Chemistry for Everyone
Some History of Nitrates Dennis W. Barnum Department of Chemistry, Portland State University, Portland, OR 97207-0751;
[email protected] Because saltpeter is a necessary ingredient in gunpowder, its history is a interesting example of the interaction between chemistry, world trade, technology, politics, and warfare. Perhaps some discussion of the history of saltpeter in general chemistry courses can increase student interest in chemistry and show its relevance to the macroscopic world. This author’s interest was piqued when he learned that the large-scale export of sodium nitrate from the Atacama Desert of northern Chile did not begin until 1830. Where, then, did all of the saltpeter come from to manufacture gunpowder for the multitude of wars before that time? For example, the European wars with Napoleon must have consumed large quantities of gunpowder and thus saltpeter. Nitrate is produced in soil by microorganisms acting on manure, urine, and vegetable waste. Ordinarily the nitrate is consumed by growing plants or washed away by rain and snowmelt. However, under dry conditions a soil can produce from three to five pounds of saltpeter per 100 pounds of soil (1, 2). A common source of dry soil was the dirt floors of stables, sheep pens, pigeon houses, cellars, and caverns. In regions of India, Tunisia, Egypt, and China, where the climate is relatively dry and large herds of animals provided abundant manure, saltpeter was produced on farms or plantations. Layers of dirt, limestone, and manure were piled four to five feet high and allowed to stand for three to five years while soil bacteria oxidized the nitrogen to nitrate (3–6). Nowadays we would call this bioengineering. A saltpeter plantation as Erker imagined one to be is shown in Figure 1. Saltpeter on Walls Saltpeter commonly forms on damp masonry walls in cellars as an efflorescence of white feathery crystals or white scale. Turriano described it “…like a wall covered with snow” (7, 8). This could hardly have escaped notice even in the most
ancient times. According to Thorwald (7) saltpeter is mentioned in an ancient Sumerian clay tablet, dated from about 2200–2100 B.C.E. and in the Ebers papyrus1 from Egypt, dated about 1500 B.C.E. There is a passage in the Bible dealing with houses with “leprosy”. It has been suggested that this refers to a saltpeter efflorescence (9, 10). If a white deposit or scale occurred on the lower walls of a house, a priest had to be consulted. If “leprosy” was suspected the wall had to be scraped clean and the house closed. If the efflorescence returned after two weeks, it was declared to be “leprosy” and the entire house had to be destroyed and disposed of in “an unclean place” (9). Perhaps the ancient Hebrews were already aware of the connection between a saltpeter efflorescence and seepage from a nearby latrine or stable.2 Ancient Uses of Saltpeter One wonders what saltpeter was used for in the era before gunpowder. There are few references to it in ancient pharmacopoeia so it apparently found little medical use. The use for fireworks in China is well known (11). Nevertheless, there must have been a fairly steady demand in ancient times as evidenced by saltpeter works found in Petra, an active trade center from about 400 B.C.E. to about 200 C.E., and also saltpeter works on the western side of the Dead Sea (12). There is at least one reference to its use for cooling wine (11), obviously based on the fact that dissolving potassium nitrate in water produces a very cold solution. Saltpeter might have been used in incense to aid burning. At least, modern recipes for incense often include saltpeter and many ancient cultures around the Mediterranean Sea, especially the Egyptians, used large quantities of incense (13, 14). Another possibility might be for making tinder. During the Middle Ages a common method to prepare tinder was to soak a certain kind of moss in saltpeter solution and then let it dry (15). Early Sources of Saltpeter
Figure 1. A saltpeter farm in 1580 according to Erker (5).
