Potassium nitrate (KNO3) is a common oxidizer in pyrotechnics. It is used most notably in blackpowder, but also in other rocket propellants, fountain mixes and much more. KNO3 can be bought from many companies, such as those in the Supplies section. It can be bought at some agricultural suppliers, but there are none of those where I live so I can't try that. I've heard you can get prices such as $20 for 50lbs! The one problem is the purity will not be as good as if you bought it from a pyro supplier. I've also heard it can be found in hydroponics stores, but I have not tried this either, the hydroponics store not too far from where I live went out of business before I tried going there.
Agricultural KNO3 pictures I've found (not my pics):
One place I have found it is at Home Depot in the fertilizer/pest control/garden section as stump remover. The brand I got is Grants, but I'm sure there are many others that work. One other brand name I've heard works is Wilsons. I bought another brand of stump remover another place, but after several failed smoke bombs and tests I realized it was not KNO3, but some other, apparently useless (for pyrotechnics) chemical. Too bad because it was already powdered, unlike Grants, which is in prills. I bought one pound (about 450 grams) of Grants for about $5. The bottle has a health warning on the back that mentions that it contains potassium nitrate, so if you can't find this same brand, look at the stump remover you find for a mention of it's contents on the label. IÔø‡m also told there is a brand of stump remover sold at Wal-Mart called Ôø‡spectracideÔø‡ or something.
Both brands of stump remover I bought:
Kno3 prills from Grants:
KNO3 must be in a powder for use, and the prills contain a binder that affects performance. In a highly unscientific comparison I did, purified KNO3 worked significantly better than unpurified. Here is the best way I've found so far to purify the KNO3, it also helps for powdering it afterwards. This does take quite a bit of time however, so I donÔø‡t think itÔø‡s really worth it. If you are still interested, read onÔø‡
Richard Nakka suggests washing the prills in methanol to remove the coating, but dissolving in water seems to work fine as well.
2 glass jars
Heat source (I used a camp stove so I could do this outside)
Pot/container (I used a coffee can)
Shallow drying pan (I used aluminum lasagna pans)
Fill one of your jars with water and heat it in the microwave (or stove, but microwave is faster) until near boiling. Pour your prills into the jar and stir with your stirring tool (it must be able to reach the bottom of your jar, the prills sink. I use wooden skewer sticks from the grocery store), stir until the prills are all dissolved into the water. Only so much can be dissolved into the water before it won't take any more, add a bit at a time and stir (but do all of it quickly before your water cools) so you don't pour in a ton and end up with half of it not dissolving. Now pour this liquid through your coffee filter into your other jar.
From the book Introductory Practical Pyrotechnics:
Potassium nitrate has a high differentail solubility in hot and cold water - 13.3 grams of KNO3 dissolves in 100 ml of 0 degree C water, while a surprising 246 grams dissolves at 100 degrees C!
Now you have to get the water out so you are left with just the KNO3. There are two ways to do this, one just a faster version of the other. You can pour the liquid into shallow pans and just set it into the sun to evaporate, but this takes at least a day, and won't work unless it's very hot. You can also only have a little bit in each pan because if you have the liquid too thick it will take even longer. This method took me almost two days of drying in hot weather, but it does make neat crystals! (If you are sun drying, you can scroll down to "Once it is all dry...")
Dried KNO3 crystals from sun evaporation:
The other way is much faster, but requires more equipment and work (but I think it's definitely worth it). I highly recommend doing this outside... Once you have your filtered liquid, pour it into a pot/container it can be boiled in. Hopefully you aren't using the same pot you cook your dinner in! Put this on your stove and boil it.
Boiling off water:
The water will evaporate off, and the time it takes will depend on how hot your stove is and how much water there is. Do not use a lid or your water will evaporate and then condense back in your container, you want it to evaporate and leave the container! Boil the liquid until most of the water is gone and the liquid has visible color. I can't give you an exact amount/percent of water to boil off, but I boiled off at least 3/4 of the liquid. Once it starts bubbling like crazy you know most of the water is gone and your KNO3 is starting to cook. Take it off the heat and pour it into a shallow pan.
Liquid after boiling in pan:
As long as you boiled off enough water you will notice it start to get thick right away, and before too long will be sludge, and then crystals. Let this dry completely (obviously this will require warm temps also, just putting it inside under a lamp may work if it's raining outside or something).
Once it is all dry it must be powdered because it will be in large crystals and blobs. This may look like a lot of work to crush, but it is much easier than crushing the prills the kno3 came as. You can use a mortal and pestle, ball mill, or whatever other method you decide on. I use my high tech ball mill of a plastic container and marbles (good thing I saved em from when I was little, I finally have a use for them!).
"Ball milling" KNO3 with my high tech method
You should crush it until it is in a fine powder. With a good ball mill it can hopefully get to flour-like size, but you can't do it that well with a container, marbles, and shaking it around in your hands.
Store the powder in an airtight container or it will absorb water from the air.