Most everyone knows that saltpetre was the primary ingredient used in the making of gunpowder and other explosives, including fireworks, from ancient times right through the Regency. But did you know that it could also be used as a cooling agent? If a Regency butler were out of ice and wished to chill the wine for a meal, he could do just as the Romans had done, and use saltpetre instead. There was no risk he would blow up the dining room, and unlike ice, if he were clever and careful, he could reclaim that cooling agent for use again, as many times as he liked.
The chilling saga of saltpetre …
More than a millenia ago, an alchemist in ancient China was seeking a formula by which to extend life for an emperor who was terrified of death. In the course of his experiments, he ground sulphur to a fine powder, added some honey to moisten it, then mixed in a substantial measure of a white crystalline powder which he had scraped from some stone walls near garbage pits, barn yards and graveyards. When the alchemist heated his mixture, it exploded in a flash of flames and evaporated in a cloud of smoke. News of this amazing discovery spread rapidly and this mixture became known as huo yao, the "fire drug." Chinese alchemists discovered by mixing different finely ground minerals and metals into the fire drug, they could make flames of different colors. These mixtures were the basis of fireworks, but it was not long before the fire drug was being used to make weapons. And thus, the results of an experiment which was meant to extend human life probably ended the lives of more humans than any other invention for nearly a thousand years.
The white crystalline powder which the Chinese alchemist had used in his experiments was potassium nitrate. It was the crucial ingredient in the making of the Chinese fire drug, what we know today as gunpowder. The formula spread from China through India to the Middle East. By the late Middle Ages it was known in Europe, where the white powder was called "China snow." But this same white crystalline substance was also known by the ancient Greeks and Romans, though they never combined it with flammable materials. The ancient Romans found this same white powder on stone walls, most often underground, in cellars and crypts. They called it sal petrae, which is Latin for "salt of stone." For many centuries the term "salt" was not limited to sodium chloride, common table salt. The term was used for any white-colored crystal which had a salty taste. It was this Latin name which gave this white crystalline powder its most common name, saltpetre.
Both the ancient Greeks and the ancient Romans, of the upper classes, used this white powder, dissolved in water, to cool their wines. It was an expensive commodity, fairly rare and difficult to find, and its use appears to have been limited only to the cooling of bottles of wine at important dinners. Though they were aware of the refrigerant capabilities of saltpetre, the Romans never seem to have used it to cool any other provisions or food stuffs. There were a number of chilled dishes on the menu at Roman banquets, but stored snow or ice was used to cool them. The knowledge of the cooling properties of saltpetre was lost in Europe after the fall of the Roman Empire and remained so for well over a millenia. However, it was known in the Arab world and was recorded in several Arabic manuscripts, some of which survived and were eventually translated into a number of European languages, including Italian and Spanish.
From the later Middle Ages and into the height of the Italian Renaissance, saltpetre was used in Europe only as the primary component of gunpowder. Harvested and stored snow and ice were used by the wealthy classes to cool certain foods, usually for brief periods for special meals. Among Europeans, particularly those of the Christian faith, there was no interest in the use of any form of long-term refrigeration, even though employing cold would have significantly reduced the spoilage of their already scarce food stuffs. Most people would have considered any type of artificial refrigeration not only preposterous, but an appalling contravention of the laws of nature and therefore of God. Cold was a mystery for which there was no obvious source, and, since it was particularly associated with death, it was much too frightening for regular use or even systematic investigation.
Men of science, especially those involved in the practice of medicine, began to take notice of the potential value of cooling by the turn of the sixteenth century. In 1530, Dr. Marcus Antonius Zimara of the University of Padua, published his treatise on the writings of Aristotle, Problemata, in which he mentions the "new" discovery of the cooling effects of saltpetre. Two decades later, Dr. Blasius Villafranca, a Spanish physician practising in Rome, provided instructions on the best method by which to cool wine and other bottled beverages using saltpetre, in his Methodus Refrigerandi ex vocato salnitro vinum acquamque, which was published in 1550. It is probable that this method had been "rediscovered" in one of the Arabic books which had been translated into Italian. Villafranca noted that all of the nobility and gentry of the city were using this method to cool their wines. Gradually, this knowledge spread north from Italy across the Continent. In 1559, a Dutch physician, Levinus Lemnius, included similar instructions in his book, The Secret Miracles of Nature, which was published in Antwerp. Dr. Lemnius wrote that " … it will so cool the wine that your teeth can hardly endure it."
