So, an island-dwelling stone-age tribe has discovered gunpowder and invaded several neighboring islands using their explosive arrows. However, a coalition of enemy tribes has formed in response to this new weapon, and is preparing to make an assault on one of the islands held by the tribe. The tribe wishes to defend their island using their gunpowder, but aren't entirely sure how to do so. They plan to set up some kind of booby-trap that will detonate when enemies get close enough for it to kill them, but are having trouble with the design of the trap. How can the tribe build landmines with only stone-age technology?
The best weapon is fear.
A fist or a spear or a bomb works while you are using it. Fear and uncertainty keep working on your enemy after he goes home. Fear is contagious - it will work on his comrades and his family as well. Your gunpowder is a scary new unknown. Maximize the ability of a weird new technology to create fear.
Below - how to trigger a primitive gunpowder bomb and then what sort of bomb would be optimal in this scenario.
This is the answer to the question as posed: a slow-burning pot of fuel is the trigger and it sets off the gunpowder bomb when tipped (into the bomb) or broken (falling from a height or crushed by a falling stone). Back when making fire was time-consuming, people carried around coals in a fire pot. This contained slow burning fuel and live coals. A device like this makes sense as a trigger: on tripping the trap the fire pot tips or breaks, releasing the coals which then trigger the gunpowder.
Also, the fire pot will be hot. It can here serve double duty containing a reservoir of sand. The sand will get very hot.
Water, sand and other heated missiles
Hot oil was considerably less common than boiling water or heated sand, which was cheap and extremely effective; even "dust from the street" could be used. These would penetrate armour and cause terrible burns. Sand, especially, could work its way through very small gaps in armour. The Phoenicians at the Siege of Tyre (332 BC) dropped burning sand down on the attacking Greeks, which got in behind the armour and burned the flesh...
On detonating, the fire pot sprays its hot sand. The tripwire should be near the punji traps so that persons coming to the aid of persons in the trap set them off. Or they could be set off manually by a hidden person watching - this in the manner of an Iraq-style IED.
The hot sand (and whatever else - hot sap, oil, etc) will not immediately kill. It is meant to hurt. A hurt warrior must be taken back to the shore by his comrades, taking 2 or 3 total out of the fight. Seeing comrades hurt and screaming is bad for morale.
Shock and awe.
Your scenario is not Vietnam. Here, gunpowder and thermal weaponry is a scary new thing, of unknown power. Capitalize on that. With the hot sand, include flour, fine sawdust or some other flammable. Explosions should be huge and fiery and spectacular.
Killing a guy who falls in a hole on a bomb is not much different than killing him with a sharp stick in the hole. It is not that scary. The idea here: psychological warfare. Leverage your tech to damage morale. Hurt your enemy with burns and scare and confuse them with huge fiery explosions. They will not be sure what the limits of your new abilities are. They will need to bring their screaming burnt comrades back to the boats. A fair number of these will live to get home, and the tales these scarred veterans tell will naturally grow in the telling. These tales will ultimately be a better defense than anything you whip up in your lab.
Given a stone-age technology, the classic percussion mechanism would be too sophisticated for these people.
OPTION 1: HIDDEN BARRELS
That's it! Bury many barrels of explosive mixed with sharp shards along the lines where the enemy is likely to move. Make sure to protect your fuses from rain, and when the enemy comes, BOOM!
OPTION 2: FOUGASSE
Similar to the barrel thing in concept -excavate a line of holes in the ground along the potential enemy path. Fill it with projectiles (stone shards), and gunpowder. Hide. Wait, Light up!
The advantage of the second is, you will not risk to waste barrels of explosives because of miscalculations or fuse defects. The Fougasse can be used when you are sure of your target, with the bonus that you can hit targets over a wider distance as well.
Use a variation of the trapping pit.
You can do this with a few simple steps.
- Dig a hole, perhaps 6 feet wide, 6 feet long, and 10 feet deep.
