium is softer than steel, it definitely isn't tougher - it will break very easily. Not to mention that steel armour (and weaponry) usually had a hard surface, but soft insides. It cannot be forged easily, and suffers horribly from fatigue. It also cannot be forge welded easily. Aluminium chainmail would tend to break quite a bit. $\endgroup$ – Luaan Sep 2 '16 at 11:23
  • $\begingroup$ As I said, I'm simplifying a lot because I'm not an engineer with the experience to really calculate the relationships accurately. Thanks for the clarification where I was making bad assumptions. However, regarding the mass of metal being the same, that is true, but the friction between the blade and the armor does matter, and increases with thickness, not mass $\endgroup$ – Kevin Wells Sep 2 '16 at 15:55
  • $\begingroup$ It's quite a complicated subject. If this were a simple collision (that is, throwing the sword instead of pushing it true), the two are almost identical. The fact that you keep pushing makes more complicated, but it still shouldn't matter unless the sword has to displace significantly more mass than it masses - which is very unlikely in any kind of viable personal armour. $\endgroup$ – Luaan Sep 2 '16 at 15:59
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    $\begingroup$ The big advantage of making your armor three times thicker is that it is much, much stiffer (on the order of 25-50 times stiffer). Means it takes a much bigger hammer to leave a dent in you. $\endgroup$ – Mark Dec 14 '18 at 20:35
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    $\begingroup$ @KevinWells, bending stiffness increases as the cube (or maybe it's the fourth power) of thickness. For similar classes of material (eg. "structural metals"), that effect quickly dominates over material-to-material variations. $\endgroup$ – Mark Dec 17 '18 at 21:28
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Chris H already has pointed out that aluminium is used is some modern armour, so this is just about weapons.

If you look at a modern climbing axe, it has an aluminium shaft. If you look at a modern javelin, it (often) has an aluminium shaft. If you look at a modern trenching spade, it has (often) an aluminium shaft. Aluminium is much better than wood for pole arms which won't break, though you probably will want the tip or blade to be something which is harder.

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    $\begingroup$ Aluminum shafts definitely have some advantages over wood, however they do tend to bend when impacted and unlike wood, it won't bend back to its original shape. I know from experience when I tried to make a trebuchet with an aluminum arm which bent in half on the first throw and never recovered. I think you're right that it would be better for javelins and possibly arrows, but for a pole arm that has to withstand impacts it may be less useful $\endgroup$ – Kevin Wells Sep 2 '16 at 16:00
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The only true benefit for using aluminum is that it is light, but in the case of body armor, that's not necessarily too good of a thing. Of course, this isn't the main purpose of armor, but heavy armor prevents your opponent from knocking you back too much. Most importantly, though, is that aluminum is used because of how easy it is to mold and bend, the exact opposite of what you want for armor.

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