But not all men of medicine felt that this new cooling method was safe or effective. In the sixteenth century, there were many, including a significant percentage of physicians, who believed that adding snow or ice to drinks was a thoroughly decadent practice which could lead to convulsions, colic, paralysis, blindness, madness and even sudden death. Most of these same learned men considered the use of saltpetre in water for cooling wines just as dangerous, if not more so. They believed that the particles in the saltpetre solution could penetrate metal or even glass bottles. Anyone who then drank this supposedly polluted wine would have their intestines burned up from the inside, resulting in an agonizing death. But not all physicians held this opinion, there were some who believed that cold drinks could be beneficial in treating some illnesses. In Italy, in particular, each summer there were many deaths by fever and these doctors reasoned that cold could counteract the heat of the fever and thus restore the victims to health.
There were other men of science, beyond physicians, who experimented with saltpetre. One of the most notable was Giambattista della Porta, an Italian scholar and polymath, who was a friend of the German astronomer, Johannes Kepler, and the great Galileo. Though Della Porta was jailed by the Inquisition for what the Church considered to be his heretical pursuits, he continued to write about his experiments even while he was incarcerated. He published perhaps the most famous "book of secrets" of the sixteenth century, his Magia Naturalis (Natural Magic), in Naples, in 1558. A compendium of popular science of the time, providing information on a wide range of subjects, such books were well-received by the European literati of the era. In his final section of Magia Naturalis, which he entitled "The Chaos," Della Porta detailed his experiments with saltpetre and snow. He wrote that he was able to produce a "mighty cold" which was twice as cold as could be achieved with either substance alone. So cold that he had been able to make ice. Magia Naturalis went through a number of editions and translations over the course of the next century. In 1658, a full hundred years after the first edition, Magia Naturalis was translated into English and Natural Magic became available to a whole new generation of English natural philosophers, as scientists were then known. Just in time for them to carry out some experiments of their own, courtesy of the East India Company.
By the late sixteenth century, gunpowder was no longer a novelty on the battlefield. It had become an essential commodity of war. So much so that saltpetre supplies were considered a matter of national security. Saltpetre was not just a crucial ingredient in gunpowder, it was also the ingredient which was needed in the highest percentage. Effective gunpowder was comprised of three-quarters saltpetre, therefore, demand for this commodity was very high. So much so that by the turn of the seventeenth century, there were men, known as "saltpetre-men," or sometimes just "petermen," who roamed England, empowered by the Crown to take saltpetre anywhere they found it, even private property, regardless of the damage they might cause. They were almost certainly the most reviled and hated men in all England for most of the seventeenth century.
Saltpetre forms naturally on walls, stones and hardened soil near and in dovecotes, barns and stables where animals are kept, as well as the walls of crypts or near trash pits. But once it is scraped away, it reforms haphazardly, with agonizing slowness, much too slow for the needs of nations often at war. It was for that reason that the English government decreed that any surface upon which saltpetre developed was the property of the Crown and the saltpetre-men carried warrants which enabled them to dig it out wherever they found it, regardless of who actually owned the property. The floors of dovecotes, hen houses, barns and stables were required to be made of packed earth to enable the saltpetre crystals to form more easily. In search of saltpetre, petermen moved privies, tore apart stables and dovecotes, ripped up the floors of houses, opened large pits and tore down walls. In addition to their warrant to gather saltpetre wherever they found it, they also had the right to commandeer carts, wagons and horses to transport their priceless freight. Many of these saltpetre-men routinely and egregiously abused their powers and the majority of them often demanded bribes to spare a property owner extreme damage as they gathered saltpetre.
Despite their continuous ravaging of the countryside for saltpetre, petermen could not find enough saltpetre to satisfy the demands of the Crown. By the first quarter of the seventeenth century, "saltpetre plantations" had been developed as a means by which to speed the production of the essential white crystals. A series of shallow trenches were dug over a large area regularly exposed to the sun. The trenches would be lined with clay and filled with a blend of soil, manure, garbage and ashes, which was heaped up until each trench had the appearance of a long burial mound. As they were filled, the trenches were heavily moistened with urine and raw sewage. Several months later, after ripening in the sun, a crust of saltpetre would slowly grow over the trenches. The fine white crystals blossomed into formations which looked like tiny white flowers. A properly-run plantation could produce five to ten pounds of saltpetre per cubic meter of soil every two years. In order to ensure enough raw materials for the making of saltpetre, in 1625, King Charles I issued a royal command that all of his subjects " … carefully and constantly keep and preserve in some convenient vessels or receptacles fit for the purpose, all the urine of man during the whole year, and all the stale [urine] of beasts which they can save." All of the materials collected were to be delivered to the nearest saltpetre plantation. Though the saltpetre plantations did significantly increase the supply, petermen still continued to roam the English countryside with their warrants, gathering saltpetre wherever they found it, until the East India Company finally rendered them irrelevant in the latter decades of the seventeenth century.