The introduction of gunpowder into warfare brought a rapid increase in demand for saltpeter. In England, Parliament commissioned “saltpeter men” or “petermen” to travel the country and dig up nitrous dirt wherever they could find it. Property owners were obliged by law, not only to allow the digging, but also to provide water and covered space to set up the necessary fires and vats. When finished the petermen could commandeer wagons to transport their vats and tools to the next site. Not surprisingly, the system generated many “complaints and vexations” despite the fact that the petermen were supposed to clean up, repair any damage, and pay six pence per mile for commandeered wagons (16–18). In France, the Farmers General3 (Lavoisier was a member) had authority to confiscate nitrous soils. They frequently seized useful material when old buildings were torn down, as well as the soil from stables and outhouses. In some prov-
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Chemistry for Everyone
inces, when saltpeter was in short supply, the Farmers General were even allowed to take the earthen floors of inhabited cottages. In Prussia, the King ordered every settlement, town, and village to have a covered shed in which broken masonry, compost, and vegetables were collected. And, in Sweden, as late as 1835 every owner of land was required, every year, to deliver a certain quantity of saltpeter to the state (5). A large portion of the British East India Company trade was in saltpeter carried to England from India and China. The Company was not eager to carry it because it was “a poor neighbor” to more costly goods in the ship’s hold such as silk and spices (12). The Company was required by the Crown to import saltpeter and store it in warehouses in England until it “pleased the King to purchase it at his own price”. When the Company complained, it was informed that it would face an export tax on silver4 or lose the favor of the King if they did not continue to import saltpeter (12). Extraction of Saltpeter from Soil The extraction of saltpeter from soil (Figure 2) has been described in detail in the older literature (2–5, 18, 19). A prospective soil was first given a taste test to determine if digging would be worth while. According to Agricola (3), the earth, “... if it is retained for a while in the mouth, has an acrid and salty taste”. According to Clarke (19), it will “...prick the tongue and taste like spice, and knowing the taste of [pure] nitre you may be the better judge”. Erker, however, recommended a trial extraction from a small test sample before extensive digging (5). In soil from stables and similar places, the nitrate was found in the top 15 to 20 centimeters of dirt. Whether from a saltpeter farm or a stable floor the salt present in soil is mainly calcium nitrate. However, only potassium nitrate is useful for gunpowder.5 To convert calcium nitrate into potassium nitrate, wood ashes, which contain potassium car-
bonate, were added to the water extract. This precipitates the calcium carbonate and leaves behind a solution of potassium nitrate: Ca(NO3)2(aq) + K2CO3(aq) → 2KNO3(aq) + CaCO3 ↓ The solution was then filtered through a bed of sand and boiled to remove most of the water. On cooling overnight large crystals of potassium nitrate were obtained. William Clarke wrote, “And I know not in what Experiment I have taken more pleasure, than to see such crystalline substance taken out of dirty earth, so beautiful a body out of chaos, such a spiritual Essence …” (19). Chile Saltpeter There are vast deposits of sodium nitrate in the Atacama Desert in northern Chile, a region almost 400 miles north to south and 30 to 40 miles wide. The nitrate occurs mainly in a caliche6 that ranges from a few centimeters to a meter and more thick. The caliche is generally from half a meter to twenty meters below the surface. Also, there are vertical veins in which desiccation cracks one to three meters deep are filled with sodium nitrate and detritus. It both cases the deposits often contain traces of iodate, perchlorate, chromate, and dichromate, highly oxidized species seldom found elsewhere in nature (20). Initially, in 1810–1812, these deposits were exploited for local consumption (1, 21) near the town of Zapiga, about 70 miles south of the present border with Peru. This is the northernmost and most accessible part of the Atacama Desert. Farther south the desert is cut off from the sea by a coastal range of mountains that rises steeply 1000 to 2000 feet directly from the sea, making the interior desert difficult and expensive to access. Even as late as 1879, the Chilean government sent a naval ship north from Santiago to explore the coast in search of a gap in the mountains through which a railroad into the interior could be constructed (22). The sodium nitrate beds were not extensively exploited until 1830 when shipments of both sodium nitrate and guano to Europe began, mainly for fertilizer rather than munitions. (21). European farmers were just beginning to appreciate the value of fertilizers at that time. For explosives, sodium nitrate was converted into potassium nitrate by dissolving it in hot water and adding potassium chloride. On cooling potassium nitrate crystallizes out: NaNO3(aq) + KCl(aq) → KNO3↓ + NaCl(aq)
Figure 2. Extraction of saltpeter from dirt in 1580 (5).
