So, I have a small tribe of island people with stone-age technology who have discovered how to make gunpowder. However, they want to make war on the neighboring islands, and they need a delivery system for their new explosive because blowing holes in your enemy's canoe is fun. They have decided to use the arrows they already use to launch the gunpowder, by packing gunpowder into a hollow stone arrowhead. However, they require some way to set off the gunpowder upon impact. How does a tribe of stone-age islanders create an impact-trigger for explosive weapons?
Historically speaking, timed fuses were far preferred to impact triggers for exactly this reason. Estimating the time of an arrow flight from the distance is a pretty straightforward affair, and if you're uncertain, you can just guess long; if the bomb sits on the ground for a few seconds before going off, it's no big deal. (It occurs to me that in a naval context, you might use airbursts, on the theory that showering enemy boats with flaming bits of wood and glass is better than missing. That would take some practice for sure, but no more than hitting a canoe from another canoe in the first place.)
For instance, the Wujing Zongyao, an 11th-century Chinese manual, suggests encasing the powder in a bag (of paper, cloth, or whatever's handy), attaching a fuse, and simply fixing the whole lot to the arrow and lighting it just before launch. Though less deadly than Greek fire or other impact-detonating schemes, it's far simpler and less likely to result in your troops lighting themselves on fire.
An Engineer of your stone-age tribe sat down one day with a quiver of arrows.
He knew that the black powder the alchemists created was a powerful weapon and that if he could figure out how to imbue that power into his arrows, his tribe would become one of the most powerful in the region.
Hollowing out several of the arrowheads, he realized that they could easily be filled with black powder and then re-sealed, but how to set them off once they reached their target?
Stumped, the Engineer placed down his arrow prototypes and pulled out his pouch of tobacco and pipe. Maybe a quick smoke would bring him inspiration.
Fishing the pieces of flint and stone (The alchemists of the tribe called the stone "Pyrite") from a pocket in his tobacco pouch, he sat back and struck them together to light his pipe, watching the sparks ignite the tobacco.
That's it the Engineer thought to himself. A bit of complicated stone carving will be necessary, but it will be worth the effort.
For several days the Engineer sequestered himself working on designs before he emerged with his completed arrow.
This is the arrowhead schematic he showed to his War Chiefs to explain how it worked:
The Arrowheads are built with a hollow chamber at the base. Inside this hollow chamber is a flint and striking stone "piston".
The rest of the space is filled with black powder.
The tip of the arrow is attached to the "rod" of the piston and is built to be retractable upon forceful impact.
When the arrow collides with its target, the arrowhead is pushed down, scraping the striking stone piston against the flint. This gives off sparks which then ignites the black powder, causing the entire arrowhead to explode.
The tribe would not need to know anything about blacksmithing or metallurgy, just that the rocks that look like this work for making a spark.
If the target is always in contact with water, they can use something like Greek fire to start the fire triggering the explosion.
Actually, they could just use the Greek fire, but if you want explosions...
Calcium oxide can be "easily" obtained by roasting limestone, tar can be found in nature. Mix them and spread the thing on the arrow tip, once it touches the water it will set on fire. If the explosive is close-by, you get a nice explosion.
A fire piston, sometimes called a fire syringe or a slam rod fire starter, is a device of ancient origin which is used to kindle fire. It uses the principle of the heating of a gas (in this case air) by rapid and adiabatic compression to ignite a piece of tinder, which is then used to set light to kindling.
Several sites have mentioned that temperature can reach above 800 degrees Fahrenheit (that's about the ignition temperature of gunpowder). Therefore I'd use the arrow's momentum to ram the piston and superheat the gunpowder in the charge. I don't think that the force of firing the arrow would be enough to set off the charge, but the impact would almost certainly be enough.
bows & arrows vs. canoes. Hmmm...
If I were the tribeleader, I'd save the explosive arrows for the land fights. I mean, if you miss your target, you waste precious explosive material. It's way more convenient to use a salvo of incendiary arrows. Those that hit will do damage, those that miss are easily repleaceable.
The best low-tech solution to fire an explosive arrow is to attach a fuse to the tip, train your archers to fire the arrows so to make them explode in the desired point. Also, you might want to add shards of chipped stone, to make sure that every explosion is even more fatal.
They will invent grenades and fire arrows on the way to the development of timed arrow-delivered explosives. And then either make water-triggered ones or invent matches on the way to impact triggers.
Step 0: Hollow Arrowhead?!
First of all... they could not use hollowed out arrowheads. Why? Because normal arrowheads are made from flint. Flint cannot be carved hollow without modern machinery and heavy cooling. With flintknapping using stone, bone and wooden tools there is no way to do this. So we have no hollow arrowheads from here...
Also, arrowheads are tiny. The amount of powder one could store in a typical stone age arrowhead, even if hollowed out would be a few grams. Which in turn is just enough to shatter the arrowhead into all directions, not to blow apart boats.
So, we need to get the powder charge a little bit back... and also make the arrow fly much straighter that way.
Step 1: Grenades
But they have clay! They can shape easily small pots and burn them, then fill them with their gunpowder. Cap with something and add a fuse, ready. But that is a rather heavy item. The Chinese had them called 震天雷, "Sky-shaking Thunder", we call them hand grenades. It looks like this (in the middle):
One could easily envision to add a loop to that design, put a piece of rope through the loop and then sling it, again, using a fuse.
