Potassium nitrate

• 1. Introduction

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  Potassium nitrate, better known for centuries as saltpetre, once cost a queen a small fortune. In 1561, Elizabeth I of England was at war with Philip II of Spain and could no longer import the salt. Her kingdom had no home production of its own. So she paid a German captain named Gerrard Honrik "300 pounds gold" for a manual called Instructions for making saltpeter to growe.

  Why would a sharp, salty, bitter white crystal matter so much to a monarch at war? Where does it come from when no mine lies nearby? And how did the same compound end up in gunpowder, fertilizer, cured meat, toothpaste and even the heat stores of a solar power plant? The answers begin with the many names people have given it.

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• 2. A Salt With Many Names

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  The consonants n-t-r appear in both the Egyptian and the Hebrew words for this salt, which suggests a shared ancestry. The Greek form, nitron, was Latinised to nitrum or nitrium. Old French turned it into niter, and Middle English into nitre. By the 15th century, Europeans were calling it saltpetre, and often Indian saltpetre, since Chilean saltpetre is a different compound, sodium nitrate. As chemists came to understand it better, it also became known as nitrate of potash.

  Other cultures looked east when they named it. Arabic speakers called it "Chinese snow". They also used the word bārūd, whose origin is uncertain, and which later came to mean gunpowder. Iranians and Persians spoke of "Chinese salt", or "salt from Chinese salt marshes", in the phrase namak shūra chīnī.

  The naming went further than salt. The element nitrogen was itself named after niter, the mineral form in which potassium nitrate turns up in nature. Americans spell it niter, while the rest of the English-speaking world writes nitre.

  In the 17th century, members of the Qing dynasty published the Tiangong Kaiwu. It described how to make gunpowder and other useful products from natural materials. Those natural materials, for saltpetre, were often stranger than a reader might guess.

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• 3. Caves and Stable Walls

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  Crystals of potassium nitrate form on the walls of caves, and bat guano piled on cave floors is another major natural source. To extract the salt, workers soak the guano in water for a day, filter the liquid, and harvest the crystals that remain in the filtered water. In Laos, this was the traditional source of the gunpowder packed into Bang Fai rockets.

  A close relative turned up in barns. Calcium nitrate, called lime saltpetre, was discovered on the walls of stables, where it formed from the urine of barnyard animals.

  In Mauryan India, the people who made saltpeter formed their own caste, the Nuniya and Labana. Kautilya's Arthashastra, compiled at some point between 300 BCE and 300 CE, mentions the salt's poisonous smoke as a weapon of war. Using it to propel anything did not appear until medieval times, and by then a stranger raw material was waiting to be put to work: excrement.

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• 4. Dung Heaps and Urine

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  A nitrary, or saltpetre works, ran on buried waste, human or animal. Workers buried the excrement in a field beside the works, watered it, and waited. Slow leaching pushed the saltpeter up to the surface, where it appeared as a powder through efflorescence. Operators gathered that powder and carried it to a boiler plant to be concentrated by boiling.

  Michael Scot, who lived from 1180 to 1236, wrote in De Alchimia that the only supply across Christian Europe in the thirteenth century and beyond was "found in Spain in Aragon in a certain mountain near the sea". That claim appears in three manuscripts.

  The nitre bed, discovered in the early 15th century, worked differently. Excrement was mixed with soil, and soil microbes converted its amino-nitrogen into nitrates through nitrification. Water pulled the nitrates out of the soil, and wood ash helped purify them into saltpeter. The method stayed very widely used until the Chilean mineral deposits were found.

  The French version was run on a grand scale. Turgot and Lavoisier created the Régie des Poudres et Salpêtres a few years before the French Revolution. Its beds mixed manure with mortar or wood ashes, plus common earth and straw to keep the pile porous. A typical heap stood about 4 feet high, 6 feet wide and 15 feet long. It sat under a cover against rain, was kept moist with urine, and was turned often to speed decomposition. After roughly a year, water leached out soluble calcium nitrate, which was converted into potassium nitrate by filtering it through potash.

  The Swiss method skipped the dung entirely. Urine was collected directly in a sandpit under a stable, and the sand was later dug out and leached for nitrates. Potash then converted them to potassium nitrate, just as in the French process.

  Both recipes would be set out in plain language in 1862, in a text written for a nation at war.

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• 5. The Confederate Shortage

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  The Confederate side of the American Civil War faced a serious saltpeter shortage. To fix it, the government set up the Nitre and Mining Bureau, which encouraged local production. That included nitre beds, and it included supplying excrement to government nitraries.

  The most exhaustive discussion of nitre-bed production is probably Joseph LeConte's text of 1862. He wrote it with the express aim of raising output in the Confederate States. Because he was appealing to rural farming communities, his descriptions and instructions are simple and explicit. He covers the French method along with several variations, and he also describes a Swiss method.

  The beds themselves were large rectangles of rotted manure and straw. Workers moistened them weekly with urine, "dung water", and liquid from privies, cesspools and drains. They also turned them over at regular intervals.

  On the 13th of November 1862, the government advertised in the Charleston Daily Courier for 20 or 30 able-bodied men to work in the new nitre beds at Ashley Ferry, South Carolina. The labor behind this industry was often forced. The National Archives published payroll records accounting for more than 29,000 people compelled to such work in the state of Virginia. In April 1864, the Confederate government forced 31 enslaved people to work at the Ashley Ferry Nitre Works, outside Charleston.

  The desperation reached into households. One Alabama official reportedly placed a newspaper advertisement asking people to save the contents of their chamber pots for collection.

  Germany would face a similar cutoff in the First World War, and it answered with chemistry rather than manure.

