Black Powder - III

In our last post, we studied some of the physical and mechanical properties of gunpowder, information which will come in handy when we study manufacturing methods in some detail. In today's post, we will look at factors that influence the rate of combustion of black powder.

As we saw in the first post of our black powder series, the ratio of saltpeter, sulfur and charcoal in gunpowders varied at different times and in different countries, but by the 19th century, many people had generally settled to using the ratio of 75% saltpeter, 10% sulfur and 15% charcoal. However, powders made by different manufacturers had different pressures and combustion properties even when they were using the same ratio of the ingredients. We aren't even talking about manufacturers from different countries, they could be manufacturers in the same country or even different powders from a single manufacturer. Clearly there must be some other factors that explain why this happens. That is what we will study about in today's post.

The action of black powder depends not only on the composition of its ingredients, but also the size of the grains, shape of the grains and the density of the grains among other things.There are other factors that influence the rate of burning, but these three are the most important. The reason is because black powder is surface-burning. Smaller grains of gunpowder will have more surface area exposed to ignition than a larger grain of the same weight, therefore smaller grain powder will burn faster than the larger grained type. However, if the powder is packed too densely, the flame cannot easily spread from grain to grain, than the same weight of powder packed in a less compact manner. Therefore, very small grain mealed powder and very large grain powder are both slower burning. The shape of the grain also will affect the burn rate, because of the surface area exposed to ignition. Shapes like cubes or spheres offer less surface area than irregular shaped grains of the same mass, therefore they burn slower. This is why laminated or flaky powders burn much faster than normal and diamond shaped grains burn more rapidly than rounded grains.

As a general rule, the larger the grain, the less violent will be the action of gunpowder (i.e.) its combustion will be more gradual. On the other hand, smaller grain powders also cause pellets to scatter much more rapidly than larger grain powders because a smaller grain powder expends all its force before the shot pellets reach the muzzle, whereas a larger grain powder causes the shot pellets to increase their velocity right up to the muzzle of the gun. Therefore, powder designed for weapons with shorter barrels, such as revolvers and pistols, must be of smaller grain, so that they can finish burning before the powder leaves the barrel. Similarly, powders meant for rifled guns are generally a larger grain than those intended for smooth bores, as a more gradual action is required to avoid putting too much strain on the gun barrel.

Since the same manufacturer often makes black powder of different grain shapes, densities and sizes for different types of guns, therefore the shooting qualities of black powder will vary accordingly. We will look at some powders from the 19th century:

Samples of different powders made by Britsh manufacturers.

Click on the image to enlarge. Public domain image.

The above image shows various black powders made in the 19th century by two large British manufacturers Curtis & Harvey and Pigou, Wilks & Laurence. As you can see, the "Revolver" powder is made of very small grains and designed to be fast burning, while Curtis & Harvey's "Col. Hawker's Duck Powder" and Pigou's "Special Punt Powder" are larger grained and designed to be used by very large bore punt guns. Similarly, Diamond #4 and Alliance #4 were generally used for hunting with shotguns, while #6, Rifle, and Martini-Henry powders were designed for rifles. Other large powder manufacturers in England included the E.C. Powder Company, Schultze Gunpowder Company, Kynoch Ltd., Hall, Coopal, Dittmar etc.

Powders made in other countries also varied in grain size, shape and density:

Black powders from different countries.

Click on the image to enlarge. Public domain image.

The above image shows some sample powders made in different countries. Of course, this is only a very small sample. For instance, in the United States in the late 19th century, there were various powder manufacturers, each making multiple types of powder for different applications: DuPont, Hazard Powder Company, Laflin & Rand, Hercules etc.

Various types of black powder made by DuPont

Various types of black powder made by Laflin & Rand.

Images courtesy of the Haglin Museum and Library

Incidentally, the reason why many of Laflin & Rand's black powder offerings were sold under the "Orange" brand name (e.g. Orange Ducking Powder, Orange Rifle Powder, Orange Lightning, Orange Extra Sporting etc.) is because their original production plant was named "Orange Mills" and happened to be located in Orange County, New York.

The quality of charcoal is also a significant factor in the burning rate of the black powder. If the charcoal is improperly charred, then the oxygen and hydrogen retained in it cause it to burn more rapidly than if it is reduced to a pure carbon. The source of wood for the charcoal is also a factor. Experiments conducted in the 19th century showed that there were significant differences in the amount of gas produced by charcoal made from different types of wood. For instance, dogwood charcoal was found to yield about 25% more gas than the same weight of charcoal made from fir, chestnut or hazel trees and 17% more gas than charcoal made from willow. This is why dogwood was preferred for black powder intended for pistols and rifles, while willow charcoal was preferred for making powder for cannons.

In our next post, we will study more into the classification of grain sizes and shapes.

Black Powder - II

In our last post, we studied the composition of different kinds of black powder as manufactured in various countries. In today's post, we will study some of the physical and mechanical properties of black powder. Gaining some knowledge of this will help understand the reasoning behind the processes of manufacturing the powder when we study that later on.

The first thing we should note about black powder is that it is a mixture and not a compound. Your humble editor will explain what that means:

A compound is formed when different substances combine with each other at a molecular level. The compound will often have properties different from its component substances. For instance, hydrogen and oxygen atoms can combine together to form water (a compound substance), which is a liquid at room temperature, whereas hydrogen and oxygen are gases at the same temperature. Oxygen can help substances burn rapidly, whereas water can be used to stop fires. So you can see that a compound (in this case, water) has quite different properties than its original ingredients (in this case, hydrogen and oxygen).

On the other hand, a mixture is when multiple substances are physically mixed with each other, but do not react at a molecular level. This means that they may be separated from each other by some physical means and mixtures often retain the physical properties of their separate ingredients. For example, you can make a mixture of iron filings, sand and sugar crystals. However, the iron filings can easily be removed from the mixture by passing a magnet over it, while the sugar can be separated out by dumping the mixture in water and letting the sand settle at the bottom while the sugar dissolves in water. Another example could be sand and glass marbles, which can be mixed together easily, but trivially separated by passing the mixture through a sieve, which will allow the sand to pass through, but retain the glass marbles. Black powder is a mixture of potassium nitrate (saltpeter), sulfur and carbon (charcoal). The three substances do not chemically react with each other at room temperature and therefore it is a mixture. Only when the powder starts to burn do the three substances react with each other and form multiple compounds.

Since it is a mixture, the various ingredients of black powder must be ground into particles of roughly the same size as each other to stay mixed together (especially before corning of black powder was invented). Otherwise, the mixture could separate out where the ingredient with the smallest size particles ends up at the bottom of the box, given enough vibration to the box. This is because the smaller particles fit in easily between the gaps of the other particles and fall to the bottom, thereby pushing the bigger particles up. The same phenomenon can be observed with a bag of potato chips (it doesn't matter what flavor of chips!). Notice that when you buy a bag of potato chips, the smallest broken chips are always at the bottom of the bag, whereas the larger pieces end up on top. This is because the bag is shaken during transport from the factory to the grocery store and from the grocery store to your home and the smaller chips end up fitting into the gaps between the larger chips, making their way to the bottom of the bag eventually and thereby pushing the larger pieces upwards. The same principle used to apply to gunpowder before they learned to cake the grains and manufacture them to the same uniform particle sizes. In fact, one of the problems of early black powders (also called serpentine powders) was that when they transported the powder to the battlefield via carts drawn by horses or oxen, the bad roads would cause the barrels of gunpowder to shake heavily, thereby moving the smaller particles to the bottom of the barrel. Therefore, if the ingredients were ground up into particles of different sizes, the ingredients would separate out into three separate layers by the time the barrel got to the battlefield, with the sulfur ending up at the bottom of the barrel and charcoal rising to the top. This is why they would remix the ingredients right there in the field before the battle commenced, which was a somewhat hazardous procedure that produced clouds of potentially explosive dust.

