Introduction
Originally published: 2006
Black powder or Gunpowder is an explosive pyrotechnic composition comprising of charcoal (C), sulphur (S) and potassium nitrate (KNO3, also known as saltpetre). Black powder is classed as a low explosive due to its relatively slow (sub-sonic) combustion or deflagration rate when compared with that of high explosives (super-sonic combustion rate) such as TNT. It can be effectively used as a propellant used in firearms, fireworks and other pyrotechnic displays.
A simplified general chemical equation of the combustion of black powder is:
10 KNO3 + 3 S + 8 C ⇒ 2 K2CO3 + 3 K2SO42 + 5 N2
Making black powder is more complex than simply mixing the three ingredients together. This is in fact called green powder or polverone which burns slowly and inefficiently leaving lots of un-burnt solids behind. Proper black powder is mixed thoroughly to a fine powder using a ball mill or with other non-sparking grinding media. This burns much more rapidly and leaves very little un-burnt residue.
The standard composition of black powder is:
- 75 parts potassium nitrate
- 15 parts charcoal
- 10 parts sulphur
All parts are by weight, not volume – the constituents of most pyrotechnic compounds stated in this convention unless otherwise stated.
Potassium nitrate can be obtained as a pure (99.9%) ground compound from a chemical supplier or as an agricultural fertiliser (<99% purity). Charcoal can be also be bought from a chemical supplier pre-milled to a fine dust or can be made at home from pure wood charcoal (the best wood to use for the charcoal is Pacific Willow). Sulphur can be obtained from a chemical supplier or as an agricultural fertiliser.
The main tool used to make home made black powder is a ball mill or the more labour intensive mortar and pestle. There are two main methods of making black powder each with their own advantages and disadvantages. Professional or commercial black powder will likely always be superior due to its more sophisticated industrial milling, pressing and corning methods.
The two methods discussed here are the ball mill method and the precipitation method. The ball mill method is far simpler than the precipitation method but produces an inferior powder.
Please note: the information provided here is for information purposes only; the methods discussed herein should never be attempted unless carried out by a person who is fully trained and is totally compliant with all relevant local laws and regulations.
Ball Mill Method
This method is potentially very dangerous if not practised with caution. The milling location should be away from any buildings/people/animals in case of accidental ignition of the powder during milling.
- Grind the raw charcoal in a mortar and pestle until small grit sized particles are obtained.
- Weigh out 15 parts charcoal and add 10 parts sulphur (powdered). Bearing in mind that you should aim to fill the ball mill to a maximum of about half full when all the substances are added.
- Ball mill the charcoal and sulphur mix for about 3 hours (without adding any potassium nitrate).
- Add 75 parts of potassium nitrate (powdered) to the charcoal and sulphur mix in the ball mill. Also add 6 parts water to the mix before ball milling, this will dramatically reduce the probability of accidental ignition of the mixture during milling.
- Ball mill the mixture for about 5 hours checking every hour or so that the mixture hasn’t dried out; if it has add a little more water and continue ball milling.
- After ball milling, empty the mixture out onto a large sheet of paper, spreading it out and allow it to dry naturally in a warm, non-humid place.
- When dry, very carefully grind the mixture in a mortar and pestle to break it up into a course powder. This should then be sieved using different sized sieves to obtain different grades of powder for different purposes.
Precipitation Method
The precipitation method is much more difficult. You first ball mill the charcoal and sulphur together (just like you would with the ball mill method), but this is followed by dissolving the potassium nitrate in hot boiling water which is then mixed with the milled charcoal-sulfur mixture. The potassium nitrate is then precipitated from the solution by mixing with ice cold isopropyl alcohol. This is followed by filtering (messy) and drying which takes a long time, and a good place with no ignition sources is required, since there is a flammable liquid involved.
- Grind the raw charcoal in a mortar and pestle until small grit sized particles are obtained. This should be done outside as it is very messy.
- Weigh out 15 parts charcoal and add 10 parts sulphur (powdered). Bearing in mind that you should aim to fill the ball mill to a maximum of about half full when all the substances are added.
- Ball mill the charcoal and sulphur mix for about 8 hours (without adding any potassium nitrate).
- While the mill is running; per 100g of charcoal-sulphur mix, place 600ml of isopropylalcohol into a large container and place that in a fridge.
- In an old pan, take 75 parts of potassium nitrate and add 40ml of water per 100g of potassium nitrate. Place the pan on the stove and bring it to the boil while continuously stirring. When the solution starts boiling add small amounts of water until all the potassium nitrate has dissolved. Then add another 10ml of water.
- Add the charcoal-sulphur mix to the boiling solution of potassium nitrate. Stir until the liquid is consistent and free from lumps. Allow to cool to room temperature.
- By now the isopropylalcohol should have cooled to about 0°C. Take the isopropylalcohol outside and pour the charcoal-sulphur-potassium nitrate mixture into the cold isopropylalcohol.
- Make sure there are no sources of ignition nearby as there will be a large amount of flammable vapour given off from the isopropylalcohol. Stir for a few seconds.
- Place this mixture back into the fridge/freezer and allow to cool back to 0°C as fast as possible.
- Now filter the mixture through an old cloth and squeeze the liquid out. Discard the black liquid.
- Spread the black mixture out on a large sheet of paper and allow it to dry naturally in a warm, non-humid place, e.g. outside in the sun light. Keep away from any sources of ignition.
- When dry, very carefully grind the mixture in a mortar and pestle to break it up into a course powder. This should then be sieved using different sized sieves to obtain different grades of powder for different purposes.