- Fill the bottom of the hole with gunpowder, as much as you'd like. As Trish pointed out, compressing this in small packets - as you might do for a gun - and placing them inside might lead to a much better explosion.
- Put a tripwire in front of the pit.
- Place a sort of crossbow, fitted with an explosive arrow, in a nearby tree, and aim it at the pit. Alternatively, rig up a pulley system that releases a taut bowstring. Attach this apparatus to the tripwire.
- Cover the pit with large leaves or something similar, so that it looks to be solid ground. Make sure the bow, too, is well-hidden.
When an enemy combatant moves through the area, they'll trigger the tripwire and fall into the pit, which will then be shot by an explosive arrow, which will in turn ignite the gunpowder.
Here's what it might look like:
The same fuse/trigger mechanism they use for the arrows could be adapted to a step-on pressure plate, with a payload delivery system that uses the victims weight instead of the impact of the arrowhead.
This methodical approach would reduce the time needed to develop the fusing/delivery system, would be intuitive since the hard legwork of getting the mechanism to function is already done, and would likely be the first 'easy' solution suggested.
Using methods I also explained here to get the arrows to detonate, two of these can be adopted to make mines:
- A ceramic jar filled with a small ammount of water is set up to break and wet the calcium oxide coated tar package, which contains a load of powder in a ceramic or bamboo container. This is a nice design, because it does not decay lightly, if glazed ceramics are used and thus stays active for a long time - but that is also the bad side of it.
- The friction ignition described in the end of the answer (pretty much an early match) is used: a stick with a friction igniting coating is stuck through a bamboo tube containing a load or fuses to several loads. Upon applying weight to the stick, it rubs the match, striking flame. This ignites the compressed charge and blows up (or the fuses to the other charges). If putting the ignition chemical on the far side of the place the force is applied to, one could make a pull-out trigger too (and possibly convert this pull-trigger into a primitive sort of stick-handgrenade). Or booby trap it to trigger on BOTH ways, making disarming the mine dangerous. The benefit of these designs is, that they shoud rot, so a forgotten mine won't endanger the own troops in the long term, as it either self-ignites or breaks apart.
Gunpowder requires some sort of fuse or ignition. So either long fuses or fire burning projectiles at the gunpowder. You could just have a line of gunpowder from your tribesmen to the gunpowder and light it from there if you want.
Gunpowder works best if contained though, personally I think grenades is a better option, contain the gunpowder in gourds or similar along with sharp stones with a fuse and throw them.
For this answer, I will be using a fire piston, basic shaped charges, and basic shrapnel. Besides gunpowder, none of these materials should be expensive or difficult to make at any level of sophistication.
I will be using a basic pit trap design. First, place a single bowl or plate of gunpowder in the center of the pit. Second, place a bamboo, animal grease, and twine fire piston on top of this bowl or plate. It would likely be a good idea to give the piston three legs, like a tripod.
Then, surround the tripod with a layer of gunpowder bundles set in thick clay bowls, facing upwards. Connect the bowls to the trigger with fuses, trails, or tubes of gunpowder.On top of this layer,place a thin mat of shattered pottery, metal, or glass debris. Atop the arm of the fire piston, place a wood platform.
When an enemy steps into the pit and lands on the platform, it depresses the fire piston. With a rush of compressed air, the trigger bowl is ignited, setting off as many of the bundles as possible. The bowls either shatter or focus the blast upwards, sending the shrapnel through the platform and into the enemy.
Borrowing Willk's burning coal idea, you could have small stoneware pots that are filled with powder; the "lid" of these pots would actually be two layers. The lower lid would be very thin, easily shattered by the weight of someone stepping on it, and concave (bowed upward) with a small indentation in the top. The upper lid would rest on top of the lower one, ideally shaped so it actually rests at the edge of the indent in the lower lid.
Here's the arming process:
The pots themselves would be placed in holes ahead of time (but not too far ahead, as rainfall could spoil the powder -- this could be another plot hook as the other tribes might figure out that the traps are foiled by rain) with the lower lids, but would be left without the top lids and not yet be buried.