Black powder can be ignited in three different ways: the first method is by contacting it with sparks or open flame, the second method is by a sharp blow and the third method is by increasing its temperature rapidly beyond a certain point.

The first method (exposing it to open flame or sparks) is the principle that different ignitions systems such as matchlocks, wheel locks, flintlocks, percussion locks etc. use. However, the source of the flame or sparks must be hot for the powder to ignite. It is possible for a shower of lower temperature sparks to fall upon black powder without igniting it, whereas a single spark of great intensity can start combustion.

The second method (striking it between two objects) is because black powder is somewhat impact sensitive. Experiments by Aubert, Lingke and Lampadius verified that black powder can be ignited by striking iron on iron, iron on brass, brass on brass, and less easily by a blow of iron on copper, or copper on copper. Of course, some of this might be explained away by the impact causing sparks which ignite the powder. Experiments in 19th century England showed that black powder is also ignited by striking brass on copper, iron on marble, quartz on quartz, lead on lead and lead on wood (a lead bullet was shot against a wooden pendulum covered with powder). Mining accidents over the years showed that striking copper on stone or even wood on stone could occasionally cause ignitions of black powder. One Dr. Dupre even showed that there is hardly any explosive, which, when laid in a thin layer on a wooden floor, will not explode, when it receives a glancing blow with a wooden broom-stick.

The third method (heating it beyond a certain temperature) has some interesting effects. Black powder may be ignited when heated rapidly above a certain temperature, even without the presence of an open flame. The temperature at which this happens depends on the nature of the powder and the proportions of its ingredients and grain size. An experiment by Horsley in the 1800s showed that black powder could be ignited by heating it to around 600 °F (about 315 °C) by heating a saucer in an oil-bath, with the temperature of the oil being taken by a thermometer dipped into it. Experiments by Leygue and Champion in 1871 used a more precise method to determine ignition temperatures and the found that a common sporting powder ignited around 550 °F (about 288 °C), while cannon powder ignited around 563 °F (about 295 °C). However, note that we said that the powder should be heated rapidly for it to ignite. What if it is heated slowly?? Leygue and Champion detail some interesting issues here: They discovered that the grains of corned black powder cake together on account of the sulfur they contain. However, note that black powder before ignition is a mixture, which means it retains many of the physical properties of its separate ingredients. When the temperature of black powder is slowly increased beyond 212 °F (about 100 °C, the temperature of boiling water), the sulfur begins to volatilize and turn into vapor. The volatilization of sulfur rapidly increases with temperature and if the temperature is slowly increased upwards, but kept below the boiling point of sulfur, then the sulfur can be completely driven out of the powder without any ignition taking place. When the sulfur is completely eliminated from the mixture, the temperature can be further increased, so that even the saltpeter melts, and the charcoal ends up floating on top of it, thereby separating out the two ingredients from each other. If, on the other hand, the temperature is rapidly increased before the sulfur is completely volatilized, then the sulfur vapor is ignited and causes the powder to explode. The shape and size of the grains of black powder have considerable influence on the temperature of ignition as well.

If a small quantity of black powder is ignited in open air, it merely burns, but if larger quantities are ignited, or if the powder is ignited under higher pressure or in a closed space, then it explodes. The larger the grain size, the slower the combustion rate. We will study more about this in the next post when we study more about grain sizes.

If good quality black powder is ignited over a sheet of white paper, it will burn rapidly and leave no residue on the paper. If black spots are found, then this indicates that either the mixture contains too much charcoal or the powder is badly mixed. The same can be said for sulfur if yellow spots are left behind. If unburned grains are found, this indicates that the saltpeter is impure. The powder should not burn holes into the paper, as only moist or otherwise bad black powder does so.

As early as 1765, Papacino d'Antoni found that lower air pressures make it more difficult for black powder to ignite. Later experiments by Munke, Hearder, Bianchi, Heeren and Sir Frederick Abel showed that gunpowder didn't explode in a vacuum tube, even in the presence of a platinum wire glowing white hot. Heeren tried to explain this phenomenon by suggesting that at normal pressures, the hot gas escaping from an exploding body would communicate the flame to neighboring particles, but under low pressure, the gas expands so rapidly on account of the lack of resistance of the surrounding air, that it cools down below the ignition temperature of neighboring particles.

On burning gunpowder under normal or high pressures, the various ingredients of the mixture combine with each other chemically and produce gases and solid residue. While this was known from the day that gunpowder was invented, the nature of the gases and solid residue was not. In fact, given the primitive state of chemistry for centuries, it was not known if the products of combustion was just one or several gases. For instance, in 1705, the great Issac Newton thought that sulfuric acid formed by the combustion of sulfur drove out the spirit of niter from the saltpeter and burned it. The same view with slight modifications, was held in 1771 by Majow, who thought a mysterious substance called "phlogiston" (thought to exist in all flammable substances) combined with the nitric acid. It was left to the famous French chemists, Joseph Louis Gay-Lussac and Michel Chevreul, to determine exactly what gases and solid residues were produced. Their experiments showed that among the gases produced were carbonic acid, nitrogen and carbonic oxide, while the solid residues were potassium sulfate, potassium carbonate, potassium sulfide, potassium thio-sulfate etc. Incidentally, Gay-Lussac was the first to prove that water is made of hydrogen and oxygen and also worked on alcohol-water mixtures, the results of which are still used to today to measure alcoholic beverages in many countries around the world (a fact that drinkers will surely appreciate!)

In our next post, we will look into the effects of grain sizes of black powder and how/why different grain sizes were used for different applications.

Black Powder - I

A while ago, we studied about black powder in two separate posts. Since we've studied the processes of obtaining the basic ingredients of black powder (saltpeter, charcoal and sulfur) in great detail in some of our previous posts in the last few months, we will study the processes of combining them into black powder in some detail in the next series of posts.

Before we start our study of black powder manufacture, let us discuss the proportions of the ingredients of black powder. While it is true that many countries had settled with the proportions of 75% saltpeter, 10% sulfur and 15% charcoal by the 18th and 19th centuries, this wasn't always true in all countries. Moreover, the proportions also varied a bit, depending on the use for the black powder. For instance, powder intended for military rifles differed in composition than powders intended for sporting applications, which differed from powders used for blasting purposes, powder used for fireworks etc. We have some information about the composition of powders made in various countries, courtesy of Oscar Guttman's book "Manufacture of Explosives" from 1895 (note that some of the countries have different names now)

	Saltpeter	Sulfur	Charcoal
(a) Rifle Powders:
Austria-Hungary	75	10	15
Belgium	75.5	12	12.5
China	75	10	15
France	75	10	15
Germany	74	10	16
Great Britain	75	10	15
Holland	70	14	16
Italy	75	10	15
Persia	75	12.5	12.5
Portugal	75.7	10.7	13.6
Russia	75	10	15
Spain	75	12.5	12.5
Sweden	75	10	15
Switzerland	75	11	14
Turkey	75	10	15
USA	75	10	15
(b) Cannon Powders:
Austria-Hungary	74	10	16
France	75	10	15
Germany	74	10	16
Great Britain	75	10	15
Switzerland	75	10	15
(c) Sporting Powders:
Austria-Hungary	76	9.4	14.6
France	78	10	12
Germany	74	10	16
Great Britain	75	10	15
Switzerland	78	9	13
(d) Blasting Powders:
Austria-Hungary	60.2	18.4	21.4
France	72	13	15
Germany	70	14	16
Great Britain	75	10	15
Italy	78	18	12
Russia	66.6	16.7	16.7

As can be seen above, many countries varied the proportions of the ingredients based on the intended use of the powder. Note that the blasting powders vary in proportion much more than the rest. This is because blasting powder's requirements were that it should be cheap and develop as much gas as possible at a high temperature. Actually, blasting powders were more varied than the table indicates because powders with different rates of burning were used for rocks of different hardness. So even though the table above suggests that the French were manufacturing blasting powder with the ingredients in 72%, 13% and 15% ratio, that was only one grade and the French Government factories actually made 3 grades of blasting powder:

	Saltpeter	Sulfur	Charcoal
Ordinary Powder	62	20	18
Slow Powder	40	30	30
Strong Powder	72	13	15

Similarly, some blasting powders in England were made of different proportions (e.g.) 65% saltpeter, 20% sulfur, 15% charcoal.