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This reaction is effective because the solubility of potassium nitrate decreases rapidly on cooling whereas the solubility of sodium chloride hardly decreases. At the time of the American Civil War, the Union blockaded Southern ports to prevent the Confederacy from financing the war with exported cotton and importing war materials such as saltpeter from Chile. The Southern states became so pressed for saltpeter that they resorted to the old method of extracting it from the dirt floors of caverns (23).7 When first worked in 1812, the Atacama deposits were part of Peru. In 1826, Peru split into two countries and the area formerly known as Alto Peru became Bolivia. The ni-
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Chemistry for Everyone
trate deposits were then in Bolivian territory, which at that time reached to the sea and extended southward to a point just south of Antofagasta. Although located in Bolivia, mainly Chilean and British companies owned the mines and the workers were mainly Chilean. Some 400 miles to the south, Chile started mining nitrate in the late 1850s. As exploration of the Atacama Desert progressed it became apparent that much more nitrate ore, as well as other valuable minerals, were located in the central desert. A border dispute arose between Chile and Bolivia. The result was the War of the Pacific (1879–1883) in which Chile faced an alliance of Bolivia and Peru. Chile won the war and in the settlement acquired the present border 8 that includes all of the nitrate-producing region (24). At the outbreak of World War I, the Allied Powers immediately took steps to prevent Germany from importing nitrate from Chile. However, the first large-scale commercial production of ammonia via the Haber process, followed by oxidation to nitric acid by the Ostwald process, had been initiated in 1912 in Germany just before the outbreak of war. Historians generally believe that World War I would have been two or three years shorter had it not been for the Haber process. Germany would have run out of nitrate for explosives. After World War I the Haber–Ostwald process became the most economical source of nitrate. Chilean mining declined and was nearly over by 1930. Origin of Nitrate Deposits in Northern Chile The origin of such a vast quantity of sodium nitrate has been extensively investigated but there is still no consensus. About the only agreement is that the Atacama Desert is so dry that however the nitrate is formed it is not washed away by rainfall or snow melt. Erickson has investigated the geology of the desert and reviewed all of the suggestions (1, 20, 25–27). Because of the well-known formation of nitrate from manure, one of the earliest suggestions was that the nitrate was produced from fossil guano (28–30). Indeed, there are some fossil guano deposits in the Atacama Desert but Erickson (1) points out that there is too little guano to account for the large quantity of nitrate. Furthermore, guano is high in phosphate and there is too little residual phosphate mineral in the desert to account for so much nitrate. Erickson concluded that only a small fraction of the total nitrate could have come from guano. More recently, a study of photochemical reactions of atmospheric nitrogen on several desert sands showed both reduction to NH4+ and oxidation to NOx− (31). The Atacama deposits were once thought to be unique yet in 1962 a similar sodium nitrate deposit was discovered along the western margin of the Ross Sea in Antarctica (32– 34). The area is dry, windswept, about 2000 feet above sea level, and contains no fossil guano. Like the Atacama deposit sodium nitrate occurs in a caliche three to five centimeters below the surface and contains traces of iodate. Actually, nitrate ion is common in desert soils worldwide. It is estimated that desert soils average 0.1 to 0.8% as potassium nitrate (1, 34). In 1912, the U.S. Geological Survey conducted a survey of desert areas in the western United States looking for nitrate (35). An area in Utah runs 1–2%