"Arming forces" would consist of pairs, one person would be carrying hot coals and the upper lids, and the other would have a small spade. When advancing enemies are spotted by scouts, the arming forces would go out. The first person would lay hot coals in the indentation and place the lids over the coals. The second person would dig up small patches of turf several feet away from the mine and gently pack it over the upper lid; not too tightly though, to allow a little ventilation so the coals don't snuff themselves out.
- For extra nastiness, smear guano or human feces on the upper lid prior to burying (more on this later)
- With the hot coal in place (which should be able to burn for a couple of hours inside its chamber), the mine is now armed and the arming forces retreat and take up more traditional weapons behind the mine line.
When the enemy advances, they'll see the pockmarked ground... but their first instinct will be to avoid the holes, not the turf. When someone steps on a mine, the pressure on the lip of the indent shatters the lower lid, dropping the hot coals into the powder and igniting it. Since stoneware is quite fragile, the explosion shatters it, so the enemy now not only has an explosion underfoot, but also stoneware shards being launched up and out. This has the potential for area-of-effect damage as well, if they are in tight formations. Lacerations might not kill, but will at the very least injure the enemy. If the lids have been smeared with feces/guano, anywhere the shards break the skin is very likely to get infected, which in the stone age would be devastating. Even if they survive the infection, the recovery period would certainly take the victim out for a significant time (assuming they can retreat or be taken to safety and not get slain in a counterattack).
Since the enemy would eventually learn the holes are actually safer, they might try to step there... so to mix it up, sometimes bury the mines under turf patches, and sometimes just throw some dirt on them so they look like the pockmarks. Now any stretch of land that's riddled with pockmarks becomes entirely unsafe to traverse!
As your gunpowder tribe continues with their warfare, they will undoubtedly refine the process, learning better ways to make the coals able to burn longer or finding other more reliable triggers, so they can bury the mines longer in advance. At some point they may just churn up entire tracts of land, plant mines, and seed the area, so the only thing the enemy can determine is "this whole area is mined" and becomes unsafe.
If there are a lot of trees in a given area or any other natural structures, it may actually be easier to make hanging mines. Especially if they're coconut trees, emptied coconuts could be used as the vessels, and hung alongside normal coconuts; ideally the meat would be scraped out too to make the walls as thin as possible for maximum explosive force. Simple twine tripwires would be set up that would drop said coconuts. The stoneware "inner lid" would shatter on impact, dropping the coal in and causing detonation similar to the underground mines.
If the trees aren't coconut trees, then stoneware vessels could be hung, and just hidden higher up in the foliage so they're not immediately visible from below. If necessary, a couple of branches from the trees could be cleverly strapped to the lowest branches to provide optical cover for the vessel. Even if the vessel explodes upon hitting a branch (assuming they're no more than 10-12 feet up) the flying shards would still cause serious damage.
Even if the mines are only like 30-50% effective at actually going off and causing severe injury or death, these weapons serve as psychological agents. Seeing your comrades -- friends and family, fellow villagers -- getting shredded by flying stoneware or having limbs blown off is horribly demoralizing, especially in tribes that aren't used to such devastating warfare. Survivors will certainly think twice about proceeding further in their attack, and will be hesitant to join on subsequent attacks.
You can get firecrackers that pop when a string is pulled. These are done by looping a piece of string back on itself and binding it tightly so that pulling it generates a large amount of friction as it slides. This is not enough to light traditional tinder on fire, but black powder is far more sensitive to brief moments of extreme heat than traditional tinder and is quite readily ignitable by this method.
The caveat is that it takes a fair amount of force and won't go off if pulled too slowly, so consider using some kind of spring mechanism attached to a more normal snare trigger that gives the string a hard yank when pulled. A bent stick should do, just like for a snare.
Of course, black powder is expensive to make with stone-age technology, so throw in plenty of less expensive traps too and only use the expensive ones where it's most effective... Like if you can make it bring down an entire hillside on the enemy.