Powders manufactured in Belgium had the following compositions depending on the purpose:

	Saltpeter	Sulfur	Charcoal
Rifle Powder	75	12.5	12.5
Cannon Powder	75	12.5	12.5
Sporting Powder	78	10	12
Blasting Powder	75	12	13
Slow Powder or Pulverin	70	13	14 & 3% wood meal
Slow Powder in cartridges	70	13	14 & 3% dextrine
Export Powder	68	18	22

In France, "pulverin" was also manufactured for use in fireworks and contained 75% saltpeter, 12.5% sulfur and 12.5% charcoal mixed together.

In the next couple of posts, we will study the grain sizes of black powder in the 19th century.

The History of Saltpeter - XIX

In the second half of the nineteenth century, people began to look for other sources of natural nitrates besides the plains of India. Today, we will study natural nitrates from South America, called Chile Saltpeter or Peru Saltpeter or soda niter.

A sample of Chile Saltpeter. Click on the image to enlarge. Public domain image.

Unlike the saltpeter we've been studying so far, which is potassium nitrate, chile saltpeter is sodium nitrate. It occurs naturally in the Atacama desert, which lies between Peru and Chile. The first shipment of Chile saltpeter to Europe arrived in England in 1820 or 1825, but they could not find any buyers and therefore dumped it at sea, so that they didn't have to pay a customs toll. However, by the time of the Crimean war (1854-1855), the demand for saltpeter was so much that the existing sources from Europe and India couldn't keep up and it became profitable to ship South American saltpeter to Europe. There is a good reason why this stuff was called "white gold".

There are a few theories about how the saltpeter forms there. The classic explanation is that in the Tarapaca region in the Atacama desert, which is a dry desert that experiences a little rainfall every six or seven years, when the rain falls, it floods the plain. However, the plain slopes gently towards the coast hills and as there is no outlet for the water, it collects there and evaporates in the desert's arid conditions, and all the nitrate that was dissolved from the entire plain is deposited in a relatively narrow area. A more modern theory states that the Andes climate was warmer and wetter about 20 million years ago and nitrates, iodine and chromium deposits were leached from the ground water. About 10 million years ago, as the Andes and the coast mountains began to grow higher and higher, the climate shifted to desert and forced the groundwater to evaporate, leaving behind the nitrates. Sea spray and fog from the ocean also drop small amounts of nitrates on the surface.

Originally, the areas where most of the deposits were concentrated belonged to Peru and Bolvia: specifically Peru's Tarapaca region and Bolivia's coastal region of Antofagasta. Mining concessions were granted to Chilean and the British companies to mine these areas. Then the Peruvians and the Bolivians attempted to control the British and Chilean companies by imposing new taxes on them. This led to the War of the Pacific (1879-1883) which ended with Chile capturing both the Tarapaca and Antofagasta regions. By the 1890s, Chile was supplying a whole lot of Chile saltpeter to the world. 

Sodium nitrate can be converted to potassium nitrate by interacting with the chloride of potassium, which could be made from kelp, wood ash, or later, from carnallite, which is a mineral that occurs in huge deposits in Europe and America, particularly in Carlsbad, New Mexico, Paradox Basin in Colorado and Utah, Stassfurt in Germany and the Perm Basin in Russia.

In a heated and concentrated mother liquor, sodium nitrate (about 95% pure from the chile saltpeter) and potassium chloride (greater than 80% purity, from the carnallite) are dissolved. Due to the chemical reactions, the solution can now contain four possible salts: sodium chloride, potassium chloride, sodium nitrate and potassium nitrate. As it happens, when the temperature of the solution is high, potassium nitrate has the highest rate of solubility (i.e. it dissolves easily in water), but sodium chloride has the least solubility. At low temperatures, potassium nitrate has the least solubility. This fact is exploited to filter out the crystals of the other salts out. The liquid is boiled for about half an hour to complete the reaction as much as possible and convert most of the salts to sodium chloride and potassium nitrate. Then it is run through a filter into shallow cooling tanks. Since sodium chloride crystals don't dissolve as well in hot water, a good amount of it gets filtered out, while the potassium nitrate (which dissolves well in hot water) passes through in the solution. The solution is kept stirred while it cools, so that the potassium nitrate may form smaller crystals. The crystals are then drained and washed with the liquors from the next crystallization using a centrifuge. The crystals still contain a fair amount of sodium chloride, so they are purified by washing in cold water, which dissolves most of the sodium chloride, but leaves most of the potassium nitrate undissolved, which reduces the percentage of sodium chloride to below 0.05 percent. Then the remaining potassium nitrate crystals are dried and ready to be used for gunpowder production.

During the First World War, demand for Chilean nitrate exports skyrocketed. Before 1914, only one-fifth of Chilean nitrates were used for explosives, but after the war started, almost four-fifth of all nitrate exports were used for military purposes. Before the war, Germany was the largest market for Chilean saltpeter. For example, in 1912, Germany imported 37.9 % of all Chile saltpeter exports, England had 5.7% and the rest of Europe had 31.6%, which means that Germany alone imported more than all of Europe combined! The United States imported about 23.6% of Chile saltpeter exports during the same year. At the beginning of World War I, German Admiral Maximilian Reichsgraf von Spee commanded a fleet of ships off the coast of Chile in an attempt to disrupt Chile saltpeter supplies to everyone else. After the German fleet was destroyed in the Battle of the Falkland Islands, the exports to Britain resumed. Germany was blockaded from accessing Chilean saltpeter from 1915 onwards, leading to the United States and the UK becoming the largest markets for Chilean saltpeter. Because of this, Germany was forced to discover a method to produce synthetic nitrates. The first breakthrough came from Fritz Haber, who discovered a way to produce ammonia from nitrogen and hydrogen in the air and water (neither of which could be blockaded by the allies). Then Wilhelm Ostwald discovered a method to convert ammonia to nitric acid and from then on, Germany's military industry was free from its dependency on Chilean saltpeter. In fact, if it weren't for these two discoveries, the shortage of munitions would have forced Germany to end the war by 1915, instead of prolonging it till 1918. 

Ultimately though, the German innovations forced the decline of the Chilean saltpeter mining industry, as more and more countries began to produce synthetic nitrates using the techniques that they pioneered. At one time, Chile saltpeter accounted for 50% of Chile's Gross National Product, but by the time the 1940s rolled around, it had fallen to practically 0. Of the 170 or so "nitrate towns" that were formed in the Atacama desert to mine for nitrates, only one of them remains open today, the town of Maria Elena.

The History of Saltpeter - XVIII

In our last post, we saw how the British secured their supply of saltpeter from the largest source in the world. In today's post, we will look at how they refined saltpeter over in England. We will study the refining process at the The Royal Gunpower Mills at Waltham Abbey.

We had actually studied a bit of this process earlier, when we studied black powder several months ago. In today's post, we will look at the process in more detail.

Waltham Abbey is a small town north east of London and fairly close to it. The river Lea flows through this area and there is a large church in town. There was once a large monastery here and the monks had diverted some of the waters of the river towards a watermill that they built to process wool for cloth production (a procedure called fulling). In the early part of the 1600s, the mill was converted into an mill for producing vegetable oils. Around 1665, the Second Anglo-Dutch war started and there was a shortage of gunpowder in England, therefore the oil mill was converted to gunpowder production. At around the same time, it was acquired by a man named Ralph Hudson. At the end of the 1600s, the Hudson family sold it to William Walton and the Walton family ran it successfully as a private enterprise for almost 100 years. In 1787, it was decided that the British government should acquire the mill from the Walton family, to ensure the supply of gunpowder and establish standards for quality, at which point it became the Royal Gunpowder Mills.