as potassium nitrate. The high concentration in the Atacama Desert is due to almost complete lack of rain, snow, and vegetation. Precipitation there averages less than one millimeter per year although there are torrential rains, on the average, about once every 25 years. Notes 1. I could find no reference to saltpeter in Ebbell’s translation of the Ebers papyrus (36). There were a few references to “black nitre” but no indication of what that is. 2. There are other references to nitre in the Bible but they refer to sodium carbonate, not saltpeter. During the Middle Ages, there was much confusion over terms such as niter, nitre, nitron, nitrum, nitri, neter, et cetera. Only in the late 16th century did the Latin word nitrium come to be used to describe sodium nitrate, which was often confused with potassium nitrate. For a discussion of the change in terms see Haberfield (37) or Hoover and Hoover (10a). 3. In France, the Farmers General had nothing to do with farming and everything to do with collecting taxes. It was a syndicate of 80 to 100 businessmen who entered into six year contracts with the king to collect taxes on certain commodities, such as salt, tobacco, and wine. The taxes collected exceeded the quantity paid to the king, resulting in substantial profit to the Farmers General. The Farmers General had ∼30,000 employees throughout France. Lavoisier received a large inheritance when he was only 11 years old that he used to purchase membership as a Farmers General. As a young man he oversaw the quality of tobacco and collection of tobacco taxes. Later, he also served on the committee for the Administration of Gunpowder and Saltpeter. The members of the Farmers General were unpopular and the system was eliminated during the French Revolution. Those members who had not fled the country were arrested and sentenced to death by guillotine. Lavoisier and 28 other Farmers General members were beheaded in just 30 minutes (38). 4. The British East India Company was created by Queen Elizabeth in 1599, however the first saltpeter from the Far East was brought back by the Dutch in 1624. For trade, England had nothing that people in the Far East wanted so the British East India Company was forced to purchase everything with silver. 5. Sodium and calcium nitrates make poor gunpowder because they are hygroscopic. Good gunpowder must stay dry. 6. A caliche is a kind of hard pan or duricrust. It is a soil layer usually found in desert regions that is hard and composed of soil mineral particles cemented together by calcite or precipitated salts. It is formed when infrequent rains rinse soluble salts down in the soil horizon followed by evaporation, or when ground water near the surface evaporates and deposits dissolved solids. 7. There is a report that in the British–American War of 1812 saltpeter was extracted from caves in Kentucky as well as Wynandote Cavern in Illinois (35). 8. For a map showing the history of the borders between Chile, Bolivia, and Peru see http://www.countryreports.org/ (accessed Aug 2003).
Literature Cited 1. Erickson, G. E. Geology and Origin of the Chilean Nitrate Deposits; U. S. Geological Survey Professional Paper No. 1188: Washington DC, 1981; p 12.
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Chemistry for Everyone 2. Stubbe, H. … Sundry Mistakes about the Making of Salt-Petre and Gun Powder are Detected and Rectified; Legends no Histories or a Specimen of Some Animadversions Upon the History of the Royal Society. University Microfilms, Early English Books; 1669. 3. Agricola, G. De Re Metallica; translation by H. C. Hoover and L. H. Hoover; Reprinted by Dover Publications: Mineola, NY, 1950. 4. Biringuccio, V. Pirotechnia; Venice, 1540; translation by C. S. Smith; Basic Books, Inc.: New York , 1942. 