As a cautionary tale for would-be amateur explosives aficionados out there, you get a similar problem storing a lot of the more accessible energetic compounds as well. It can crystallize under a screw on lid and blow your hand off if it doesn't kill you outright. It can degrade and become more and more sensitive. There's plenty of news stories about teenagers blowing their hand off because they thought Acetone Peroxide was all the rage, but it's easy to overlook that while it appears simple on the surface, there can be plenty of hidden hazards that are unintuitive. You don't know what you don't know and the learning process can leave you pulling your socks off to count past 7.
Stiff cardboard tubes, packed clay, and low explosives like plain black powder, flash powder, etc provides all the confinement you need for a bang and a flash. Thermite is also fun, and so long as you're not igniting it on top of a car battery still filled with acid, it's quite a bit safer.
You can also go the slow pyrotechnic route with making smoke bombs. It's not as simple as it may appear, but from a hazard point of view nothing is going to explode. So long as you take some basic fire safety precautions like doing it outside away from structures and flammable materials on a hot plate it's safe and entertaining. Couple of safety tips, if you're grinding up your oxidizer in a coffee grinder (definitely recommend, mortar and pestle suuuuuucks) make sure you never use it for anything other than oxidizers in the future. If you use it to grind up a fuel later on there's a very good chance you won't get every last trace of oxidizer out so you could wind up with a bit of powdered fuel and oxidizer mixing together in the tiny joint between the blade and the bowl and completing fire triangle bingo. Another foundational precaution is to have a good understanding of what your ingredients are because getting it wrong can be quite hazardous, not just disappointing. For example a friend of mine in high school wanted to make some smoke bombs and was asking me for directions. I told him to find some stump remover from the hardware store and make sure it was plain potassium nitrate. He couldn't find it on the bottle and assumed it probably was and figured it would either work or it wouldn't. That bottle was filled with sodium metabisulfite instead of potassium nitrate and when he tried to heat it with sugar to melt it together it started decomposing into sulfur dioxide gas which combines with moisture in mucous membranes to form sulfuric acid. He had chemical burns in his throat and was coughing for a bit afterwards but luckily no lasting damage.
Another fun misadventure with the same friend. We came up with the brilliant plan of stuffing an entire gross (144) of bottle rockets into a huge pickle jar that was the perfect height to hold the body up over the mouth of the jar. We sprayed the fuses all over with starter fluid (ether, not lighter fluid), lit the bundle, and ran back. Moments after they started launching the heat from the exhaust (and the ether might have been a contributing factor) shattered the pickle jar and now all of the remaining rockets are splayed out in every direction horizontally. We ran and dove with rockets going everywhere and these were the ones with a healthy report at the end too so it's basically just launching fire crackers at us.
I'm not going to try to tell anyone not to play with fire, but please show some modesty and act responsibly. Everyone makes mistakes and overlooks things. Plan for this, expect it, and position yourself so the fallout for things going sideways doesn't involve the fire department or the coroner especially when your latest escapades involve showing your friends. They're idiots, they will do dangerous things out of ignorance. Don't put someone else in a position where they need to know anything other than stay behind that tree and don't come out if it doesn't go off. They aren't going to stop drop and roll, they're going to panic and make things worse like running with a flaming gas can spreading fire all over the place. Aggressively plan for failure because things will go wrong and you need to be able to deal with that when it comes up.
Flower (and I guess sugar) can be explosive like a rudimentary fuel bomb. It has to be aerated first. If it’s tightly packed it’ll just smolder. If it’s aerated enough a shockwave will go through and oxidize it all at once. It’ll be one hell of an explosion. With cocaine you could probably be safe by standing up wind of the burn.
You pretty much had it right. It seems the two things you missed were:
1. Milling the powder makes it a lot more effective. My buddies and I would make ball mills with old washing machine engines, and whatever non-sparking material we could find for the balls.
2. If you want it to explode rather than burn you have to granulate it. Wet it and let it dry, and grind it a little bit and pass it through some mesh to grade it.
The raw powder is good for making rockets, but it won’t very readily explode.
I wonder: they mention that the powder can ignite prematurely when you place the top anvil. Could you prevent that by putting a piece of paper between the anvils? It seems to me that the paper should prevent sparks by preventing direct contact between the anvils, but be weak enough that it shouldn't affect the deflagration too much.
Then again, I'm not an expert in this kind of thing. Would this somehow make things way more dangerous in a different way?
Safe is relative. I'm such a genius that I managed to injure myself doing a reaction with a single reagent, namely, making amorphous plastic sulfur. I accidentally caught the sulfur on fire, you see. If you ever find yourself in this situation, resist the temptation to try to blow it out; you will regret inhaling near a sulfur fire.
My father demonstrated the combustion of a mixture of potassium nitrate and sugar to me when I was 7 or 8 years old, and I was instantly hooked. The effect is spectacular: thick white smoke, hissing pink flames, a huge charred mass of fluffy carbon, and the smell of burnt sugar in the air. My interest in chemistry up to that point had centered around growing crystals. After that first smoky demonstration I wanted to make fireworks.
It's still possible to have that bonding experience today if you have some open space where there is nothing flammable. I see that Amazon has multiple sellers of saltpeter (potassium nitrate), the key ingredient. As recently as last year I have also seen it sold as stump remover in the gardening section of Fred Meyer (Kroger).
Potassium nitrate plus various fuels like charcoal and/or dusting sulfur, sugar, and iron filings is great fun and relatively benign. Do any grinding by hand in a porcelain mortar and pestle and there's really no risk of mechanical ignition. The mixtures don't burn fast enough to risk explosive effects unless ground very finely or strongly confined [1]. I had to photocopy the black powder and pyrotechnics sections of The Chemistry of Powder and Explosives from the library, but today you can find a full scan of that book (and much more) online.
[1] You can also make more vigorous mixtures with potassium nitrate by using powdered metal fuels, but you're not going to find those fuels just sitting on the shelves of ordinary retailers.
It's a good addition to any suite of destruction, but creating any kind of fire is a higher amount of risk for collateral damage than the other methods. Plus, unless the combustion is thorough, you may still be able to discern information off the paper.
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