Our description of the process at Waltham Abbey comes from a book written in 1915 by Arthur Marshall, who served as a Chemical Inspector, Indian Ordnance Department.

The Royal Waltham Gunpowder Mills acquired its saltpeter exclusively from India. The crude saltpeter (also called "grough saltpeter") was produced in India and then packed in jute bags and shipped over to England. At Waltham Abbey, the process was as follows:

The crude or "grough" saltpeter is dissolved in a larger pot A, which has a capacity of 500 gallons and is fitted with a perforated false bottom, which prevents the saltpeter adhering to the vessel. For each batch, about 25 cwts. (2800 lbs. or 1270 kg.) of grough saltpeter are taken, and 5 cwts. (560 lbs. or 254 kg.) of crystals recovered from liquors, and 5 cwts. of crystals left in the crystallizing cisterns. This is all dissolved in about 280 gallons (1060 liters) of the washings of the purified saltpeter, which also contains a considerable amount of the salt. The fire is lit under the pot, and in about two hours the saltpeter is dissolved and the liquid boiling. Just before it boils, a thick scum rises to the surface, consisting mostly of impurities. This is skimmed off and the false bottom is removed, and cold water is added from time to time to induce fresh scum to form, if it will. The fire is then withdrawn and the the liquid is allowed to settle for about two hours. Then a hand pump is lowered into the pot and the liquid is pumped into filters B, where it passes through linen cloth. From here it runs to shallow copper crystallizing troughs C. As it cools down, the liquid is stirred by a workman in order to make the saltpeter separate into small crystals, and the saltpeter "flour" as it forms is drawn up on to an inclined draining platform D, and from there  is passed to a washing vat E. After the temperature has fallen to about 32 °C. (90 °F.), the solution is no longer stirred and any crystals that form after that are treated as grough niter.

The washing vat E is about 6 feet long, by 4 feet wide, by 3.5 feet deep, and is fitted with a false bottom made of wood with small  holes bored in it. Below the false bottom is a plug which can be removed to allow the washings to flow away. First, the charge is washed with 70 gallons (264 liters) of water sprinkled over it by means of a rose, the plug being left out so that the washings can drain away into liquor tank F. After draining half an hour, the plug is inserted and the saltpeter is covered with fresh water, which after standing for half an hour is also allowed to drain into F. Finally the salt is washed by sprinkling 100 gallons (378.5 liters) of water, the plug remaining out. The saltpeter is now allowed to drain all night and is then removed to the store house where it is allowed to dry spontaneously. In about three days, the moisture has fallen to 3 or 5 percent.

The mother liquors and other impure solutions are boiled down to about a quarter of their original volume. Any scum or deposit that forms during the boiling should be removed and water then be added. The solution is now filtered and allowed to crystallize. The crystals are treated as grough saltpeter and the mother liquor returned to the evaporating pots.

The author mentions that the methods of refining adopted in France, Germany and other countries are substantially the same as that of Waltham Abbey. He also mentions that a small amount of size (a gelatinous solution) is often added to the pot to assist in the formation of scum.

In our next post, we will look at the production of charcoal in some detail.

The History of Saltpeter - XVII

A few posts ago, we left off with the Portuguese discovering a sea route to India and Asia, followed by other European powers, notably the English East India Company, which started exporting saltpeter from India to England. In today's post, we will look into some of the history of how the English managed to secure their sources.

During the first few decades of European exploration in Asia, most countries were interested in trade mainly. In fact, most European countries were not interested in colonization until the late 18th century. The only reason that some of them started spending for military hardware initially was to protect their trading interests from their other European rivals. The one exception to this rule was the Dutch, who were already fighting a war in Europe against the Spanish and the Portuguese in the 1600s and attacked their overseas bases as well, aiming to destroy them and cut into their trading networks. Gradually, the Dutch took over most of the Portuguese bases in the East Indies and replaced them as the largest European traders in Asia.

Meanwhile, India was ruled by many rulers, each with his own small kingdom, and the English negotiated with many of them separately and established small trading posts in all their kingdoms. There were occasional diplomatic spats with some local rulers, but on the whole, peace reigned in about 30 different trading posts around India, and other European countries also had their trading posts in the same areas as well. Then, over in Europe, the War of Austrian Succession (a.k.a., the Seven Year War) started and France and England started attacking each others' overseas settlements as well. Over here in North America, the 3rd French and Indian war (a.k.a. King George's War) was part of this conflict. In India, the English didn't have much manpower at all (they had less than 2000 men distributed over 30 different trading posts in the entire country), and the French attacked and captured the British port of Madras in southern India without too much trouble, as it was defended by fewer than 100 Englishmen at that time. One of the English employees of the East India Company in Madras at that time was a young clerk named Robert Clive, who had only arrived a couple of years earlier. He managed to escape the French prison camp and headed off to another British post in the area. Remember that name, as we will hear of him again shortly:

Lord Robert Clive, after he became famous. Public domain image.

Soon afterwards, a local ruler, who was allied with the English, attempted to recapture Madras on their behalf. However, his large army of 10,000 men were easily defeated by the small French forces in a little known action called the Battle of Adyar River. This particular battle had some very important implications for the future. The French used 300 of their own soldiers and 700 soldiers recruited from the local Indian population and they used a classic European tactic of lining up three ranks of men to successively fire salvos of musket fire and charge up the field, quickly scattering the much larger enemy army facing them. Using local people alongside European soldiers was not a new concept as most European countries had done it already: the Portuguese and Dutch recruited the Ambonese in Indonesia and Spanish allied with the Pampangas in the Philippines, English and Dutch armies allied with Iroquois in North America, while the French did the same with the Huron, Shawnee, Ojibwe etc. Many also used native converts to Christianity and half-Europeans as well. However the main difference was that until this battle, these native troops fought with their own traditional tactics and weapons and were used as auxiliary troops rather than regulars. What the French had done for the first time was train local Indian soldiers to use European weapons, uniforms and tactics. In effect, they had shown that local people could be reliably trained with European tactics and were just as effective in battles, which reduced the burden of bringing over larger numbers of troops from Europe. Meanwhile, the English were watching and learning. Soon after, they started doing the same thing as well, forming units of Indian troops trained in European tactics. Meanwhile, in Europe, the Seven Year War ended with the treaty of Aix-la-Chapelle, whereupon the English returned Louisburg to the French in North America and the French returned Madras back to the English in India (much to the disgust of Joseph-Francois Dupleix, the French general in India at that time).

Over in Bengal, the English and the Dutch formed agreements around the 1720s, to buy saltpeter from Indians together as a single company, so that they wouldn't bid up against each other and drive up the prices. In 1735, the French showed up in the area as well, and the English and Dutch tried to convince the local ruler to restrict the amount of saltpeter to be sold to the French. Meanwhile, larger Indian traders got a whiff of what was going on in Europe and started to move into the business as well.  Therefore, in 1736, the three countries agreed that no Indian traders in their area would be allowed by buy saltpeter around the town of Chapra, where they had their bases. However, in 1740, a rich Indian merchant named Omichund (also spelled as Omichand, Umichand, Omychand and Amir Chand (probably his real name) in some documents) was involved in some intrigue, which resulted in the English and the Dutch in Chapra supporting two opposing local factions and as a result, started bidding for saltpeter separately, contributing to a rise in the price. Moreover, this Omichund had a brother named Deepchund, who became the local big-wig in Chapra by 1745 and forced the local suppliers of saltpeter to sell their stocks to him alone. This meant that the English and Dutch were forced to buy from the brothers. Another saltpeter merchant king of that area was an Armenian named Khwaja Wajid, who had started as an agent of Deepchund. Between the three of them, they manipulated the saltpeter market to their benefit. The English did not forget this easily.

