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