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Though glass knife existed and are still used in modern electron microscopy, glass is known for its brittleness and a sword need to be able to parry an attack. But the idea of a transparent glass sword is very appealing aesthetically to me.

So if plausible, in what ways would a glass sword be useful in an alternate 19th century setting where gun ownership began to rise? Can it be reinforced with other materials? Is it possible to be a weapon of assassination or is it only possible to be a ceremonial weapon?

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    $\begingroup$ While it might be possible with some of the newest glass technology from Corning, not with 19th or 20th century glass making tech. $\endgroup$ – pojo-guy Aug 16 '18 at 11:15
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    $\begingroup$ Well, this guy tried to make a sword from melted obsidian - even the best attempts had cracks. $\endgroup$ – Kreiri Aug 16 '18 at 11:38
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    $\begingroup$ As long at this sword never bumps into anything hard, like another sword, stone or metal... then you can have it for stabbing-purposes. Apart from that... no. $\endgroup$ – MichaelK Aug 16 '18 at 11:43
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    $\begingroup$ @Tonny to be fair, every time they come up with a stronger version of Gorilla Glass, manufacturers turn round and make the glass in their screens thinner so the overall strength stays much the same. I think you'd notice the difference more if the thickness of the glass stayed constant. $\endgroup$ – walrus Aug 16 '18 at 13:01
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    $\begingroup$ If you want to go with a transparent blade, you could consider transparent aluminium. $\endgroup$ – T. Sar - Reinstate Monica Aug 16 '18 at 14:02

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Blades are made of metal, even though metal isn't as hard as some minerals, because metal is tough. Hardness is a term in material science that represents the amount of energy it takes to cause an object to deform. Toughness is a term that represents that amount of energy it takes to cause an object to fracture.

In comparing fracture toughness, we see that Aluminum is perhaps 20x as tough as glass, while steel more like 50x as tough.

There are very tough glasses out there, but they will never be as tough as metals. Here is one company that advertises a mostly transparent glass/ceramic product. They don't advertise the fracture toughness of the material (because it is low) and instead concentrate on hardness, bending strength, and stiffness (which are comparable to steel). The best their ceramic products do is 7 MPam$^{1/2}$, which is about 1/3 of Aluminum and 1/7 of steel.

So there do exist transparent glass products out there, reinforced with Silicon Nitride, that take 3 times less energy to break than aluminum and seven times less energy to break than a regular old steel knife.

What does this mean? Well for stabbing a squishy meat-bag of a human, they will do just fine and be quite fatal. But if the person is wearing some sort of armor underneath their clothing, these hardened glass knives are as likely to punch through the armor as steel (because they are just as hard and stiff) but seven times more likely to break while trying to punch through.

So I guess the answer is...maybe?

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    $\begingroup$ Bah, while I was out searching for and reading up for Fracture Toughness, you beat me to the punch. +1. :-P $\endgroup$ – MichaelK Aug 16 '18 at 11:41
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    $\begingroup$ you have a very similar answer to what I would have used. I would maybe add laminated glass with layers of 7 or more (like bullet proof glass). One layer may shatter in a fight but it should be easy enough to add a new one after, making the "sword" reuseable $\endgroup$ – Reed Aug 16 '18 at 13:20
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    $\begingroup$ And in a sword fight against a steel sword, you'd better avoid parrying or getting parried :/ $\endgroup$ – Matthieu M. Aug 16 '18 at 17:58
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    $\begingroup$ A melee weapon with a blast radius - that has to be worth something. $\endgroup$ – Xenocacia Aug 17 '18 at 2:57
  • $\begingroup$ Hardness does not represent any sort of energy. The theoretically simplest measure of hardness is the Young's Modulus, which has units of force per area (in SI newtons per square metre also known as pascals - although Youngs Modulus is more often quoted in MPa.) $\endgroup$ – Martin Bonner supports Monica Aug 17 '18 at 12:49
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It would presumably be less practical than the Aztec Macuahuitl, unless those reinforcements are amazingly effective.