By 1756, the local ruler of Bengal began to have some disagreements with the English and captured Calcutta. The young former clerk, Robert Clive (remember him from above), had risen in the ranks of the East India Company to become an able commander and he was tasked with recapturing it. Meanwhile, the French and English were at war in Europe again and therefore, their bases in India were also at war with each other. After recapturing Calcutta, Clive headed off to the nearby French port of Chandernagore to try and capture it, along with Admiral Watson. Naturally, the ruler of Bengal, Siraj-ul-Dullah, attempted to come to the aid of the French, as he was opposed to the British at that time. Meanwhile, his commander-in-chief, a man named Mir Jafar, planned to depose him and rounded up a few fellow conspirators and started negotiating with the English for their support in this plan. Clive sent a local trader to negotiate between Mir Jafar and the English and the person he sent was (surprise, surprise) Omichund! As it turns out, this Omichund tried to work out a deal for himself, threatening to reveal the whole plot to Siraj-ul-Dullah unless the agreement included a clause for a substantial payment to him. To dupe him, Clive prepared a second document with this extra clause included in it, which he signed (but Admiral Watson flatly refused to). Clive then led a mixed army of 1100 English and 2200 Indian soldiers (they had learned well from the French that native troops could be effectively trained to fight with European tactics) against the ruler of Bengal in the Battle of Plassey. Mir Jafar and two other traitorous commanders ordered the soldiers under their commands to stay away from the battlefield, allowing the English to win the battle and depose Siraj-ul-Dullah. This allowed Mir Jafar to become the new ruler of Bengal. He stuck to his bargain and soon afterwards, the East India Company were granted much more favorable trading rights than their European competitors in the Bengal region (remember, this is the part of India that had the best quality saltpeter production in the world). They also managed to cut out Omichund, his brother Deepchund and the Armenian merchant, Wazid, out of the saltpeter trade altogether. This was the beginning of the British getting involved in political matters in India.

A few years later, the British deposed Mir Jafar and took over the whole of Bengal and started to spread their influence throughout India, but it was the one-time clerk, Robert Clive, who kicked off the whole thing. Robert Clive amassed a fortune and became a Lord in England (though he had to admit the embarrassing situation of forging the document that he showed Omichund to the English parliament later in life) and the rest, as they say, is history.

As the British gradually started extending their influence through India, they could now dictate to the saltpeter suppliers to not sell it to anyone else. And since Indian saltpeter was cheaper than anywhere else in the world, they could effectively price competitors out of the market as well. Soon, they had a monopoly and used it to supply their allies (Sweden, Portugal etc.), while keeping it out of the hands of their enemies (French, Russians etc.). During the US Civil War, they were able to supply saltpeter to both sides and control the action. In fact, a diplomatic spat between the Union and Britain during the Civil war, caused the British to delay a gigantic order of thousands of tons of saltpeter to be sitting at the docks, instead of being shipped to America. This caused C.H. Davis of the US Bureau of Ordnance, Navy Department, to draft a letter to the US Congress in 1862, urging them to fund exploration to find their own sources of saltpeter.

In our next post, we will look at the refinement process of saltpeter in England at the Royal Mills at Waltham.

The History of Saltpeter - XVI

In our last post, we saw a brief description of how crude saltpeter was refined into a higher quality product in India. In today's post, we will study a detailed description of the process from an actual factory. Our source of this description is an article by Mr. David Hooper, published in The Agricultural Journal, Volume 3, 1905. The author says that his observations are from visiting a refining factory in a village called Jajmou near the city of Cawnpore in the United Provinces of India (now the suburb of Jajmau in the city of Kanpur). Later on in his article, he also mentions the owner of the factory as one Mr. Gurmuk Rai Durga Pershad (or more properly spelled as Gurmukh Rai Durga Prasad). As was mentioned in our previous post, European merchants found it more convenient to deal with the larger Indian refiners, who had well-established connections with the saltpeter men (nuniahs) in all the small villages for generations.

Your humble editor has reproduced the article almost verbatim in red italic font, but has taken the liberty of correcting some spelling from British to American English and has also translated some of the units of measure to both imperial and metric units for the benefit of readers of this blog from around the world. Furthermore, your editor has also in-lined some of his own notes about the process in blue italic font

The Refining of Saltpeter

A saltpeter refinery consists of a large fenced yard with office and godowns and sheds for the factory occupying several acres of land. One portion of the yard is covered with earth suitable for crude niter production. When a refinery is first established, niter earth is obtained (from the nuniahs) and spread on a part of the yard. The salt from the nitrous earth obtained from this area is extracted in the ordinary way with water, and the exhausted earth is spread out on this portion of the yard to receive the furnace ashes and nitrous by-products from time to time. The ashes, soil, and washings are mixed intimately, and fresh niter is constantly generated from the "factory soil." It is a common opinion that such earth is better than new earth collected from outside. At any rate, it is a continual source of crude niter to the refiner, and it enables him to use to the best advantage, all the products of his factory which otherwise might be wasted. (Like the nuniahs, these refiners also tried to extract the maximum amount of nitrate possible from the soil)

A saltpeter refining factory in Kanpur, India. Click on the image to enlarge. Public domain image.

The accompanying is a sketch of a refinery in Jajmow, Cawnpore, in the United Provinces (now called the suburb of Jajmau in the city of Kanpur in the state of Uttar Pradesh, India). The yard is enclosed with a high mud wall and gate. One portion of the yard (M) is covered as described above with lunamatti or niter earth. At the left corner, there are two pairs of filters or kurias (KK) for extracting crude niter from nitrous earth. A well (W) supplies the water for this process, as well as for making solutions for the refining process. There are four iron evaporating pans (PPPP) supported on masonry fireplaces. Here the niter liquor is boiled. Near each pan is another empty pan or wooden vessel to serve as a settling tank. From this, the liquid is transferred to the crystallizing tubs (C) arranged under the sheds. These tubs are so arranged that each day as two or more are filled, two or more are emptied, and the crystals collected. The round tubs are for making crude niter or small refined crystals; the larger oblong vessels are for the production of the higher quality or kalami saltpeter. One of the most important utensils in the refinery is a boiler or iron pan for evaporating the liquor. The pan is from 10 to 12 feet (3.05-3.66 meters) in diameter and costs Rs. 260; if well made, one will last ten years. It is supported on a brick-and-chunam (limestone) furnace, which is 25 feet long, 15 feet broad and 4 feet deep (or 7.62 x 4.57 x 1.22 meters). Two sloping slides enable men to carry the crude niter to the pan. In the front is the door of the furnace. At the other end nearest the sheds is a cistern of solid masonry or a spare pan. Under the sheds are arranged the crystallizing vessels, which are wooden oblong tanks, 7 feet long, 5.5 feet broad and 2 feet deep (2.13 x 1.68 x 0.61 meters), where the niter crystals form.

The process followed varies in different refineries and in different parts of the country. But as the chloride of sodium is the principal impurity and as its solubility is practically constant, all the processes followed are based on the varying solubility of nitrate of potassium in hot and cold solutions. (We talked about how potassium nitrate  and sodium chloride dissolve differently in water of different temperatures in our previous post.)

