The History of Saltpeter - IV

In our last couple of posts, we saw the basics of saltpeter extraction and also an insight into the bacterial and chemical processes involved. As we saw previously, it was the job of saltpeter men to go around the country and locate soils rich in nitrates, so that saltpeter could be extracted from them. They often found these lands close to places where organic materials decomposed, which were sheltered from rain and sun. This usually meant digging in cellars and stables of various farms. However, the formation of saltpeter in these areas was a slow process and the supplies could not keep up with the demand. Therefore, people began to prepare special areas to produce saltpeter. These areas were called niter beds (or nitre beds, if you're used to British spelling). Other names for these include nitraries and saltpeter plantations. We will study how these worked in today's post.

Niter Beds. Click on the image to enlarge. Public domain image from a woodcut from 1598.

Workers would prepare long trenches lined with clay and pile on heaps of manure, rotting leaves, plants and urine, arranged with layers of limestone and ash in between, and small twigs, branches and straws in the middle, to give the mixture sufficient porosity. Such heaps can be seen as C in the image above. The sides of the various heaps protect each other to some degree from wind and weather. Every week, the workers would keep the heaps moist by adding more urine, dung water, water from drains etc. The idea was to keep the heaps moist, but not too wet. Urine from drinkers of beer and wine were in much demand, as it was thought that this resulted in superior quality of saltpeter. The process needed to be somewhat carefully controlled because if it was overdone, the production rate of saltpeter would actually decrease.

Meanwhile, the workers would collect pure rainwater in a large vat (A in the image above), as it is a relatively pure source of water without any minerals dissolved in it. They would also collect wood (D in the image) to be used to prepare ashes and for boiling the liquids later on. After about a year, the heaps would be ripened enough and a saltpeter digger (E in the image above) would dig into them and take them into buildings B and A for processing.

Click on the image to enlarge. Public domain image.

Inside, they would use the pure rainwater to dissolve and leach the saltpeter crystals from the compost heap and then use the wood to boil the water and extract the crystals from it, as described in the posts previously.

As can be imagined, running a nitrary (saltpeter plantation) meant that the smell was very nauseating. In fact, one of the major qualifications to be a nitrary manager or worker was to be able to tolerate the incredible stench produced. That's why many of these operations were located out in the countryside, away from most people.

The remains of what once used to be the largest nitrary in Dijon, France. This was originally out in the countryside, but is now a residential neighborhood of the city. The only hint of what used to be here is the street sign. In the inset, it says "Rue de la Raffinerie" (i.e.) "Street of the [saltpeter] refinery."

Click on the image to enlarge.

These nitraries were dedicated to producing saltpeter and some countries (e.g. France, Sweden, Germany, colonial-era America etc.) introduced laws requiring people to set aside X amount of land in each village for niter beds. A well run operation could produce, every two years, about 2-4.5 kg. (about 5 to 10 pounds) of saltpeter per cubic meter of dirt. It was a slow, painstaking, dirty and labor-intensive job, but for several countries, it was necessary for their survival.

In our next post, we will look into the works of a certain Professor Joseph Leconte, who wrote a pamphlet on the production of saltpeter during the Civil war.

The History of Saltpeter - III

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.