Sulfurless Black Powder

In my last post, 2 years ago, I said I might write about this.  Since it has been so long, I feel like I owe it to you to make good on that threat.

First, I have only ever read about this technique for making black powder in one place (I no longer recall the source).  This is superior to any other technique I know of, and supposedly it was used around Civil War times for making the highest grades of military gunpowder.  I will describe the use of this process for making sulfurless black powder, because this is what I make with it.  The original purpose of the sulfur in black powder was to make it ignite easily with a spark.  I am using this for rocketry, where I don't need it to ignite so easily, so there is no reason that I need the sulfur.  In addition, sulfur is the most expensive and difficult to obtain ingredient, and leaving it out makes the gunpowder less volatile and thus much safer to handle.  The side effect is that the rockets are extremely difficult to ignite without commercial rocket igniters (like the ones that come with Estes rocket engines).

To begin, we need our ingredients.  Traditional black powder is made from charcoal, saltpetre (potassium nitrate), and sulfur.  As we are leaving out the sulfur, we only need the charcoal and potassium nitrate.  Now, note that we cannot just use a bag of charcoal briquettes from the store.  These typically contain bits of unburnt wood and some oils to make them light more easily.  These will ruin the gunpowder.  In one of my earlier posts, I explained how to make charcoal, but I will briefly go over it again.

Charcoal is made by burning wood without oxygen.  This is easily done by placing pieces of wood in a metal container that is not quite air tight, and then placing this in a very hot fire for a while.  As the container heats up, it will vent volatile gasses (which will ignite if they are near the flames).  Once these gasses have stopped venting entirely, the charcoal is probably done.

The potassium nitrate may be more difficult to find.  I actually had great difficulty finding it initially.  One likely place to find it is a store that sells meat processing supplies, as it was traditionally used to cure meat.  Many of these places sell more modern curing chemicals though, instead of potassium nitrate.  Conveniently though, it turns out that the powdered stump remover sold in most hardware stores is typically pure potassium nitrate.  Before buying it though, ask for an MSDS for it.  This will list the ingredients.  Some stump removers have one or two additional chemicals which happen to be poisonous and which would probably produce toxic smoke when burned.  Only use it for gunpowder if the only ingredient listed is potassium nitrate (I use Spectracide, but I would recommend getting the MSDS even if you do get this brand, just to be safe).

Now, you need to grind the charcoal to be incredibly fine.  I don't know how fine they got it for traditional gunpowder, but the finer you get it, the more efficiently it will burn and the more power you will get out of it.  The best way to do this is with a ball mill.  Since most people do not have easy access to this kind of equipment, I will tell you how to make your own.  Buy a small rock tumbler (don't get one of those tiny plastic ones that comes with those cheap educational kit though; get a 3 lb. one at least).  Also buy a couple packs of glass marbles.  If there are any marbles with unusual surface textures, remove those, as they could break (I also removed some cool looking ones that I wanted to keep).  From the rest, measure out around 2 lbs. (if your tumbler is bigger than 3 lbs. adjust this accordingly; don't put in more than 2/3 the maximum weight of the tumbler).  These marbles will be the balls for your makeshift ball mill.  Place these in the drum of the tumbler, and then fill it the rest of the way with charcoal (it works best if the charcoal is already crushed up a bit).  It might be a good idea to check the weight at this point, to make sure you are not exceeding the maximum weight.  If you are, remove some charcoal and/or some marbles.   Now, run the rock tumbler for a while.  The time required will depend on the size of the tumbler and the hardness of the charcoal.  I used fir for the charcoal (supposedly hardwoods are better) and it took only a day before most of the charcoal was an extremely fine powder.  Next time, I will probably run it for two days, because there were still a few small chunks left (though, running this through a fine sieve would easily remove those).

You will need a third ingredient.  Get some of the highest percentage rubbing alcohol you can find.  I used 97%, but I have only seen it a few times.  Make sure you get enough.  I don't know exactly how much this is, but one cup of gunpowder needs more than a 1 pint bottle of alcohol.  Put the alcohol in the freezer the night before you want to make the gunpowder.

