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.

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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.

The History of Saltpeter - XII

In our last post, we studied a few details of the regions where saltpeter could be found in India, as well as the establishment of European trading posts in India. We will study more about this, as well as the production process in India.

As we saw in our previous post, the English East India Company started importing saltpeter in a small way in the 1630s. In the early days, raw nitrated earth was being used as ballast in the East India Company ships, but the fact that it took eighteen months for a ship to make a round trip from London to India and back meant that the company needed to place orders and make payments for saltpeter well in advance of delivery, and there was not a guarantee of finding a buyer later either. However, after the English Civil War in the 1640s, it became profitable to start shipping saltpeter and in 1643, the East India Company found it profitable to refine the raw nitrated earth into saltpeter in their Indian warehouses, before shipping it out. By 1644, they established their own refinery in their trading post at Ahmedabad in India's west coast. However, the saltpeter in this region was of a rather coarse quality. Reports of high-quality saltpeter being produced around India's east coast in the Bengal region began coming in, therefore the East India Company directors sent a message to its traders in December 1650, telling them to establish a post in this area, to trade in "saltpeter, silk and sugar". The importance of saltpeter in establishing the new trading posts is evident, because the directive was to invest at least half the capital stock in saltpeter alone, and this was the only commodity for which any debt was to be allowed. With large orders of saltpeter coming in from the English government in the 1650s, the company had a guaranteed buyer and made the decision of building trading posts in India's eastern parts, with saltpeter being a major item of trade here.

By establishing trading posts in India's east coast, the English came into competition with the Portuguese, who had already established trading posts in this area since the 1530s, and the Dutch, who had also established trading posts here earlier. The regions of Bihar and Bengal were already producing saltpeter in the 1450s, well before the first European had landed in India, as mentioned in our previous post. The towns of Patna and Chapra in Bihar were two important centers of saltpeter trade (Patna is now the modern day capital of Bihar state in India, while Chapra is now much less important than before) and the East India Company set up in Patna in 1658. The person in charge of purchasing the saltpeter for them in was one Job Charnock, who later went on to set up the company headquarters in a little place called Calcutta, which later grew to become a large city and the seat of British power in India.

Around the same time, the area around Patna and Chapra had saltpeter refineries established by the Dutch, Portuguese, French and British, all vying for a bit of the business. We have the account of a French traveler, Jean-Baptiste Tavernier, from 1666, where he mentions that on a visit to Patna, "The Holland Company have a house there, by reason of their trade in saltpetre, which they refine at a great town called Choupar (Chapra). Coming to Patna, we met the Hollanders in the street, returning from Choupar, who stopped their coaches to salute us. We did not part till we had emptied two bottles of Shiras wine in the open street, which is not taken notice of in that country, where people meet with an entire freedom without any ceremony." In another section of his memoirs, he notes that "The Dutch have established a depot at Chapra; and the saltpetre being refined there, they send it by river to Hoogly. They imported boilers from Holland, and employed refiners to refine the saltpetre for themselves; but have not succeeded, because the people of the country, seeing that the Dutch wished to deprive them of the profits of refining, would not supply them any longer with whey, without the aid of which, the saltpetre cannot be bleached, for it is worth nothing at all, if it is not very white and very transparent."

We have some European accounts of the production process in India. The earliest details of the process date  from the 1620s, from a Dutch merchant named Francisco Pelsaert and an English merchant named Peter Mundy. Pelsaert was based in Agra, Northern India for 7 years, on behalf of the Dutch East India Company, while Mundy was in Surat on India's west coast in 1628. They note that the process started by collecting black earth and placing it in a shallow reservoir, which was flooded with water and the earth was trodden by workers until a paste was formed. The water was allowed to stand for two days after which it was run off into a second shallow reservoir close by, where a deposit of crude saltpeter settled. The liquid was then evaporated by boiling in iron pans, with some impurities being skimmed off the surface. The reminder was then placed in large earthen jars and left overnight, whereupon the heavier impurities sank to the bottom of the pots. The pots were then broken to free the saltpeter crystals, which were then dried in the sun.

We also know that the superintendent of the Patna factory, the above mentioned Job Charnock, was very interested in increasing the quality of saltpeter exported from India's eastern parts. As early as 1665, it was reported that the quality of saltpeter from the factory under his supervision was much improved than before:

the best that has gone from these parts, of twise boyled, occationed by the convenience of a warehouse which Mr. Charnock has built on the river side, neere our petre men, that now he viewes all they bring in, if bad, returnes it to be by them boyled over againe. Also, the whole yeare they may be bringing it by water. So that now, if [we] had monyes, 1000 tonns might easily yearly be procured.

 For his efforts, he was promoted to senior merchant in 1666 and became third in the company hierarchy for Bengal by 1676 and second in charge by 1679.

The yield of saltpeter from Bihar and Bengal was huge and by the late nineteenth century, over 20,000 tons of saltpeter were exported annually from Bihar alone. One John Stephenson observed in 1835 that practically every village in Bihar was producing saltpeter for centuries. Many of these villages were small and only produced about 5 maunds (375 lbs. or 170 kg.) per season, but since there were so many of them, the total saltpeter production added up to a huge quantity. The climatic and soil conditions in many parts of India were suitable for saltpeter production, but it was particularly so in Bihar and Bengal, where the procedures were well established in the villages for centuries. In the next post, we will study the process in detail.