I wanted some potassium nitrate because I had spent 3 years touring Europe with a theatre company, and part of my job was to let off enormous firework mortars. I got to love these mortars so much, that I learnt how to make them. Did you know, for instance, that the whistle in whistling fireworks is not created by gasses passing through a mechanical whistle? They are made simply by adding aspirin to the black powder mix! Aspirin actually whistles when burnt!
Those 'stars' which blink and change colour in the chrysanthemum-shaped mortar bursts are all made by hand - what you do is begin with a nut of gunpowder which is held together with gum arabic. The mix of that nut contains the chemical which adds the colour that you want. You then coat the outside of the nut with a layer of gum arabic, and this causes the slight pause or 'blink' between colours. You then roll the wet nut in another layer of gunpowder which contains a different chemical for a different colour, followed by another coating of gum arabic - and so on, for as many times as you need. The trouble is that those chemicals are usually chlorates or perchlorates and are very unstable after a period of around a year. Many amateur firework makers have been blown to pieces (along with their families) by making a whole batch of fireworks containing chlorates about 9 months before the actual launch event, then storing them in the basement where they quietly sweat away until they reach a critical state of chemical change...
Many professional firework manufacturers have died the same way too. I personally knew one who was blown apart by his entire factory stock in Kent, where I used to buy the giant mortars. I was notified of his death via a Christmas card from the company, which started, "We regret to inform you..."
Entering a firework factory is a bit like entering a Catholic church. In the same way that there is a little bowl of Holy water on the door jamb of the church which you put your finger in before passing the threshold, there is a copper plate outside every black powder factory which you religiously touch as you pass through. This plate is securely earthed to the ground outside, and touching it helps to discharge all your accumulated static electricity, and so prevent a lethal spark between your body and the materials you use in every day work. They also avoid wearing nylon, silk and hob-nail boots.
I got so far down the gunpowder making process before I stopped through fear. The ingredients of gunpowder are Sulphur, Charcoal and Potassium Nitrate. They have to be milled together for a long time, until they become a homogeneous powder. In order for the powder to become explosive, it has to be turned into granules so that oxygen can circulate between them, increasing the burn speed to almost instantaneous. The granules are made by heating the powder on a sheet of steel until the sulphur melts, and after it has cooled down (slowly), the solid sheet can be crushed into the right sized granules. You can see why I stopped.
Being a shotgun certificate holder, I am automatically granted a licence to buy and store black powder at home, but I don't. The difference between modern, smokeless nitro cellulose in modern cartridges and black powder, is that one is a propellant and the other is an explosive. That's a big difference if they are under your bed.
When I was a kid - before all those naughty terrorists got busy - you could buy absolutely lethal stuff over the counter in Boots. So one day, having realised how easy it is, I decided to make some Nitro Glycerin. I went to my local Boots the Chemist and bought a bottle of nitric acid and a bottle of glycerin, then took them home.
As I held the two bottles over the glass receptacle with shaking hands, something in my 8 year-old self (maybe my guardian angel?) suggested that this might not be such a good idea, and I carefully replaced the caps on the bottles, never to open them again. This is why I am able to tell you this story, even though my hands are sweaty even thinking about it.