  • A solid core with glass edges might be the most practical solution, but of course this goes against your desired visual effect.
  • It might be limited to a single stab, not a fencing duel where blades clash repeatedly. That sounds like an assassin's tool, but unlike a dagger a longer sword would be hard to hide.
  • In a world prior to metal detectors, a non-metallic sword would be no easier to hide than a metallic one.
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    $\begingroup$ A solid core with glass edge might not be OP's desired visual effect, but it would probably still look awesome. Imagine a dark steel blade with a leading edge of sharpened, polished glass. Such a weapon would gleam in the sunlight. $\endgroup$ – MindS1 Aug 16 '18 at 17:12
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Synthetic sapphire

What you want from your glass sword is aesthetics; it doesn't seem essential that it be technically a form of glass. What I propose is actually a crystal.

Sapphire is the pure crystalline form of (otherwise very common) aluminium oxide. It is extremely hard -- much harder than steel, and nearly as hard as diamond -- and quite tough. It isn't as tough as steel, but similar to cast iron, and very much tougher than glass. It is also much more resistant to high temperatures than steel, so at white heat it is actually tougher than steel. And it is -- or can be made -- water clear, far more transparent than ordinary window glass. It is about half the density of steel: light, but still much denser than glass, plastics, or light alloys, so perhaps making for a sword that it fairly lightweight, but has some heft for powerful blows. Alternatively, the lower density could be "spent" on making the blade thicker and wider, for greater strength.

It's no surprise that natural sapphire is a prized gemstone, but in the mid-twentieth century, we learned how to make synthetic sapphire fairly economically. By the end of the twentieth century the technology had progressed to the point where we could make huge synthetic sapphires -- potentially sword sized, perhaps even bigger.

Although this technology was developed well after your period, the basic tool required -- the hydrogen blowtorch -- did exist in the nineteenth century, and it is quite plausible that in an alternative timeline, synthetic sapphire may have been discovered far earlier.

Why would someone want a sapphire sword? One or more of the following reasons might apply:

  • Use at very high temperatures (how the wielder survives is an exercise left to you!);
  • Superior corrosion resistance in many environments (but not in alkalis, which damage it rapidly);
  • Not affected by magnetic fields or electric currents;
  • Superior hardness and extreme sharp edges (but the very devil to re-sharpen if you damage the edge);
  • Coolness for a social elite: the hoi polloi can have their common steel pig-stickers; us gentry use only the purest crystal swords.
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    $\begingroup$ Sapphire is nowhere near as tough as cast iron. Toughness given at Azom is 1-1.5 MPam$^{1/2}$. Cast iron is 10-24 MPam$^{1/2}$. Off by an order of magnitude. -1. $\endgroup$ – kingledion Aug 16 '18 at 13:08
  • $\begingroup$ It's still glass, so one good whack would probably destroy it whether you used it against a sword, a rock, or anything remotely hard. $\endgroup$ – Clay Deitas Aug 16 '18 at 13:16
  • $\begingroup$ Hardness is the wrong measure. A sword's durability stems from its fracture toughness, and sapphire isn't much better than glass on this point. $\endgroup$ – Mark Aug 16 '18 at 19:49
  • $\begingroup$ Some useful problems expressed in the comments. I like the idea — but it wouldn't work in the albeit terse conditions expressed in the question. You're talking synthesis of nonmetallic composites: a variety of things can be done with them, but only in 20 years recent, or thenabouts, has the cost of manufacture allowed them to be used in anything other than proof–of–concept. $\endgroup$ – can-ned_food Aug 17 '18 at 4:38
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    $\begingroup$ @kingldion: my reference says differently. 1.5 MPa $m^{-1/2}$ is very much on the low end -- actually it gets up closer to 6 MP $m^{-1/2}$. This isn't quite as high as cast iron, but as engineering materials cover some 3 1/2 orders of magnitude, its close -- that's why I said "similar". $\endgroup$ – Securiger Aug 19 '18 at 12:06
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Let's use obsidian for our starting point. Obsidian has been and still is used for knives. These knives are poorly shaped, and in modern times only used in low impact high detail settings. It is unsuitable for making a sword or pretty knife.

