I am building a world with a small island nation trying to defend themselves against a large continent with medieval (1200-1300) level technology. They lack metal and use obsidian and stone for weapons and tools, similar in technology level to the Aztec around 1500. How could they feasibly synthesise an explosive and how would they go about doing so?
Yes they can.
First, we need to know what Gunpowder is. Luckily we have Wikipedia, they will need an inventor. Will make a good story on how to get to gunpowder, methinks.
Your island(s group) are very lucky. A few of them have huge bird colonies. Those big bird colonies create a lot of guano. So you can feed a lot of people from the small area you have. Good, you will need your people to be numerous. But you can create part of gunpowder from guano.
Next step is sulfur. Again, you have hit the jackpot! A volcano with a Sulphur Mine. You might want to create a lower caste to get this stuff. It really smells. You know, like hell or another bad place.
The last step is charcoal. Now that is easy, we get that from burned wood. Now wood will be a bit of a resource bottleneck on an island. But for the greater boom, it will need to be done.
Oke, now we have Gunpowder, how do I use it?
In the west, it is best known for its use in cannon and rifles. Those things are not available to you, for lack of metal (working).
So I suggest, go with mines and grenades. Both will need fuses. You have the material to make those, so no biggy.
Mines are best used for guerrilla activities. Blow up their boats when they are close to the shore. Explosions work better under water. You do need to do some waterproofing. I think you can use animal fats for that (thank you birds).
Grenades will be a tennis to basketball size affair. An explosion is nice, but it is the shrapnel that will kill. So use a hollowed out coconut for the shell. Make it as thin as possible. Then line it with stones. Sharp ones are preferred. In the middle, you place the gunpowder. Attach fuse, light, and throw at an enemy. Make those foreigners sorry they ever came this way!
Wars were fought over islands that were covered feet deep in seabird guano. http://pigeonsformeat.com/index.php/utility-pigeons/10-gunpowder
The wars were fought because guano is a rich source of potassium nitrate, which is the oxidizer in gunpowder.
You need sulfur for gunpowder. If they are on a volcano they are in luck as regards sulfur - they can just go pick it up. A coral atoll, not so much. How cool is this volcano with burning sulfur I found at nationalgeographic.com?
The third ingredient is charcoal. A beach, a hole, a fire and a bunch of coconut shells will make you fine charcoal.
Those are the ingredients for gunpowder. I would not encourage the obsidian cannon.
Gunpowder is within their realm of options. It requires three ingredients.
- Sulfur, this will be the hardest of the three. It must be mined. Mining without metal is hard but not impossible. Can be found near volcanoes. Those seem fitting for an Island nation.
- Saltpeter could easily be extracted from guano with a process quite similar to potash. Hardly a modern and sophisticated technique.
- Charcoal, come on it's likely they know about charcoal. But if you want to mix it up. Pure enough alcohol or Sugar could suffice.
So there is your basic gunpowder. Not to make it useful without metal is a bit harder. There is evidence of bamboo cannons but it's likely those are at best unreliable. But wooden or bamboo barrels might work with stone projectiles.
If you can make cannons, you can make bombs. Fill a bladder or wooden barrel with gunpowder mixed with shrapnel. I guess granite might do the job. Obsidian seems to brittle to be of use and regular stone not sharp enough. But granite chips would cause a nasty wound.
You could use guncotton.
First, have your alchemists heat Melanterite, and then add water to the resulting sulphur trioxide to get sulphuric acid. Then get new alchemists (it's a strong exothermic reaction) and have them add some saltpeter to your sulphuric acid to produce nitric acid. The resulting mixture of nitric and sulphuric acid is mixed 15:1 with cotton (which means you need tropical or subtropical climate on your island). After a few minutes your guncotton is washed with cold water and then dried and ready to use. Expect to lose a few alchemists before they figure out the importance of washing the guncotton.
Better use what you already have: any flammable material, ground finely enough, is explosive. No need to resort to complex chemistry likle gunpowder.
It's all a matter of surface area. A lump of coal burns slowly; grind it up and it burns much faster. Suspend coal dust in the air and you have a highly explosive mixture that has destroyed many mines. See https://www.cdc.gov/niosh/mining/features/coalmineexplosion.html
Similarly, flour mills and silos are frequently destroyed by exactly this type of fuel-air explosion when powdered material ignites in the air. https://www.independent.co.uk/news/uk/home-news/cheshire-flour-mill-explosion-four-missing-workers-named-10400399.html
The challenge is to weaponise this effect. Modern thermobaric or enhanced-blast weapons do it with powdered aluminum packed around conventional explosive. Your stone-age natives will use flour or similar, and will need a clever means to scatter it in the air and ignite it -- but the explosive itself does not require anything so tricky as gunpowder and can be made easily enough with stone-age technology.
There are a large number of explosives out there, but let's go with the one that comes top to everyone's mind...Gunpowder
You need 3 things to make traditional gunpowder. Sulphur, which might be readily available on a volcanic island. Saltpeter, which can be harvested from urine. Finally, you need Charcoal, which you can get if you have fire and trees. Mix in the right proportions and you have a basic black powder. A large enough volcanic island and large enough population and you have all the bases covered.
As for Metal, it's not necessary at all. Firecrackers are pretty much gunpowder in a tight paper sleeve. Everything else is just a matter of scale.
Metal turns out to be useful when you are trying to generate shrapnel or to make firearms, but there are other ways you can get some devastating devices without having to resort to metals.
The Korean Hwacha is a pretty terrifying device. It's basically a rack of around 200 heavy, rocket propelled arrows. You can look up some cool videos on it, and the Mythbusters one is very cool. If you have obsidian to use as arrowheads, the effect could be very scary.
Next we have Grenades. They used to be a clay pot with a narrow neck, a plug and a fuse. Imagine the iconic picture of a ball shaped bomb with the fizzing fuse. Those were bad enough. Now imagine an old time grenade with obsidian shards embedded into the clay. Nasty!
At any rate, metals aren't necessary, but they tend to give more options.
As others have said, gunpowder.
However, I disagree about cannons not being possible. They won't be good and they will be fixed mount but if the access to your island is limited that need not be a showstopper.
Take a wooden gun barrel (yes, it can't last) and bury it in a lot of stones/sand of mixed sizes. It won't be perfect but most of the force will still propel whatever you are shooting. It's the mass piled on top that resists the force, not the gun barrel itself.
If you can make concrete (doesn't need metal, doesn't need any more chemistry than gunpowder--it was discovered earlier) you won't have a problem with your gun burning up. I don't think you'll be able to do heated shot even then, though. (Heated shot is normally done by preparing the gun minus cannonball, getting the cannonball as hot as you possibly can, you load it and immediately fire, before it's heat gets through your wadding and lights the gunpowder.
This may need an ignition start. It is made up of clubmoss spores. It has been used in fireworks for centuries. Found naturally, just collect it.