Category Archives: Weapons

Early Bio-technology and Potassium Nitrate for Gunpowder

Early BioTechnology and the “Making” of Saltpetre

In the previous post, we discussed Gunpowder, a mixture of 75% Saltpetre (Potassium Nitrate), 15% Carbon (charcoal) and 10% sulfur.

Let’s now look at acquiring the Saltpetre.

The pre-technology (but not pre-biotechnology) source of Saltpetre is aged urea-soaked organic material (yes, this is probably considered a bit gross in these days of out-of-sigh, out-of-mind toilet habits). The aging of this material allows bacteria to convert the ammonium compounds into nitric acid. Note: this material must be protected from rain or flooding, as the desired compound is water-soluble.

To convert this nitrogen-rich organic material into Saltpetre, we can boil the source material with water. Then add a source of potassium (such as ashes from your fireplace). The resulting liquid can be evaporated to get crystals of Saltpeter (1).

We can mimic the process used by our pre-industrial ancestors by using the following steps from “The Do-It_Yourself Gunpowder Cookbook” (2):

1. Construct a filtering container by punching small holes in a 5 gallon bucket and forming a filter at the bottom by using a layer of cloth, a layer (1/2 cup) of wood ash, and another layer of cloth.
2. Fill the bucket with screened, nitrate-bearing soil/manure/guano. Leave room at the top.
3. Place the bucket over a collection container. Pour—very slowly—a gallon or so of boiling water over the soil in the bucket. You want the hot water to permeate and percolate uniformly through the soil.
4. Allow the drained solution to cool and settle. Then pour into a heat resistant container and boil the collected solution for 2 hours. Discard any salt that forms during this step.
5. After the solution has boiled down to half its original volume, let cool for ½ hour. Add an equal volume of alcohol (applejack, 12 year old scotch, or cheap vodka) and stir briefly. The small white crystals that form are Saltpetre. Collect and dry.
6. To refine further, re-dissolve and repeat the boiling step.

Unfortunately, this uses up your medicinal alcohol supply. We’ll save the production of alcohol for another post.

Another pre-industrial method is from the 5th Foxfire book (3):
1. Make a filtering container using wood slats in a “V” shape (wide at top, narrow at bottom). Line with straw-type material. At the bottom have a collection bowl.
2. Fill the V-container with nitrate-bearing soil/manure/guano. Pour boiling water over the material. Allow to permeate and percolate. Collect the leachant.
3. Repour the leachate through the nitrate material.
4. Combine the leachate with woodashes (which provides potassium hydroxide). A white precipitate (calcium hydroxide) forms and settles.
5. The woodash leachate is then boiled down until saltpetre (potassium nitrate) crystals form.

Conclusion: if you’re writing an alternative or speculative history novel, your characters will have access—through the above procedures—to the major ingredient of gunpowder. The next steps are:
1. Finding elemental sulfur or extracting it from minerals.
2. Charcoal (the way easy step)
3. The metallurgy required to smelt iron and forge gunbarrels (the ‘whew’ step).

As always, let me know if I missed something (or outright missed reality)!


(2)… The Do-it-Yourself Gunpowder Cookbook

(3)… Foxfire 5


Gunpowder developed as a weapon in China about 1000 CE (common era, formerly designated AD) although it has been used for pyrotechnics as far back as 100 BCE (before the common era, formerly BC).

Weapons use probably started with fire arrows, small tubes of bamboo attached to arrows that fizzed smoke and fire. A logical next step was bombs, essentially large firecrackers.

The next development in gunpowder weaponry may have been fire lances, larger tubes of burning gunpowder that spew fire and sparks at enemy combatants.

With better quality and purity, gunpowder grew more explosive, resulting in the easy step of adding shrapnel to the fire lance. Soon, the bamboo tubes would be replaced with metal ones.

By the end of the thirteenth century, an enclosed projectile could be thrown by the exploding gasses confined in a tube. The gun was born. No longer was the smoke and fire the resulting weapon. The projectile was the payload.

This technology had reached Europe by the thirteenth century. Its use only limited by the quality of ingredients and by metallurgical constraints of the gun barrel.


Gunpowder is comprised of only three ingredients: potassium nitrate (saltpeter), charcoal and sulfur in a ratio of 75%, 15% and 10%.

Sulfur ignites at a low temperature (261 degrees) starting the explosive process. This starts the charcoal burning and breaks down the saltpeter which, consisting of Potassium and Oxygen, releases pure oxygen. This then accelerates the burning/exploding of the charcoal. The heat of this reaction produces massively expanding black gases. If constrained, this becomes an explosion.

The charcoal is easily made from wood: just burn with insufficient air. The physical structure of the wood is important for the charcoal to easily pulverized as well as to have a low ash content. Willow is a common source (and is used as a traditional artists’ medium) as well as alder, hazel wood or grape vines.

Sulfur was more difficult to find. It’s an element, so it’s either geologically available or not. Prior to refining sulfur from petroleum products or natural gas, it had to be found as a solid in the ground near hot springs and volcanic regions.

Sicily was an important source in the medieval world. Its purity was variable, but it could be easily purified. One method was to heat impure sulfur in a clay pot, the vapors given off were directed by a clay pipe to another pot where the fumes condensed into pure sulfur.

Saltpeter was even more difficult. Its chemical name is potassium nitrate, having a potassium ion attached to a nitrate ion. The nitrate ion consists of three oxygen atoms bonded to one nitrogen. Saltpeter is very soluble and is released from organic matter. One common source was privies and compost heaps from which, if protected from the weather, small amounts of saltpeter leached to the surface. The resulting white crust was saltpeter.


Once the ingredients were obtained, the next step was mixing them into the actual gunpowder. First the ingredients were mixed by mechanical or hand blending.

Next the mixture is ground into a microscopically fine homogeneous powder. Considerable heat can be produced, so this material was always kept moist.

In the third step, the material is pressed into cakes, producing a denser compound.

Finally, the cakes are granulated between crusher rolls. Screens are used to separate the grains. The end product is uniform mixture of black powder in the appropriate grain size. This last is important in that the size of the particles determine the rate of burning. For example, in larger cannon, larger grained gunpowder was used while rifled guns used finer grained powder.

Final Thought

Making gunpowder was a highly technical development. The chemistry wasn’t understood until the 19th century when it was discovered that the nitrate provided oxygen for the reaction.

The critical development in the use of gunpowder was metallurgical, the development of iron of sufficient quality to contain the exploding powder long enough to allow a projectile to be accelerated out of a gun or cannon.