5. Ercker, L.Treatise on Ores and Assaying; Prague, 1574; translation by A. G. Sisco and C. S. Smith; University of Chicago Press: Chicago, IL, 1951; Chapter 5. 6. Williams, A. R. Ambix 1975, 22, 125. 7. Thorwald, J. Science and Secrets of Early Medicine; translation by R. Winston and C. Winston; Harcourt, Brace & World, Inc.: New York, 1962. 8. Turriano, Book of Machines; “XXI Libros de Maquinas” Madrid B.N. MS 3372-3376; translation by A. G. Keller; 1560. 9. Lev. 14:33–57. 10. Agricola, G. De Re Metallica; Froben, 1555; translation by H. Hoover and L. H. Hoover; The Mining Magazine: London, United Kingdom, 1912; (a) Footnote 5, p 558. (b) Footnote on p 562. 11. Partington, J. R. A History of Chemistry; Macmillan & Co. Ltd.: New York, 1961; Vol. 2, pp 113–114 refers to the use of saltpeter for cooling wine; subsequent pages refer to ancient Chinese fireworks. 12. Salt—Economics & Monopoly. http://www.salt.org.il/ india.html (accessed Aug 2003). 13. Groom, N. St. J. Frankincense and Myrrh: A Study of the Arabian Incense Trade; Longman Group: London, United Kingdom, 1981. 14. Herrmann, P. Conquest by Man: The Saga of Early Exploration and Discovery; translation by M. Bullock; Harper Brothers Publishing: New York, 1954. 15. Salt— Saltpeter. http://www.salt.org.il/saltpet.html (accessed Aug 2003). 16. Anonymous. … a Proclamation for the Calling in and Frustrating all Commissions for the Making of Salt-Peter; University Microfilms, Early English Books; 1595. 17. Anonymous. An Ordinance of the Lords and Commons Assembled in Parliament: For the Making of Salt-peter [sic] in the Kingdom of England and Dominion of Wales; University
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Microfilms, Early English Books; 1644. 18. Coke, Sir E. Diverse Resolutions and Judgements … for Digging of Saltpeter; University Microfilms, Early English Books, 1656. 19. Clarke, W. The Natural History of Nitre. A Philosophical Discussion of the Nature, Generation, Place, and Artificial Extraction of Nitre with Virtues and Uses; E. Okes: London; University Microfilms, Early English Books; 1670. 20. Erickson, G. E. Am. Sci. 1983, 71, 366–375. 21. Bermudez, M. O. Historia del salitre desde sus origines hasta la Guerra del Pacifico; Ediciones de la Universidad de Chile: Santiago, Chile, 1963; p 456. 22. Pissis, A. The Nitrates and Guanos of the Atacama Desert; Taylor and Francis: London, United Kingdom, 1878; pp 1–28. 23. Ziemke, P. C. J. Chem. Educ. 1952, 29, 466. 24. Talbott, R. D. A History of Chilean Boundaries; Iowa State University Press: Ames, IA, 1974. 25. Eriksson, E. Tellus 1952, 4 (3), 215–232. 26. Erickson, G. E. Geology of the Salt Deposits and the Salt Industry of Northern Chile; U. S. Geological Survey Report No. 698: Washington, DC, 1963. 27. Goldschmidt, V. M. Geochemistry; Oxford University Press: New York, 1954; p 449. 28. Rondizzoni, T. In Nitrate and Guano Deposits of Atacama; Taylor and Francis: London, United Kingdom, 1878; pp 48–53. 29. Sieveking, D. P. The Nitrate Beds of the Northern Part of the Province of Atacama. In Nitrate and Guano Deposits of the Atacama Desert, Chilean Nitrate Beds; Pissis, A., Ed.; Taylor and Francis: London, United Kingdom, 1878; pp 38–43. 30. Guano—Verwendung und Handel. http://mindepos.bg.tuberlin.de/lager/wipki/guano.htm (accessed Aug 2003). 31. Schrauzer, G. N.; Stramped, N.; Hui, L. M.; Palmer, M. R. Proc. Nat. Acad. Sci. U.S.A. 1983, 80, 3873–3876. 32. Claridge, G. G. C.; Campbell, I. B. Nature 1968, 217, 428. 33. Johannesson, J. K.; Gibson, G. W. Nature 1962, 194, 567– 568. 34. Mueller, G. Nature 1968, 219, 1131–1134. 35. Gale, H. S. U.S. Geolog. Survey Bull. 1912, 523. 36. The Papyrus Ebers.The Greatest Egyptian Medical Document; translation by B. Ebbell; Levin & Munksgaard: Copenhagen, Denmark, 1937. 37. Haberfield, P. J. Chem. Educ. 1985, 62, 58. 38. Poirier, J.-P. Lavoisier. Chemist, Biologist, Economist; translation by R. Balinski; University of Pennsylvania Press: Philadelphia, PA, 1993.
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