Then there's regular pane glass. These shards can be very sharp, but tend only to exist in large but fragile broken shapes. you could create a shaped blade by etching the glass and breaking it out, but it would be very useless and look bad. You could also mold it into the shape of a weapon, but it would risk breaking and hurting yourself and could only be ceremonial. (Depending on the material glass can be a number of colors, but all tend to have the same properties as the most common silicon variety.)

Next we have tempered glass. Oh beautiful tempered glass. Take the mold from the regular glass and make sure there's a wide flat blade with a sharp tip and a sturdy handle. Temper it into safety glass of the highest kind, or at least a moderate temper. the idea is that a wide flat blade can be easily snapped, rupturing the temper and causing the blade to become a bunch of glass shards inside of any assassination target. Good luck saving them. Plus, the handle will crumble away, and provided you wrapped it in a cloth or wore a glove to protect your hand, there will be no physical evidence to trace back to you except a bunch of small bits of glass. If you forget to protect your hand you may get some small cuts.

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  • $\begingroup$ I think it is worth mentioning that swords got pretty banged up through regular usage and if you chip a tempered glass sword it is likely to poof. $\endgroup$ – Marie Aug 16 '18 at 12:25
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    $\begingroup$ That's why I recommend it as an assassins blade. if you purposefully break it while it is inside someone they will get a belly full of glass shards which are hard to remove and don't stop the bleeding like a regular knife when left behind. $\endgroup$ – Clay Deitas Aug 16 '18 at 13:01
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    $\begingroup$ Actually, since tempered glass doesn't form shards, but rather blocks, the pieces in the wound would be relatively benign, and could be mostly cleaned out. $\endgroup$ – WhatRoughBeast Aug 16 '18 at 17:21
  • $\begingroup$ Still, they will be hard to remove completely considering the number, especially when racing against a large amount of bleeding. And every piece has the potential to become stuck, cause internal bleeding or infection. The stabbing does most of the damnage, but the shards are just an extra annoyance for anyone who may try to save them. $\endgroup$ – Clay Deitas Aug 16 '18 at 20:16
  • $\begingroup$ @ClayDeitas Still seems like a weak justification; You'd think a potential assassin might just invest some time to get better at stabbing people where it counts instead. $\endgroup$ – Cubic Aug 17 '18 at 14:20
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In Brandon Sanderson's Mistborn series glass daggers were a weapon of choice for certain fighters. The reason was simple: some people could manipulate any metals, thus any metal armour was impractical. Metal weapons were out too for the same reason, so all you were left with were either wooden weapons, or something brittle (glass, bone, etc). Since wood alone isn't that good for cutting, you used glass. And since your opponents typically had no armour, glass was practical even for open confrontations, not merely assassinations.

In other words, make metal unsuitable for combat for some reason, and you will find glass weapons useful.

Of course, those still were daggers, not swords, as you wouldn't want to parry with them.

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  • $\begingroup$ +1 for the precedent, but still weak justification. If I had the choice between a glass or a wood weapon I'd take the wood every time. Wood can be sharpened well enough to cut at least a bit, doesn't shatter easily and even without an edge you can still use it for bludgeoning. You can make fairly adequate (in the absence of metals) spears and clubs with just wood and rocks. $\endgroup$ – Cubic Aug 17 '18 at 14:23
  • $\begingroup$ @Cubic Well, wooden weapons were used there as well, but mainly as bludgeoning weapon. While wood can cut, as far as I know it won't be as sharp as glass can be (with a glass shard you can cut your arm to the bone with just a glancing movement, while even sharpest wooden edges I've seen require a noticeable pressure to make a cut through skin, and wooden edges dull fast). If for some reason (blood magic? poison?) external bleeding is extra bad (as opposed to internal = bruises), that would provide extra incentive for cutting instead of clubbing. Spears are still better with wood though, yes. $\endgroup$ – Alice Aug 17 '18 at 14:38
  • $\begingroup$ Also, that doesn't really explain why one would want it all made of glass, as opposed to something Macuahuitl-like. $\endgroup$ – Alice Aug 17 '18 at 14:38
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You said you are interested in it for aesthetics. Therefore, I'm going to go with an answer that explains why glass weapons exist without them necessarily being the most practical weapon. I think the glass sword would be:

The weapon of choice for a gentleman's duel to the death.

