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In a post-apocalyptic world where disease wiped out most of society over 100 years ago, where could you find explosives?

The situation is that an army is coming to attack a group of survivors and the survivors plan is to use explosives to attack and destroy the access way to their camp (in order to delay the army) by destroying a stone bridge (preferably with some of the army on the bridge). They are near an old city which was abandoned before the collapse of the old civilization so the question is where would they be able to find still-working explosives?

As to how they know about explosives, one of the group of survivors has studied old texts and thinks that explosives would be just what they are looking for.

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    $\begingroup$ I don't know of any long-term stable explosives. Dynamite only lasts about a year and semtex is specifically designed to last only five. $\endgroup$ – Frostfyre Apr 14 '15 at 18:22
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    $\begingroup$ DIY? I mean why find when you can "make"? $\endgroup$ – Ghanima Apr 14 '15 at 18:33
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    $\begingroup$ @Frostfyre: Dynamite actually lasts longer, but becomes unstable. The area where I live has a long history of mining, and every couple of years or so there's a news story about some old miner dying, and the bomb squad blowing up the shed where he'd stored his old box of dyanmite. $\endgroup$ – jamesqf Apr 14 '15 at 19:01
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    $\begingroup$ @Frostfyre My understanding is that dynamite is actually stable for much longer than this if properly cared for--which includes turning the box over every so often. $\endgroup$ – Loren Pechtel Apr 14 '15 at 23:07
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    $\begingroup$ If I knew the location of 100-year-old high explosives, I would know where not to go. $\endgroup$ – WhatRoughBeast Apr 15 '15 at 2:49
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Unexploded ordnance is a potential source of explosives. That kind of explosive is made to withstand the shock of being fired from cannons etc. It might be hard to find, and you get the extra thrill of trying to get it out of a shell or bomb that should have exploded but didn't. Areas in Belgium (iirc) are still off-limits because of dangers from WW1, and bombs from WW2 are still being found and defused (with large areas being evacuated during the process)

And when you have the explosive, you get to the next problem; blowing it up when you want it to.

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    $\begingroup$ If you don't mind blowing yourself up from time to time, UXO is a good source of explosives. The problem with century-old explosives of any sort is that most of them break down over time into unstable, shock-sensitive forms. $\endgroup$ – Mark Apr 21 '15 at 1:43
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Make your own!
Fertilizer bombs can be pretty powerful, assuming you know some chemistry and have the ingredients.

Apparently they won't go boom if you don't do it right though.

I'd look up the exact ingredients, but I'd prefer not to end up on some list :)

You (and your character) could probably dig up an old copy of the Anarchist Cookbook, as I believe it has the instructions.

If you don't have the stuff to make a fertilizer bomb, there are several recipes for napalm which are really easy.
If I'm remembering Fight Club correctly, the three ways to make napalm: One, you can mix equal parts of gasoline and frozen orange juice concentrate. Two, you can mix equal parts of gasoline and diet cola. Three, you can dissolve crumbled cat litter in gasoline until the mixture is thick.
There are also recipes for plastic explosives in there, but require you to have something like nitroglycerin first, which isn't something you want to make if you don't know what you're doing.

Regarding Fight Club. Reportedly the recipes given in the book would work, but they changed them for the movie to cut down on the number of idiots blowing themselves up.

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    $\begingroup$ Not only that, the movie had a van with several hundred gallons of nitroglycerin in it! A real testament to the state of road repair in the movie's city. $\endgroup$ – WhatRoughBeast Apr 14 '15 at 19:27
  • $\begingroup$ @WhatRoughBeast In the book I think it was plastic explosive made by mixing nitro and paraffin, which (according to the narrator) is not very dependable if you want to blow something up. Bomb doesn't go off. $\endgroup$ – AndyD273 Apr 14 '15 at 19:39
  • $\begingroup$ Seconding ANFO -- it's about the cheapest and easiest-to-make of the industrial grade blasting explosives out there; it's also something you can handle reasonably without blowing yourself to kingdom come (unlike, say, nitroglycerine). $\endgroup$ – Shalvenay Apr 14 '15 at 23:17
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    $\begingroup$ @AndyD273 First, I think we're all already on some list. Second, I thought you weren't supposed to talk about Fight Club... $\endgroup$ – Frostfyre Apr 15 '15 at 0:03
  • $\begingroup$ Heh, you could also have the characters find an old copy of the novel, use it as a bar for experimentation. $\endgroup$ – AndyD273 Apr 15 '15 at 5:04
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Let me get this straight. Civilization collapsed 100 years (5 generations) ago, and the current survivors know how to use high explosives - they just don't know where to get them? With all due respect, I suggest that you need to rethink this. The limits on the suspension of disbelief are not working in your favor. Why would your group have any idea of what HE can do? 100 year old legends?

With that out of the way, I'll have to say that you also need to change your target. Nobody in their right mind stores any quantity of explosives in cities, for pity's sake. There are far too many people who might get hurt. Major explosive storage areas are as far out in the country as folks can get.

Finally, explosives don't store well on the time scales you're talking about. The same reactive tendencies that make go boom make them degrade fairly rapidly, and the presence of even tiny amounts of impurities has very bad effects. I'm not sure if ultrapure explosives would age any better, but in the event, nobody makes them. It's just not cost-effective.