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• 6. Purity and Industry

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  In 1270, the chemist and engineer Hasan al-Rammah of Syria set down a purification method in his book al-Furusiyya wa al-Manasib al-Harbiyya, or The Book of Military Horsemanship and Ingenious War Devices. He began by boiling barud, the crude saltpeter mineral, in as little water as possible, and he kept only the hot solution. Next he added potassium carbonate in the form of wood ashes. That caused calcium and magnesium to drop out as carbonates, leaving a solution of purified potassium nitrate that could be dried. The terminology he used indicated that the gunpowder he described had originated in China.

  Mineral supplies came later. By at least 1845, nitratite deposits were being worked in Chile and California.

  From 1903 until about the First World War, factories made potassium nitrate for black powder and fertilizer on an industrial scale. They started from nitric acid produced by the Birkeland-Eyde process, which used an electric arc to oxidize nitrogen from the air. Then the war cut Germany off from its Chilean supply of sodium nitrates. The Haber process, industrialized in 1913, was combined with the Ostwald process after 1915, and Germany could make nitric acid for itself.

  This reversed an older order. Historically, nitric acid came from mixing sulfuric acid with nitrates such as saltpeter. In modern times, nitrates are made from nitric acid produced by the Ostwald process.

  Today there are several routes. Ammonium nitrate can be combined with potassium hydroxide. A route that avoids an ammonia by-product pairs ammonium nitrate, the ingredient in instant ice packs, with potassium chloride, which is sold as a sodium-free salt substitute. Neutralizing nitric acid with potassium hydroxide also works, and the reaction is highly exothermic. On an industrial scale, makers use a double displacement reaction between sodium nitrate and potassium chloride.

  All that effort was largely aimed at feeding fire.

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• 7. Fire, Metal and Heat

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  Until the late 1880s, blackpowder provided the explosive power for all the world's firearms, and potassium nitrate was its oxidizer. After that, small arms and large artillery increasingly turned to cordite, a smokeless powder. Blackpowder survives in black powder rocket motors. Mixed with fuels such as sugars, it becomes "rocket candy", a popular amateur rocket propellant. It also goes into fireworks such as smoke bombs.

  On its own, the salt is not explosive and is not poisonous, though it can react explosively with reducing agents. It is moderately soluble in water, and more so as the water warms, but it will not dissolve in alcohol. Its crystals are orthorhombic at room temperature and change to a trigonal system at 128 C.

  Smaller jobs rely on its oxidizing power too. Manufacturers add it to cigarettes to keep the tobacco burning evenly. It ensures complete combustion of the paper cartridges used in cap and ball revolvers.

  Heated to several hundred degrees, it can color steel in a process called niter bluing. The finish is less durable than other forms of protective oxidation, but it allows specific coloration of screws, pins and other small firearm parts. Metalworkers also use a molten bath of potassium nitrate, usually with sodium nitrite, for medium-temperature heat treatment. A similar bath makes the durable blue-black finish often seen on firearms. Its oxidizing quality, solubility in water and low cost make it an ideal short-term rust inhibitor.

  A molten potassium nitrate bath can also strengthen glass and make it more scratch-resistant. In condensed aerosol fire suppression systems, it is the active ingredient, producing potassium carbonate as it burns with the free radicals of a flame.

  At the Gemasolar Thermosolar Plant, sodium and potassium nitrate salts are kept molten to store solar energy collected by heliostats. Adding calcium nitrate or lithium nitrate, to make ternary salts, has been found to improve the storage capacity.

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• 8. Table, Field and Folklore

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  Salted meat has contained potassium nitrate since antiquity or the Middle Ages, though widespread use is more recent and tied to large-scale meat processing. In processed meats, the salt reacts with hemoglobin and myoglobin to produce a red color. Cooks have mostly dropped it, because it works slowly and inconsistently compared with sodium nitrite preparations such as "Prague powder" or pink "curing salt".

  It still turns up in salami, dry-cured ham and charcuterie, and in some countries in the brine for corned beef. In the Shetland Islands, it is used to cure mutton for reestit mutton, a local delicacy. In the European Union, the additive is labeled E252. The United States, Australia and New Zealand also approve it, and the last two list it as INS 252.

  Since October 2015, the WHO has classified processed meat as a Group 1 carcinogen, based on epidemiological studies. In April 2023, the French Court of Appeals of Limoges confirmed that the food-watch group Yuka was legally entitled to describe potassium nitrate E252 as a "cancer risk". The court rejected an appeal by the French charcuterie industry.

  In the field, the salt supplies two macronutrients, nitrogen and potassium. Used by itself, it carries an NPK rating of 13-0-44. Some tree stump removal products are about 98% potassium nitrate, which speeds natural decay by feeding nitrogen to the fungi attacking the wood. In the Philippines, growers use it to induce flowering in mango trees.

  Medicine has borrowed it too. Some toothpastes for sensitive teeth have used it since 1980, although the literature does not strongly support its efficacy. It was used historically to treat asthma, and in Thailand it is the main ingredient in kidney tablets for cystitis, pyelitis and urethritis. It was once used as a hypotensive against high blood pressure.

  Folklore adds its own claims. Many believed the salt induced impotence, and it is still rumored to lurk in institutional food such as military meals. There is no scientific evidence for this.

  In 1776, John Adams asked his wife Abigail to make saltpeter for the Continental Army. She eventually managed it, in exchange for pins for sewing.

  Farmers in the 1800s Corn Belt told of drought-killed corn in manured fields that collected so much saltpeter that, when a stalk was opened, it would "fall as a fine powder upon the table".

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