Gentlemen have to have a good way to have a duel to the death after all, and when it comes to such things the ideal weapon is a weapon the requires real skill, and not necessarily because it is practical. I'm imagining that in your world a short glass sword (glass dagger/rapier?) has become the weapon of choice to permanently settle matters of honor, and all such fights are fights to the death. The duel itself has evolved to account for the ease with which a glass dagger can break. Some thoughts off the top of my head:

  1. Intentionally breaking an opponents sword is considered dishonorable
  2. Finishing the duel, dispatching your opponent, and doing it without breaking your own sword is considered an especially impressive victory.
  3. There will no longer be any parrying at all - doing so will obviously be an easy way to break your sword
  4. Stabbing will be the goal. You could probably build a cutting edge with a glass sword, but slicing your opponent will probably have a high chance of breaking your sword.

I'm trying to figure out what such a duel might look like:

  1. If there is an edge on the blade, I could imagine there being special gauntlets/armor worn on the arms that are used to deflect a stab/cut from the opponents weapon. Perhaps this is the only armor that combatants are allowed to wear.
  2. Presuming that the weapon is for stabbing only, I imagine a shorter blade will be preferable so that you can stab and then pull your weapon out of your opponent quickly, minimizing the chances of your weapon breaking if your opponent moves/falls.
  3. Good aim and knowledge of anatomy will be important - hitting an opponent's bone may kill them (depending on what else you hit) but probably has a much higher chance of breaking your sword.
  4. Allowing your sword to be broken would be considered poor form because at that point in time your only recourse is to effectively beat your opponent to death, and that is neither skillful nor honorable.
  5. If both opponents break their swords then you basically have two men trying to beat each other to death with their bare hands. Perhaps the relative "indignity" of this act leaves even the victor in a state of dishonor.
  6. As a result, the goal may be to end the match with a single quick thrust into vital areas/organs, quickly and painlessly (relatively speaking) killing your opponent with a minimum of fuss. After all, that is the only way an honorable gentleman would kill another (irony intended).
  7. Depending on weapon length I could see this leading to a match that is effectively a grappling contest, with each contestant trying to line up their "sword arm" for a single quick and clean stab into just the right soft spot on their opponent...

I'm not even a fan of violence but I almost want to watch such a match myself :) Glass swords: the weapon of choice for a gentleman when the court system is not available to give you the "justice" you deserve.

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If bullet proof glass exists, then I think there is at least some kinds of glass that can be used as a traditional weapon. Remember that even when using swords, wielders try to avoid clashing weapons. So if the glass is bullet proof, and is only impacting soft flesh as per the 19th century soldier types where heavy armor was not common, then I don't see a problem with using glass weapons.

The technology and manufacturing process might not exist in this time period however.

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  • $\begingroup$ Bulletproof glass is a composite, typically of a hard glass and a tough plastic. It's also quite thick -- you could make a club out of it, but not a sword. $\endgroup$ – Mark Aug 16 '18 at 19:51
  • $\begingroup$ Likely any "glass sword" would not be 100% glass. Just like swords are not 100% iron. Having a little bit of plastic or some other material would not be a game changer since the goal of the poster seems to be to have a transparent sword. $\endgroup$ – Tyler S. Loeper Aug 16 '18 at 20:22
  • $\begingroup$ Most "bulletproof glass" is a multi-layer (typically 3 layers) of polycarbonate (GE's product is Lexan). The "bulletproof" version is usually about 1-inch thick. It is extremely puncture resistant but actually quite easy to cut. $\endgroup$ – nurdyguy Aug 16 '18 at 22:12
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Mayhaps a "Prince Rupert's Drop type construction so the sword shatters instantaneously into 'dust' when stressed at the appropriate location but, until then, is "reasonably strong".