Their lack of cast iron makes them unable to make flying-cloud thunderclap cannon" (飛雲霹靂炮; feiyun pili pao) type of ammo, that relies on a specific type of gunpowder and the hollow metal shell to create sparks to ignite half a pound of powder.
Step 2: Flaming Arrows
Now, if we started harnessing the power of the explosives in pots... let's downscale a bit and look for a lighter shell. How about a pouch of cloth, wrapping around the arrow shaft? Extra points for burning, smoke and sparking midair, though no boom at the target. YET.
Step 3: timed Exploding arrows
Now, they might find this special grass on their island. This hollow grass that makes containers just by growing. The large compartments are perfect to fill with power, cap with a fuse and then throw... and then there are thinner ones that could fit onto an arrow. Hust a hole at both ends, push the arrow through the filled rod, add an arrowhead and fuse... An arrow-delivered small grenade from bamboo! Also, dry bamboo can be turned into charcoal or to heat the homes, build rafts and bows, it is a super material indeed!
Branch 1: Combine 2 & 3
It's easy to wrap a burning cloth around a compartment containing gunpowder and light that, triggering a fuse to light the compacted interior load after a short while.
Branch 2: Water ignition
By roasting limestone, Calcium oxide can be created. This has a very warm reaction with water, enough to set a mix of sulfur and tar on fire. If this mix is used to glue and seal our bamboo compartment onto the arrow shaft, we created a devious weapon that doesn't go out if it hits the water but instead detonates then as well!
Step 4: Matches
The next step is to get wood super easy to ignite with a tiny bit of sparks or heat by impregnating pinewood with sulfur, which we already have. I believe other wood works too, but China used pine. At least these make igniting the arrows' fuses from a tiny source of wood on board the canoes safe.
Now, we need to make that into friction matches... for this, we need antimony pentasulfide - which luckily has the same chemicals as Stibnite. To add more sulfur into the mix we add the sulfur and melt it in airtight pots to get antimony red.
Next: Potassium chlorate... That might be tricky, but their volcano might provide a steady stream of hot chlorine gas from a small vent. Bubbling that through a solution of KOH - caustic potash, the result of mixing potash from burnt with slaked lime. And both of these are natural minerals.
Mix these two together, add some gum to glue that mix to the sulfur soaked wood sticks and we have friction matches. These will very easily ignite if rubbed over any rough surface...
Step 5: Impact/Friction ignited grenades
...such as the interior surface of the compacted powder in the grenade arrows from step 4. Now, our grenade-bamboo was formerly affixed to the middle... but we can do better! Add a prevention for the capsule to slide back, but allow movement forward. Add some area just in front of the bamboo grenade with some of the friction-ignition composite (sulfur, potassium chlorate, antimony pentasulfide, gum) so that the bamboo can slide forward on impact... and have its interior surface rub over the stuff. As soon as it rubs, the thing ignites the mixture, then the powder and BOOM!
The real benefit? the thing can be secured with a tiny wooden splint between the rubbing surface and the container, and then wrapped in a towel to keep friction from the ignitor.
It made me quickly think of snaps fireworks, they contain small amount and gunpowder and some shards of flint. When rubbed together or thrown to the ground, the spark make it explode. Stone age tribes could easily manage this. It would consist of a little pouch containing gunpowder and flints shards attached to the arrow. On the impact, it would blow. Setting fire to a bit would be pretty strange to me because it may turn off during flight due to the high speed of the arrow.
If you don't mind doing it in a few steps you could approach the problem like this. Hollow out arrow heads, fill them with gun powder, and seal off the arrow (so powder doesn't fly out of it on its path and become wasted in the water). On impact, the arrowheads will shatter and the gunpowder will spill on the ship/canoe. A secondary squad of archers will follow up with flaming arrows, which is easily made during any time period. On impact of the flaming arrows, the gunpowder will light as well.
Combining the powder with flaming arrows seems like a superior assault due to the three steps that damage will be done. The first powder arrow, the powder explosion, and the fire that will damage a canoe whether or not the gunpowder arrows landed correctly.
If you really want impact detonation, then put a small hole in the tip of the arrow head and put a small amount of non-combustible material in it to keep the powder from falling out. (a thin, clay disk would probably work.)
When shooting the arrow, the archer first sticks a wooden rod with a lit bit of slow match on it into the hole. On impact the lit match is driven through the non-combustible barrier and into the charge.
Just lighting a fuse will be more reliable, but impact detonation might be slightly more useful for penetrating armor if that's a concern.
The two obvious chemical choices are fulminate of mercury and nitrogen triiodide. Both are reasonably easy to synthesize in a modern country (the latter you could make using only the supplies in a well-stocked janitor's closet), but stone-age? Dunno.
From a simple standpoint of setting it off, I think Perkins has the only thing they're going to be able to come up with.
However, we have a problem here. They can't make anything even approximating a pressure vessel unless they live where something of the sort grows. Without such confinement gunpowder just makes a flash of fire. Light a pile of it on fire and you just get burnt, not blown up.
I imagine a stone age approach would be to simply fill a walnut, coconut, ball of tar, or some other combustible object with gunpowder, light it, and fire it before it is too late. On impact, it will break and detonate. The objects would serve as filled arrowheads. Another option would be to attach a gunpowder soaked thread to the arrow, to serve as a rapid fuse.