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If you go to your bathroom and check the ingredients in your well-known brand of sensitive toothpaste, you may well find that the recipe contains the active ingredient potassium nitrate. Also known as saltpetre or nitre, this naturally occurring mineral is found in foods as a preservative (e.g. corned beef), and used in fertilizer, cigarettes, blood pressure medicines and fireworks. Since medieval times it has formed one of the main ingredients in gunpowder, and it is this connection that has also given potassium nitrate a long association with teeth and gums.Many of the seventeenth and eighteenth century recipe books in the Wellcome Library's manuscripts include treatments for gunpowder burns, but some also proposed that gunpowder could be therapeutic. Katherine Jones, Lady Ranelagh (sister to the famous chemist Robert Boyle), recommended a ‘little gunpowder’ applied in a linen cloth to ease toothache. On one page of Anne Brumwich’s recipe book (Wellcome MS 160, p.83) we can find nine recipes for toothache remedies written in two different hands. One, ‘An aproved medecine for ye toothake’ (approved meant that it worked) required gunpowder, aniseed water and lint, mingled together to ‘make a litell thing’.Once the sufferer had picked their tooth very clean, the recipe instructed them to push the preparation into the tooth, taking care not to allow any of the mixture down the throat.A century later, in A Treatise on the Scurvy (1795) David Paterson introduced his fellow naval surgeons to a wonderful, and apparently unknown remedy for scurvy: during a voyage in 1784, he claimed, he had restored the health of eighty sick seamen not with lemon juice, fresh fruit or vegetables, but with the potassium nitrate extracted from the gunpowder in his ship’s stores. Paterson’s remedy was soon forgotten, until in 1828, a desperate surgeon named Charles Cameron, having used up all his supplies of lemon juice, remembered Paterson’s recipe. Cameron was stranded in the calms near the equator and he was faced with a ship’s hospital full of scorbutic convicts, less than half way through the voyage to Australia. He extracted the nitre from the powder, dissolved some of it in vinegar, and mixed some more with vinegar and lime juice. He also added a little sugar (to taste?!) The effects were ‘miraculous’.For the Navy, if Cameron was right, this was a money-saving opportunity; nitre was cheap and did not decompose over time. In the following decades surgeons continued to experiment with different remedies for scurvy until, in 1840, the Admiralty decided to perform a large-scale experiment to determine once and for all the best scurvy remedy. Over the next four years the surgeons of sixty ships transporting fifteen thousand convict men from Britain and Ireland to Australia received crystallised citric acid, potassium nitrate, and lemon juice. Their instructions clearly forbade the surgeons from trying to cause scurvy during the voyage but if the disease did appear, the patients were to be divided into three groups, each group receiving one of the remedies. Of course, the surgeons often had their own ideas, and often altered, combined and varyed the doses according to their own personal favoured recipe. So, while Surgeon Deas mixed some nitre with lime juice and some with citric acid, and felt that both mixtures were useful, Alexander Bryson gave each group the remedies mixed in a glass of wine, water and sugar. After many of the convicts developed severe scurvy, Bryson finally decided that potassium nitrate was ‘objectionable’. The surgeons had come to very different conclusions about the value of potassium nitrate but the results of the experiment were clear; potassium nitrate was abandoned as useless, lemon juice was in for good.In the mid 1970s, dental researchers – in laboratories this time, rather than on ships – began to report a strange occurrence: mixing potassium nitrate with toothpaste seemed to reduce dental sensitivity in sufferers.  More work confirmed the compound’s beneficial effects, but the scientists still admitted that they were unclear why it should work; being soluble, it seemed that it should simply dissolve in water and wash out of the teeth at first rinse.Jump forward again to the present, and potassium nitrate is often used as the active ingredient in products for sensitive teeth. So we have come a long way in medical understanding since women like Anne Brumwich stuffed aching teeth with gunpowder soaked lint, or Victorian naval surgeons dosed their convicts with nitre in the certainty that it helped with scurvy, and yet nitre has proved persistent: these earlier ideas about potassium nitrate’s ability to reduce not only the pain of toothache, but the symptoms of scurvy – a disease so commonly experienced in the mouth and gums – are worth wondering about.&#160;Katherine Foxhall is a Wellcome Postdoctoral Researcher in History at King’s College London. Currently working on a history of migraine, Katherine has worked in the past on the history of migrant health, maritime quarantine, and illnesses including scurvy, cholera and typhus. Her book, Health, Medicine and the Sea: Australian Voyages, c. 1815- 1860 has recently been published with Manchester University Press.