Once the charcoal is finely powdered (mine was finer than powdered sugar), you are ready to make the black powder.  According to Wikipedia on sulfurless black powder, "They typically contain 70.5 parts of saltpetre and 29.5 parts of charcoal."  This should be by weight.  Measure out the amounts of each that you need before starting.  According to the Wikipedia page on potassium nitrate, the saturation point of potassium nitrate in water at boiling is 2460 g/L.  This is 2460 grams of potassium nitrate for each liter of water.  Since you are probably not using grams and liters, use Google to convert the units for you (entering "2460 g/L to oz/gallon" gives you "328.475119 oz / US gallon").  Use this to figure out how much water you need, then add few extra tablespoons to that.  Put the water in a pan, add the potassium nitrate, then heat the water to boiling.  If you did your math right, once the water gets close to boiling, all of the potassium nitrate will have dissolved.  Now, let it boil and watch it carefully.  When potassium nitrate crystals start to appear in the water (not just at the edges), pour all of the charcoal powder in and stir it in.  Right before you do this, take the alcohol out of the freezer and pour it all into a large bowl (it needs to fit all of the gunpowder in there as well).  My charcoal did not want to mix in well.  If this happens, just keep stirring until it does.  Leave the burner on while you do this (don't worry, the gun powder will not ignite; besides the fact that it is wet, without the sulfur, it is extremely difficult to ignite, even with an open flame).  Once the mixture has turned into a black paste, with all of the charcoal mixed in, turn off the burner, and pour the mixture into the chilled alcohol.  This is the part I always have problems with.  The first time I did it, it went fine, but the second time, I did not have enough alcohol, and I poured the powder too fast.  For this to work properly, the alcohol needs to remain chilled (if you don't have enough, the hot mixture will warm it up).  Also, if you pour too fast, you will end up with a pile of the mixture at the bottom, where the middle is insulated from the cold by the outside.  The fast cooling is essential for optimum quality.

Leave the mixture in the alcohol until it has cooled completely.  It should now be fairly solid, though easy to break.  Pour the alcohol off and save it (you can reuse the alcohol several times, even if it is dirty from the previous batch).  Break up the chunks of black powder (don't crush them into fine powder though), and place them somewhere they can finish drying.  In a warm climate this should only take a day (give it more time in a cold climate).  I don't know how long it will take in a humid climate.

Once it is dry, you can pack it into rocket tubes to make engines.  This powder will not make a good explosive, and don't bother trying to use it in an old gun.  The lack of sulfur makes it so difficult to light that even an open flame may take half a minute to light it.  Really, it is not very good for anything but confined burns like in model rocket engines.  Given that this process was originally used for making powder intended for guns, it would probably work fine for a more traditional gunpowder mixture that includes sulfur.

Now, I want to talk about why this produces such high quality black powder.  Black powder is a oxidizer/fuel mixture.  The charcoal is the fuel and the potassium nitrate is the oxidizer.  The efficiency of the powder depends on how well the burning charcoal can get the oxygen from the potassium nitrate.  Two things will improve this.  First, the smaller the particles, the more surface area they have and the better the oxygen can transfer.  Second, the more intimately the ingredients are mixed, the better oxygen can be transferred.  Milling the charcoal with the ball mill will get those particles very small.  We could stop there and mill the potassium nitrate in with the charcoal (this is the common way most rocket enthusiasts make black powder now).  Potassium nitrate dissolves in water though, which gives us another option.  What we did above was to supersaturate the water with potassium nitrate.  This means that we increased the amount of potassium nitrate that the water could hold by heating the water, then we dissolved the maximum amount in it.  As the water cools, the potassium nitrate will start to crystallize out.  The faster the water cools, the smaller the crystals will be.  In addition, crystals tend to form first around rough spots.  The charcoal is very porous and thus has an enormous amount of rough spots.  So, when we poured the mixture into the chilled alcohol, the potassium nitrate formed extremely small (microscopic) crystals all over the rough charcoal particles.  This is about the most intimate mixture you can get without rearranging individual molecules.  This means that the mixture will burn almost completely (coarser mixtures will have a lot of partially burned particles), which means we maximize the energy output.  This creates an extremely efficient rocket fuel when we use it to make sulfurless black powder.

I hope this is useful to someone.  Like I said, I have only seen this technique described once.  I have not been able to find any information about it since.  Because I know the chemistry behind it, I know that it works.  By publishing this information, I may be preventing this from becoming a lost art.

Lord Rybec