The storage of ammunition serves as a decent example, since the active ingredient is nitrocellulose, a nitrated hydrocarbon. I've never seen a recommendation to buy ammo that's more than 10 years old, so that should tell you something. At the other extreme, I have run across reports of ammunition reliably functioning at 50 years, but with somewhat reduced power.

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    $\begingroup$ Ammo will last more than 10 years. I have some 30 & 40 year old surplus ammo I purchased for hard to find calibers and it still fires fine. You might not want to use it in an international shooting competition. But for your standard Road Warrior scenario where you're tooling around in the desert in dune buggies while wearing hockey gear, it'd do the trick! But +1 to the answer in general. $\endgroup$ – GrandmasterB Apr 15 '15 at 4:05
  • $\begingroup$ 100 years isn't really enough to make something a "legend". In fact, you would probably have people alive who grew up listening to their grandparent talking about 'the old days' they grew up in before the collapse. If you are 20 years old when this happens, and you live to 80, you'll be telling people about civilization 50-60 years after the event. If you are 20 when grandpa passes on, you've heard the stories many times and 100 years after the event you are still 60 years young. $\endgroup$ – Bryon Apr 17 '15 at 13:58
  • $\begingroup$ Yes, you will. But without experience to provide background and context, the meaning will get distorted in all sorts of ways. How do you explain TV to someone who has never seen it or even heard radio? And in this case, how many grandparents had EOD experience? Have you ever heard anybody talk about the details of setting off blasting explosives? Do you know the difference between fuse and det cord? How do you connect to a blasting set? Do you understand the dangers from static buildup, and know the safeguards? If you don't know this stuff, why do you think others will? $\endgroup$ – WhatRoughBeast Apr 17 '15 at 20:16
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A really powerful boom can be had by igniting a could of flammable dust. This happens with some regularity in grain silos and coal mines, but going to YouTube you can also see various other mixtures ignited to create huge clouds of flame or the occasional explosion.

Thermobaric weapons use a variation of this principle, but AFAIK they rely on the uniform dispersal of explosive or flammable aerosol and ignition when the proper air/fuel ratio is achieved. Like other DIY soutions, this requires some expertise or the ability and willingness to do lots of experiments to establish the correct conditions and ensure reliable ignition and detonation when you need it. That part will be rather....exciting....for the people involved in the test program.

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  • $\begingroup$ Actually, while true, that's unrelated to the question as asked. Dust / fuel-air explosives have their uses, but bringing down a bridge isn't one of them. And for what it's worth, trying to use the principle on this scale probably requires a bigger dispersal charge than is required to take down the bridge in a "normal" manner. $\endgroup$ – WhatRoughBeast Apr 17 '15 at 20:19
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There are two reasonably-improvisable explosive materials that'd be useful in such an environment: ammonium nitrate/fuel oil aka ANFO (which is commonly known/understood in industry, but is considered a blasting compound as it isn't cap-sensitive, requiring a booster charge to set off) and urea nitrate (which is slightly harder to make and more sensitive than ANFO, but can be set off with a blasting cap). Of course, there's nothing wrong with using urea nitrate booster charges to set off an ANFO main charge.

Detonators would be the main problem -- primary explosives/initiating compounds are much harder to make/more sensitive than even nitroglycerine (lead styphnate is extremely sensitive, mercuric fulminate requires handling mercury salts, and lead azide requires the preparation of some other azide salt, which is also quite toxic), and exploding bridgewire detonators aren't easy to reinvent or set off without the right machinery. You might be able to get away with a pyrotechnic ignitor (such as those used for airbags and model rocketry), though...

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Not an expert, but I believe you can get a simple form of black powder from just charcoal and saltpeter. You need just the right type of charcoal, so that might take some experimentation to get right, but it's essentially burnt wood, so it should be easy to experiment with. Saltpeter can be obtained from human waste, so while distasteful, it's certainly doable.

But this isn't something they could do on the fly, they would have to have been working on this for a long time already, but maybe they have just perfected their mixture and so you can still get some drama of them rushing to mix a big batch from their stockpiled saltpeter and a big effort to make the 'right' charcoal, followed by very carefully attaching a make-shift bomb and laying a trail of powder Wile-E-Coyote style so they can ignite from a distance.

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  • $\begingroup$ Charcoal, preferably from willow trees, Saltpeter obtained from a latrine of straw kept above ground and specifically used to pee in, and flowers of sulphur make black powder, certainly powerful enough to damage a 100+ year old bridge. BTW the above recipe, sans proportions, was available in a book on western expansion I read in the library, It always stuck with me. $\endgroup$ – Paul TIKI Jan 6 '17 at 20:53
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Napalm, like Molotov cocktails, will set things on fire but not blow them up more that regular fire would.

Aerated substances explosions are difficult to manage, but some jugs with gasoline will go boom. A tank with a dust of flour is harder to set off.

"The access way to their camp" is what? Made of what? How supported? How old? Taking out a wooden corduroy road into a swamp island is much different than the approach to the Hellgate Bridge they live on. How much time do they have to destroy it? They may not need explosives.

I wonder how many aerosol cans would still be dangerous?

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