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In a world consisting of dark-age-esque technology, how would an alchemist go about making a low-budget "flash grenade?"

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    $\begingroup$ You say "alchemist" - are you envisioning someone in modern times, in a medieval or Renaissance setting, or what? $\endgroup$ – Cadence Mar 26 '19 at 10:18
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    $\begingroup$ How is a "flash grenade" different from fireworks and other pyrotechnic devices? Fireworks were made and used in China since the 9th century, and in Europe since the 14th. $\endgroup$ – AlexP Mar 26 '19 at 10:24
  • $\begingroup$ I guess I'm thinking of a deployable container that can quickly dissipate large amounts for a very short time. A flash grenade was the best modern equivalent that I could think of. I guess the best thing I can think of right now is burning magnesium traces in a transparent container (not even sure if that would be viable...) $\endgroup$ – LogCapy Mar 26 '19 at 10:26
  • $\begingroup$ Yes, I'm thinking of a more medieval era $\endgroup$ – LogCapy Mar 26 '19 at 10:27
  • $\begingroup$ Getting magnesium with medieval tech is improbable... Glass is far from low-budget expendable. You should better look into fireworks $\endgroup$ – Artemijs Danilovs Mar 26 '19 at 10:43
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Arrowroot.

It's been around and in various forms has been cultivated for at least 7000 years as a food and food additive (thickener).

The tubers, containing starch are washed and cleaned then are ground up with a mortar and pestle and then beaten to a fine paste a little water is added and mixed in, the milky liquid would then be passed through cloth into a collecting flask. The starch, settles out on the bottom, - the water is discarded and the starch dried in the sun or a cool oven to form a fine powder.

Black Powder

  • Saltpetre (extracted from manure run-off and known to the Chinese in the mid first-century AD), brimstone (found in deposits around volcanoes), and ground-charcoal mixture would be prepared.

  • A fuse would be prepared by soaking string in saltpetre and then drying.

  • A flask would be filled with the black powder, the fuse added, trailing out the top which would then be threaded through a fine hole in the stopper. The stopper is then secured in the flask.

  • A bowl-shaped thin sheet of lead is then partially filled with the arrowroot , the flask nestled inside, surrounded and covered with the white powder until only the top with the fuse can be seen. The lead can be gently tapped to fold in around the neck of the jar and secured with string.

The flash occurs when the exploding flask mixes the arrowroot with air and the heat source of the black powder, deflagrating in a bright yellow/orange fireball. Copper powder can be added to the black powder to give a blue/green effect if wanted.

I suggest arrowroot, but ordinary flour would work too. Lycopodium - a moss-extract was first extracted and used for this specific purpose, but may be more difficult to obtain.

There's a great thread in the chemistry stack, the "An Explosion in a Custard Factory." one which elucidates.

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    $\begingroup$ I feel like the real trick to making such things in the medieval era would be a reliable fuse and lighting system. $\endgroup$ – TheLuckless Mar 26 '19 at 15:44
  • $\begingroup$ ah, this is super cool! thank you! $\endgroup$ – LogCapy Mar 26 '19 at 19:25
  • $\begingroup$ Question, any reason why you chose to lead for the outer covering? $\endgroup$ – LogCapy Mar 26 '19 at 19:37
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    $\begingroup$ @Estilus Yes sure, lead is freely available - sheets particularly from flashing on church/castle/palace rooves (Apothecaries had rich sponsors if they were any good), it's easy to beat into a thin sheet, when thrown in the form I describe with it's contents, it will deform on landing absorbing some of the shock, and should therefore keep the payload intact - the flask with the explosive. That's it realy. $\endgroup$ – Tantalus' touch. Mar 26 '19 at 21:08
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    $\begingroup$ @Estilus A clever Apothecary might (having gotten the idea for saltpetre from the Orient), find a way of using resin or latex impregnated paper after the fashion of Origami instead, just a thought. $\endgroup$ – Tantalus' touch. Mar 26 '19 at 21:21
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In our own history, the first explosives (other than gunpowder) produced in the Western world were fulminates. These primary explosives are easy to make, have few ingredients, and are far more brisant than gunpowder.

The only problem is, their explosion produces little light relative to the amount of pressure/noise. A fragment the size of a pea is described as having, unconfined, blown a hole in an iron spoon -- but such a piece would barely produce a perceptible flash.

The brightest compound available to a Dark Ages alchemist (at least in our history) would have been an early version of gunpowder. By fiddling with the mixture (increasing the charcoal slightly at the expense of sulfur) and confining the material (and inventing a way to ignite confined powder in its sealed container) one can make a mix that would produce a moderate flash and pretty good bang with only the confinement of a hard fired ceramic flask.

In fact, this is much the way medieval and renaissance grenades and petards were made (aside from using powder optimized for its explosive power, rather than for light production), once functional fuses (black match, usually) had been invented.

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  • $\begingroup$ Duly noted, thank you for the help! $\endgroup$ – LogCapy Mar 26 '19 at 19:25

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