To start a refinery, the niter earth obtained from the factory soil is filtered in the two pairs of kurias. The crude niter solution obtained from these is boiled down, clarified by sedimentation and set out to crystallize. In from six to ten days the crystals are extracted and the residual tor or mother-liquor is then available for future use. Crude saltpeter is dissolved in this mother-liquor to which sufficient water or washings are added to keep up the volume. The main supply of crude niter is obtained by purchase from small manufacturers (the previously mentioned nuniahs, who were responsible for making nitrous earth and crude saltpeter crystals and sold their products to the larger refiners, who had business connections with them for generations). A well near the center of the yard supplies sufficient water, usually of a saline character for the operations. When crude niter is dissolved in tor or mother-liquor and the solution is concentrated by boiling in the large evaporation pans, a dirty white granular substance known as sitta falls into the bottom of the pan. The sitta as it forms is removed by means of a large iron spade fixed to a handle 6 feet (1.82 meters) long. The sitta thus obtained is sometimes washed and the washings are returned to the pan. In Bihar, where sitta is not excised, it is mixed with the refinery earth. About 2.5 maunds of sitta is separated from each pan of liquor. At a  factory near Cawnpore, the proportion of sitta was said to be 20 percent of the crude saltpeter.

The evaporation of the liquid in the pan is continued at the temperature of boiling water. In some factories, the froth or scum, called zag, zoga, mail or phain, is removed from the surface at this stage, in others it is removed after transfer to the settling tank. After boiling for three hours, or until the liquid changes from a dark to a light yellow color, the concentration is considered complete. The liquid is emptied out of the pan by means of an iron scoop known as a dal hung at four corners by ropes. Two men stand on opposite sides, each holding two ropes. They deftly raise the liquid in the dal from the pan and pour it into the wooden trough which leads it to the settling tank. Here the hot liquid is allowed to settle for about 2 hours. The scum or zag is taken off with an iron perforated jhara, and the clarified liquor is decanted, or siphoned off with a bent brass tube, into one or more crystallizing vats. At the bottom of the settling tanks is found a substance called matiaree, which is a by-product containing nitrates, and is accordingly carried off and mixed with the nitrous earth in the factory yard. The crystallizing vats under the sheds are filled with niter liquor to about 6 inches from the top. In the United Provinces, on the surface of each is floated a trellis work made of interlaced bamboo sticks (called tattis in Cawnpore). This device facilitates the formation of good crystals. (This is because the bamboo stick framework provides good nucleation sites for crystals to form) After seven days, the bamboo frames are removed and the adhering niter crystals are shaken or picked off, and the crystals at the bottom and sides of the trough collected into a heap and drained. At Kheora, Cawnpore, troughs of two sizes are used. There are some 3 by 5 feet (0.91 x 1.5 meters), which require the liquor to remain eight days, and others, 6 feet (1.82 meters) square, where the liquor remained ten days. The larger the vessels and the longer the liquor stands, the larger and longer are said to be the crystals. 

The damp saltpeter is contaminated with the mother-liquor adhering to it, and minute crystals of salt (i.e. sodium chloride), and these must be removed by washing before the salt is ready for the market. Plain water is used for this purpose. Alum is occasionally used for the same purpose as indigo blue to whiten the saltpeter. Alum is also used in admixture with saturated nitrous liquor before it is run into the crystallizing vats, in order to precipitate matter in suspension in the liquid. Bags containing the refined substance are placed over an empty tub or vat, which is slightly tilted to allow the liquor to drain. Cold water is sprinkled from time to time upon the saltpeter through the open mouth of each bag. The water trickles slowly through the saltpeter crystals carrying with it inferior salts in solution. Some saltpeter is also dissolved, but the loss is not great. (This is because potassium nitrate does not dissolve as easily in cold water as it does in hot water; whereas the solubility of sodium chloride is more or less constant. We discussed this in our previous post.)   After the washing, the refined saltpeter is spread out and dried, and after remaining a few hours, is conveyed to the store godown.

The mother-liquor or tor from the crystallizing vats and all washings of the refined saltpeter, and of the settling and setting vats and of sitta are returned to the evaporating pans and used for dissolving fresh crude niter. It is thus seen that the utmost economy is practiced at every stage of the refining process, and, practically speaking, no niter is wasted.

One evaporating pan is capable of dealing with two boilings (40 maunds (3291.44 lbs. or 1492.97 kg.)  of crude niter) per day. The boiling begins early in the morning and is finished by midday. It is calculated that one maund of crude niter according to its quality will yield from 15 to 23 seers (37.5 to 57.5 percent) of refined niter.

The fuel used at Hansi is cotton stalks, and costs Rs. 1 per day. At other factories, other cheap fuel, such as dried castor stalks and wild shrubs, is used.

The total output from the refinery described is 2,800 to 3,000 maunds (230,400-246,858 lbs. or 104,508-111,973 kg.) in a season, but the output from any refinery will vary with the quality of the crude niter.

The author goes on to mention that a sample of saltpeter refined from a factory similar to this yielded the following in a chemical analysis:

• Insoluble substances: 0.08%
• Water: 1.97%
• Sodium Chloride: 0.69%
• Potassium sulfate: 0.04%
• Potassium nitrate: 97.22%

This was sufficient purity for manufacturing cordite in the Government factory, which only specified that the sample must contain not less than 95% potassium nitrate and less than 0.85% sodium chloride.

The average amount of saltpeter (both crude and refined) exported from India to the world kept rising until it reached a high mark in the 1850s of about 30,000 tons yearly, falling down to about 20,000 tons yearly by the early 1900s, with the advent of smokeless powders and the discovery of nitrate deposits in Chile. In our next couple of posts, we will study how the British managed to secure their supplies of saltpeter in India, as well as the refinement process of crude saltpeter from India in England's Royal Waltham mills.

The History of Saltpeter - XV

In our last couple of posts, we saw haw crude saltpeter was produced in various parts of India. In today's post, we will study the process of refinement that was undertaken there.

Crude saltpeter produced by using artificial heat sources for evaporation is generally more pure than that produced using solar energy for evaporation. This is because when artificial heat is employed, some impurities can be removed by skimming off the scum that floats to the top of the liquid when boiling it, and other impurities precipitate at the bottom of the vessel when the concentrated brine is allowed to settle and can be removed by passing the liquid through a fine cloth filter. By contrast, when the heat from the sun is used to evaporate the liquid, nearly everything that can crystallize is collected together, which includes many impurities as well.

Crude saltpeter produced by the processes detailed in our last two posts had a wide variety of purity. An analysis printed in the Agricultural Ledger of India (Volume 12) in 1905 of  55 samples of crude saltpeter from different parts of India, showed that the potassium nitrate content varied from as low as 26.8% to as high as 80%, with the average percentage predominantly at around 53%. The chief impurity in all the crude saltpeter samples was common salt (a.k.a. sodium chloride or NaCl). Some samples from Bihar showed a bit of sodium sulfate and many of the inferior samples contained quantities of dirt and other insoluble substances. Because of the impurities present in it, crude saltpeter is yellowish in color, instead of white.

A pure saltpeter sample. Click on the image to enlarge. Public domain image.

In India, the refinement of the saltpeter was generally carried out by the Mahajan community (the word 'Mahajan', while being a last name in India, is a generic title for people involved in money lending and financial services.) The Mahajans would advance money yearly to India's saltpeter men (the previously mentioned nuniahs) at 12% interest and during the hot months of April, May and June, they would be on the lookout for any crude saltpeter produced by the nuniahs and carry it off to their own factories for final refinement. If a nuniah happened to produce more crude saltpeter than what could cover his loan advance, the extra stuff would be sold off clandestinely to other petty purchasers. Since the trading networks were spread out too widely across India, to make it worth the while of Europeans with capital to attend to; it was mostly left in the hands of large Indian merchant houses, who had their small branches in every tiny village in the area and would collect the crude saltpeter, refine it and then resell it to Europeans. This trade practice was in use for hundreds of years by several generations of Mahajans, and the English were content to leave the existing system in place (as per the diary of a French adventurer, Jean-Baptiste Tavernier, some Dutch merchants unsuccessfully tried to cut into the refiners business and soon found that no one was willing to sell to them.)