Obtaining a "sword shaped" Prince Rupert's drop 'may take some work'.

Applications might include -

  • Causing a weapon to vanish after it has been used to commit a dastardly deed.

  • Poison or other substance delivery system - an aerosol cloud is generated when the sword self-dismantles.

Stills from this Youtube video

Frame at 1m46s - Head end is "sharp hammer head proof"

enter image description here

But "tweak the tail" and it explodes. (1m51s)

enter image description here

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The question was pertaining to swords, but hinted also interest as an assassin´s weapon: As an assassin´s weapon of choice it would do wonders! My history teacher told us about the venician glass daggers. But i could only find references in fiction: for example "The Glassblower of Murano". Maybe an italian speeker can help out =).

The idea of glass stilletos is so interesting because they are expensive to make -> sending a message, take great skill to use -> brittleness, and make the victim suffer,a lot. Since hitting anything remotely hard, breaks the weapon and it is almost impossible to remove the tiny shards completely, also the . Think about a certain Superhero with a magnet in his chest, but the magnet being useless.

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Pure glass through and through wouldn't work in the traditional way we use blades, as while its sharpness is very much comparable, the flexibility is nowhere near what steel's is. However! If you slightly change the way you think of swords, the Macuahuitl is a club that has been embedded with obsidian shards and used like a wakizashi (pull edge across skin, not poke or hack). It's not going to survive an actual swordfight without further alteration, but as a close-combat weapon, it would produce deep lacerations if slid over skin or armor.

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Glass isnt as brittle as people think. For example I regularily walk passed a door with a glass panel with a 40kg rhodesian ridgeback that throws his full force against it and scratches the glass, but as of yet the glass hasnt broken or scratched.

Using a graph from this PDF, 12th "slide": https://www.google.nl/url?sa=t&source=web&rct=j&url=https://glassproperties.com/references/MechPropHandouts.pdf&ved=2ahUKEwjFjIXi7PHcAhUrM-wKHUnyCFYQFjAAegQICBAB&usg=AOvVaw0b7mMzd29Xp1D6r6uIo7fp

You can see that tempered glass has a yield strenght of 500 while hotrolled steel is 580. That should be close enough for a few glass weapons unless someone has a better idea. Ofcourse if you stretch the definition you could go for anything you can see through and then you can go for very interesting materials.

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  • $\begingroup$ Yield strength is only marginally less wrong than hardness when evaluating something's usability as a sword. If you want to know if your sword will break when hitting armor, or another sword, or a brick wall, you're looking at the fracture toughness, and steel is far better than glass by this measure (by around one to two orders of magnitude). $\endgroup$ – Mark Aug 16 '18 at 19:57
  • $\begingroup$ The context is the 19th century. There is little or no armor in this time period. $\endgroup$ – Tyler S. Loeper Aug 16 '18 at 20:19
  • $\begingroup$ @TylerS.Loeper But also no swords... $\endgroup$ – kutschkem Aug 17 '18 at 8:15
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As far as how useful a glass sword would be, Larry Niven answered the question in regards to a dagger in the short story "What Good is a Glass Dagger", originally published in The Magazine of Fantasy and Science Fiction, September 1972.

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    $\begingroup$ I'm afraid that this doesn't answer the question - without reading the story (and I haven't) it's impossible to know what it means. Can you find a relevant passage from the story to quote (or summarise the relevant plot points)? $\endgroup$ – walrus Aug 16 '18 at 23:12
  • $\begingroup$ From memory it wasn't useful. The story has been reprinted in collections of Niven's short fiction. Please the relevant technical details of the glass dagger with respect to a glass sword. Have fun! $\endgroup$ – a4android Aug 17 '18 at 2:09
  • $\begingroup$ Actually, it was useful. Being made of a transparent material, it was able to be hidden in a stream and used to kill the story's baddie. $\endgroup$ – WRSomsky Aug 17 '18 at 16:13
  • $\begingroup$ would you mind quoting such publication? or parts of it? $\endgroup$ – DarkCygnus Aug 17 '18 at 18:33

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