In our last post, we studied one of the techniques of producing saltpeter. We did promise there that we will study another method using niter beds. But, before we dive into that topic, let's cover a few more things that were left out in our previous post.
First, in case the reader is wondering, it is not possible to simply pick up a pile of fresh dung and urine and manufacture saltpeter that way. The dung and urine need to ferment for a while (at least 6 months or more), with sufficient moisture and shelter from rain and sun, and the soil needs to be alkaline and have certain compounds and not grow any crops on it.
Since we have the benefit of modern chemistry and biology knowledge, let us try to understand what is actually happening. First, when animals and people produce a large amount of urine and dung, under the right conditions, certain bacteria will turn the urea into ammonia and then combine that ammonia with oxygen in the air to produce nitrate ions. Now, these nitrate ions are looking to combine with other minerals. If the soil contains minerals such as calcium carbonate (limestone), magnesium carbonate or potassium carbonate, these minerals with react with the nitrate ions being produced by the bacteria, to form calcium nitrate, magnesium nitrate, potassium nitrate etc. Now if plants are allowed to start growing here, they will absorb the nitrates and grow tall and green, but there will be no saltpeter produced, since the plants use up the nitrates in the soil (nitrate compounds have been used as fertilizer for this reason). However, in stables, the animals will eat any plants and therefore, the nitrates can continue to form under the floorboards and walls of adjacent buildings. Similarly, if the ground is porous and dry, plants won't grow well and therefore nitrate crystals can form on it.
Now remember that we said that nitrates such as calcium nitrate, magnesium nitrate, potassium nitrate etc. are formed. Of these, only potassium nitrate is useful to us for gunpowder. The other two nitrates absorb too much water from the air and this reduces their explosive strength. The trick is to separate the potassium nitrate from the others. An Arab scientist from Syria named Hassan Al-Rammah described how to do so, in his book from 1270 AD titled "al-Furusiyya wa al-Manasib al-Harbiyya" (i.e. "The Book of Military Horsemanship and Ingenious War Devices"!) The details in his book seem to indicate that the method is of Chinese or Indian origin. The method described consists of dissolving all the nitrates in water and mixing in a lot of wood ash (which contains a lot of potassium in the form of potassium carbonate). The potassium ions in the wood ash replace the calcium and magnesium ions in calcium nitrate and magnesium nitrate and leave behind potassium nitrate, which can be crystallized. The knowledge of this method went westwards via the Italians and spread to the rest of Europe. A version of this method was described in the previous post.
All of this seems very obvious to us readers, who have had the benefit of knowledge passed to us by hundreds of scientists and inventors over many centuries. However, for people living in the 13th to 18th centuries, who did not have the knowledge of chemistry and biology that we do, the formation of saltpeter crystals was practically magic. They understood that decomposition of organic matter might have something to do with it, but they could not explain why saltpeter crystals would form under stables and cellars, but not in the open fields where dung is also found, and also why saltpeter crystals would not always form under stables if the weather conditions weren't right (the role of certain types of bacteria in the nitration process wasn't fully understood until the late 19th and early 20th century). The source of dung also had an effect on whether saltpeter was produced or not. While it was known that seabird guano was good for fertilizer, it wasn't as good for saltpeter production. However, some soil found under dove cotes (i.e. houses for pigeons and doves) was found pretty rich in nitrates. Stables provided with porous floors, such as straw and ashes, allow the formation of saltpeter. Some caves with limestone floors and filled with bats, would provide good saltpeter from the bat guano piles which were centuries old. Another puzzle was why sunlight and rain affected the production of saltpeter. Again, they didn't fully understand the roles of bacteria, soil chemistry, other minerals etc. This is why the role of the saltpeter men in various countries became so important, because people couldn't guarantee that saltpeter crystals would form at any particular location and it was left to saltpeter men to sniff around and find rich deposits of saltpeter. Since it was such a vital ingredient of gunpowder, they were given permission by their country's rulers to dig literally anywhere and given state protection from angry landowners.
In our next post, we will start looking into niter beds.