An experienced person can determine fairly accurately, the amount of potassium nitrate in a crude saltpeter sample, just by looking at it, since potassium nitrate crystals are small elongated prisms, whereas common salt (sodium chloride) crystals are shaped as cubes. David Hooper, in his notes about Indian saltpeter production in 1905, mentions that he witnessed an an expert dealer, a Mahajan from Kanpur, purchasing some crude saltpeter and declaring that it was of superior quality and paying an appropriate price for it. When a sample of his purchase was later chemically analyzed, it turned out that the expert dealer was pretty accurate on his estimate of how much potassium nitrate was in it (the lab test showed 67.73%). He also mentioned that the prices of crude saltpeter tend to vary depending on the season and their cheapest price is just before the rains, because the crude saltpeter produced at this time is of inferior quality, owing to the conditions of temperatures under which it is produced. This is also the time that the refiners would buy extensively, because crude saltpeter could not be made during the rainy season.

In a saltpeter refinery, the process was similar to that carried out by the nuniahs to extract crude saltpeter, as was described a couple of posts ago. They would use nitrated earth, which was worked over and over again, with weak liquors being thrown on to it. However, after the strong liquors were collected from the filters, they were not evaporated by themselves. Instead, additional crude saltpeter crystals (which were largely bought from the nuniahs) were dissolved into the liquid at boiling point. Potassium nitrate dissolves differently in water at different temperatures, whereas sodium chloride (the primary impurity) dissolves at a constant solubility. This means that at lower temperatures, potassium nitrate doesn't dissolve in water that much, but at higher temperatures, potassium nitrate has great solubility in water and sodium chloride has much lesser solubility. Therefore, at boiling point, the potassium nitrate dissolves into a saturated solution, whereas most of the sodium chloride crystals remain undissolved, along with other impurities (dirt, minerals etc.) that don't dissolve in water. Then, it can be filtered and allowed to settle and cool down, to deposit more purer crystals, which can then be washed in cold water to clean them (which dissolves some of the potassium nitrate, but not all, and the cold water can be recycled to extract the nitrates). The resulting crystals are about 95% pure potassium nitrate.

In our next post, we will look at a more detailed description of the refinement process, with sketches of a typical factory.

The History of Saltpeter - XIV

In our last post, we studied how saltpeter was produced in the eastern regions of India, namely Bihar and Bengal. In today's post, we will study how it was produced in some of the other drier regions in India in the northern, central and southern parts.

Saltpeter Crystals. Click on the image to enlarge. Public domain image.

In our last post, we noted that the saltpeter production in the eastern regions of India was mainly done by a caste/tribe of people called nuniahs or luniahs, who specialized in this type of work. In the other regions of India, saltpeter production was done on a part-time basis by farmers, potters etc. These people would sell the high quality saltpeter crystals to others and keep some of the nitrated earth for themselves, to use as fertilizer for their fields. 

The following description is taken from The Agricultural Ledger, Volume 12, published in 1905, by the Office of the British Government in India, based on a report by David Hooper. The author based his description on a personal study of the saltpeter industry at Hansi, in the Hissar district of northern India, during the hot summer of 1902.

In this district, the nitrated soil is collected at Hansi fort (also known as the Asigarh fort, because of a historic sword manufacturing factory within its walls), an ancient ruin existing since about 700 AD or so, and rebuilt in the 12th century.

Views of the ruins of Asigarh fort at Hansi, India from different angles. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution-Share Alike 4.0 International License by Amrahsnihcas 

A painting of Asigarh Fort and its surroundings at Hansi by Sitaram in 1815

The soil around the walls and moat of this old ruin contains nitrates and a sum of 300 per annum is paid to the Government for the privilege of collecting it. The earth is transported by donkeys, to the factory situated by the side of a main road outside of town. The arrangement of the beds for leaching the nitrous earth and evaporating the nitre liquor is shown in the following sketch:

A = Beds or filters (kurias) for the filtration of the nitrous earth.

B = Beds (patas or kiaris) for evaporating the nitre liquid.

C = Channels to conduct the liquid to the evaporating beds.

R = Jhela or reservoir.

P = Pit for storing the saltpeter

W = Well for supplying water.

The kurias (or filters, we encountered this word in our previous post) are 25 to 30 feet (7.6 - 9.1 meters) in length, 6 feet (1.8 meters) in width and 1 foot (0.3 meters) in depth. There are two of the kurias, which are sometimes sub-divided and arranged in two rows, running parallel, and situated on a broad hillock raised 3 to 4 feet (0.9 - 1.2 meters) above the ground. The beds are made of plastered clay or lime and are practically water-tight. The two evaporating beds are built on the level ground, and have concrete floors and sides. These are about 6 inches (0.15 meters) deep  and 25 to 40 feet (7.6 - 12.2 meters) square. They communicate with one another, and the smaller bed, which is raised slightly above the larger ones and is nearer the mound, serves as a reservoir for collecting any nitre water that is not required by the other beds.

The nitrated earth is carried to the kurias and is packed in them to a depth of 8 inches (0.2 meters). It is sometimes mixed with ashes in order that the soil may remain open and porous when the water is added, and possibly also with the object of decomposing the calcium and magnesium nitrate with the carbonated alkali. When the packing of the earth is complete, the water from the well (W in the figure above) is baled up by earthen pots and poured over the nitrous soil and is allowed to filter slowly through it in order to dissolve the saline matter. The saturated liquor flows off in a small stream, through the concrete channel, into the large shallow evaporating beds. Meantime, the other bed is filled as described with earth and water, and filtration and drainage go on regularly in rotation in the filters until enough liquid is obtained to fill the lower evaporating beds. The exhausted earth is removed from the kurias when the water extract has been fully drained off.

As the weather in this part of India is dry and hot during the summer, the liquid is allowed to evaporate in the shallow beds (B in the figure above), due to the heat from the sun. As the yellowish liquid in the evaporating beds becomes more concentrated, the nitre begins to crystallize at the sides and bottom, and after about seven days, most of the nitre has solidified and it is raked together into parallel ridges along the length of the bed about 3 feet (0.9 meters) apart. The mounds of crude crystals, after further drying, are collected together into heaps and then carried in baskets to a pit made in the ground a short distance away. The evaporating kurias are never allowed to become quite dry during the working season, in order to avoid cracking; as soon as the damp crystals are removed to the pits, fresh nitre liquid is run in from the reservoir, and evaporation is continued. Each kuria is said to yield 20 to 30 maunds (1645 - 2470 lbs. or 745 - 1120 kg.) of crude saltpeter per week. The nitre prepared in this manner is placed in the storage pit until it is sold. 

This method of preparing saltpeter using the heat of the sun, was practiced in the drier parts of India, where the climate permitted it. However, the crude saltpeter obtained by this method was not considered to be of as good quality as the crude saltpeter produced by artificial heat (which we studied in the previous post).

The crude saltpeter produced was then refined in a larger factory. We will study the process of this in the next couple of posts.

The History of Saltpeter - XIII

In our last post, we saw how various European countries realized that the eastern regions of India were capable of supplying good quality saltpeter and they all fought to establish trading posts there. In today's post, we will study the process of manufacture there in some detail and also why this region produced so much high quality saltpeter.

A few weeks back, we'd studied some details about the conditions necessary for saltpeter crystals to form. We will revisit that now in some detail. The main factors needed are:

1. Organic material: Large supply of plant and animal organic material that must be decomposed: cattle manure, plant parts, rotting wood etc.
2. Temperature: The bacteria that perform the nitrification process become active at 54° Fahrenheit (12.22° Centigrade) and the activity increases as the temperature rises, until the temperature hits about 99° F (37.22° C), where the nitrification process reaches its maximum rate. Higher temperatures than this reduces the rate of the nitrification process. Under the right conditions, all other things being equal, 10 times as much saltpeter is obtained at 99° F, than at 54° F.
3. Moisture: Water is necessary for the bacteria to proceed with the nitrification process. If there is drought, this will stop the nitrification process. The water holds dissolved saltpeter and other salts in the solution, and as the water evaporates in hot weather, the salt deposits on the surface of the soil.
4. Oxygen: This is also essential for the bacteria to do their job. If the surface soil is sufficiently aerated, it allows the formation of saltpeter for a few inches of depth.
5. Darkess: In the initial part of the saltpeter formation process, the bacteria should not be exposed to direct sunlight, in order for optimum performance.
6. Calcium Carbonate (limestone): The presence of calcium carbonate in the soil aids the process of nitrification. 
7. Time: The nitrification process of the bacteria takes a few months to complete, during which all the above factors should be present. 

As it happens, large parts of India were agricultural in nature, which meant availability of large amounts of wood, plants and cattle. This was particularly so in the eastern regions of Bihar and Bengal, which are plains with the Ganges river flowing through the middle of it, where most of the land was cultivated agricultural country, with a high proportion of domestic animals. These regions have the characteristics of hot summers, strong monsoon rains and presence of limestone in the soil. The farmers living in these areas were also working in conjunction with saltpeter makers for centuries and they had the whole process of manufacturing in sync with the rhythm of the seasons. Other regions in India also had many of these factors, but not as well as Bengal and Bihar. In particular, just three districts in Bihar: Tirhut, Saran and Champaran, were responsible for over two-thirds of all the saltpeter being exported from Calcutta to England annually in the nineteenth century. We have many accounts of the process written by authors such as R. W. Bingham, David Hooper, Arthur Marshall, Leather, Mukherjee and others, about how the industry worked.

In Bihar and Bengal, a special caste of people called Nuniahs or Luniahs (depending on region) were in charge of crude saltpeter production and another group called Mahajans were engaged in refined saltpeter production. In other parts of India, such as northern Punjab or the United Provinces, ordinary villagers undertook to produce saltpeter. In some other regions of India, potters would work on saltpeter production as well. The cycle went as follows:

During the rainy season, which lasts from June to October, the process of nitrification happens on the warm, moist soil, aided by the addition of organic refuse. This refuse was typically cow dung, wood and straw. Since cow dung and wood were used for fuel in rural India, villagers would collect pats of dung and straw and stick them onto the sides of huts to use later. The nuniahs would also plough special areas in the villages and spread an earth called 'sithi' on top and they had agreements with the richer villagers to let their cattle stand there for half an hour each morning and evening, so that their dung would drop on these special areas. Typically, the soil in these special areas would be so salty that ordinary crops would not grow there anyway, and the nuniahs wouldn't have to go digging about in the regular fields where agricultural crops were grown, thereby keeping everyone happy. The soil's natural supply of inorganic bases was increased by the villagers throwing fuel ashes onto the soil outside their houses and in the special areas. The villagers were paid by the nuniahs in advance, for doing this work during these months and allowing access to the specially prepared areas on their land. 

During the end of October and early November, while the rains stopped and fine weather returned, the nuniahs would visit the villages and scrape the walls of the huts and the surface of the soil of the prepared areas up to the depth of one or two inches, using ordinary spades, broken tiles or even shards from a broken pot. They would carry this nitrated soil away in baskets to be stored elsewhere. The nuniahs would return to each prepared field every four days or so (in some areas, they would come every fortnight) and repeat the process, until they had collected as much nitrated earth from the leased areas as they could before the rains returned. They would store this soil (which is rich in nitrates) in 2-4 feet high conical heaps under long sheds about 40x25 feet (or around 12x7.5 meters) where it would be protected from the weather and the precious stuff could not be washed away by the monsoon rains. The sheds would also protect the nitrated earth from direct sunlight and allow the bacteria to do their work for a few months. The amount of nitrated earth collected during this season was enough to easily keep the nuniahs and their families busy for months afterwards, doing the next stage in the process.

During the hot season months of April, May and June, the filtering and boiling process would start. The nuniahs would build earthen filtering chambers (percolators) called "kuria" or "kothi" with wet mud, which was allowed to dry. Each earthen chamber would either have circular walls about 5 to 6 feet (1.5 - 1.8 meters) in diameter or oblong walls, and a floor which slopes slightly from back to front. In the front wall is a hole at the level of the bed, under which a large earthen pot is buried, and the hole allows the nitrate liquor to drain into the pot. Above the bottom of each earthen chamber, a false bottom is laid, consisting of brushwood, bamboos and matting placed on a few loose bricks. 

A set of earthen filtering chambers (kothis) used for filtering the saltpeter. Click on the image to enlarge. Public domain image.

In the above image, you can see the man building a series of circular filtering chambers. The false bottom is clearly visible in the chamber in front. Also note the series of buried pots on the left side of the image, one in front of each filtering chamber, to collect the liquid coming out of the filters. The nitrated earth which was stored in the sheds for the previous 5-6 months would now be carefully packed into these chambers, as shown in the image above. Stones are removed from it as far as possible, and the nitrated earth is put in slightly moist and trodden down so as to leave no channels, through which water can run too rapidly or settle in. Wood ashes are generally mixed with the earth, so that the potash in them may convert into saltpeter, the nitrates of lime and magnesia. A small piece of matting is  placed on top of the nitrated earth and water is cautiously poured on top. The purpose of the matting is so that the water that is added distributes evenly. After about an hour or so, the water  passes through the filter and becomes a very strong nitrous brine solution, which trickles out of the hole in the bottom into the collecting pot. The first batch of liquid that percolates through the filter is more concentrated. More water is poured to the top of the filter after the first batch comes through, and subsequent batches of liquid are collected until the liquid trickling out of the bottom is deemed too weak to work, at which point the exhausted soil is taken out of the filter and thrown into a heap, which will be reused later (as we will see shortly). 

Meanwhile, the stronger liquid from the first few batches is transferred to a large earthenware or iron pan called a karahi, which is supported by a brick fireplace, where the liquid may be evaporated down (in other parts of India, such as Punjab, the liquid would  be transferred to shallow masonry trays, in which the concentration of the nitrates takes place through the action of the very dry air and the heat of the sun).

Evaporating the liquid in an iron pan (karahi). Click on the image to enlarge. Public domain image.

You can clearly see the shallow iron evaporating pan in the above image, placed on top of a fireplace. The pan is generally constructed from iron sheets which are riveted together. The liquid is boiled in the pan and lighter impurities, which float to the top, are skimmed off periodically. The liquid is boiled for about 7 hours until it is sufficiently concentrated. To determine this, a drop of the liquid is taken out and placed on a thumb nail. If crystals appear at once, this means the boiling process is complete. The hot boiled liquid is then transferred to open vessels made of clay and allowed to cool and the crystals that are formed are collected next morning, by filtration. The crystals are separated from the liquid and allowed to dry in the sun, but the liquid left behind is not wasted. Instead, it is combined with the next batch of liquid from the earthen filters and the solution is again boiled in the pan to get more crystals and so on. 

After several cycles of boiling, the remaining liquid becomes very saturated with salt. At this point, it is thrown on the heap of exhausted earth that was left behind by the filtration process (mentioned three paragraphs above) and left exposed to the air for a few days. This allows the exhausted earth to become nitrated once again, at which point it is filtered and boiled to extract more saltpeter and so on. The nuniahs were very careful to not lose any of the nitrates, because they often owed money that needed to be paid to the villagers and landowners for the next season. Therefore, they always strived to get the maximum yield of saltpeter possible. At the end of the hot summer season in June, the nuniahs would go back to the villages and pay the villagers an advance fee to use their lands for the next season.

The saltpeter refined by this process was somewhat crude and contained some impurities such as sodium chloride. However, it was pure enough (about 50-70% potassium nitrate) to be used for reasonable quality gunpowder, or it could be sold to other people for further refinement to produce the high-quality gunpowder.

In other parts of India, where the climate was drier (such as Punjab in northern India and parts of central and southern India), the crude saltpeter could be extracted by the heat of the sun instead of boiling artificially. We will study that process in the next post, followed by another post or two about the process used to produce refined saltpeter.