Though it receives little attention from most scholars today, probably the single most important commodity which the East India Company encountered in the Indian sub-continent was saltpetre. Vast flats of very fine saltpetre were discovered conveniently located near the port of Bengal, on the east coast of India. This high-grade saltpetre was exported to England by the East India Company beginning in the mid-seventeenth century, right through the first decades of the nineteenth century. Once saltpetre was imported regularly from India, the saltpetre-men gradually became superfluous and the domestic production of saltpetre eventually ceased, giving the East India Company a monopoly. The mark-up on the textiles which the East India Company imported into England was seldom more than 200% while the mark-up on saltpetre was always well over 400%. It was not just its high profitability which made saltpetre the ideal commodity. It was an imperishable chemical which was impervious to the roughest handling and made a most excellent ballast for East Indiamen on the return trip to England. Since it is both anti-insect and anti-bacterial, this valuable ballast had the added benefit of reducing the rate of decomposition in the hulls of the ships which carried it.
Well into the eighteenth century, the English government contracted with the East India Company for almost their entire output of saltpetre, making saltpetre the "largest revenue yielder" in the company’s trade. Having a regular, uninterrupted supply of high-quality saltpetre was one of the major factors which ensured English superiority at sea, since they always had the best quality gunpowder available. No other European maritime power could provide their ships with a steady supply of such reliable gunpowder. Records of the time show that, at the Battle of the Nile, in 1799, the French were using gunpowder which was adulterated by well over twenty-five percent because they could not get pure saltpetre. Bengal saltpetre was one of the reasons that Napoleon was keen to wrest India from the English. And one of the reasons why England retained and continued to expanded their control of the Indian sub-continent.
By the early 1660s, only a couple of years after Natural Magic was translated into English, and saltpetre was regularly imported from India, Robert Boyle began experimenting with the white crystals. By 1665, Boyle had determined that sodium chloride, common table salt, provided more powerful cooling if used with ice or snow, but when ice was not available, saltpetre gave much better results. Saltpetre was used for medical purposes, most often as diuretic, when a small amount was dissolved in a large glass of water. It was also used in the treatment of asthma and hypetension. Saltpetre was particularly effective in the preservation of meats, since it would retain the reddish color of meats such as beef or ham which had been soaked in salt brine. Throughout the eighteenth century, various experiments were conducted with saltpetre, most often with regard to its cooling properties, and the results of these experiments were often published in journals and magazines, a number of which were widely circulated. By the Regency, well-read people with even a passing knowledge of popular science would have been aware that saltpetre could be used for cooling.
Saltpetre was most often used to cool wines during the Regency, by cooling the water in which the wine bottles were immersed. A large wooden tub, preferably of a cylindrical shape or, better still, wider at the top than at the bottom, was the ideal shape of a vessel to be used for cooling with saltpetre. This cooling tub should be lined with sheet lead or zinc and should also have a close fitting lid which would exclude as much of the warmer ambient air as possible. The thicker the surrounding wood, the better the cooling mixture would be insulated. A cooling tub with a capacity of ten to twelve gallons should be filled with four or five gallons of water. The cooler the water, the better, so water just pumped or drawn from a well would be most effective, since the water temperature would be about 75º Fahrenheit. Five to seven pounds of saltpetre should be pulverized to the finest powder possible. This finely-powdered saltpetre should be slowly sprinkled into the water and allowed to dissolve. Within about fifteen minutes the temperature of the water would drop twenty-five to thirty degrees, within a half hour the temperature would drop another four or five degrees. At that point, the temperature of the water would remain steady for over two hours, so long as the lid was kept on the tub as much as possible. After that, the water would begin to warm at a rate of about three or four degrees per hour, unless more powdered saltpetre was added to the water.
Five gallons of saltpetre-chilled water can cool five to six bottles of wine. The wine should not be decanted, but left in the bottle. The bottles should be put into the cooling liquid about a half hour after the saltpetre has been dissolved. The cooled water should come up over the shoulders of the bottles in order to ensure complete cooling of the contents. Since cold sinks, any wine in the bottles which is not below the cold water line will not be chilled. There were multiple schools of thought on how the bottles should be handled once they were immersed in the cold water. Some thought the bottles should be moved around in the water, usually held by the neck between the palms and rolled between them, keeping the filled portion of the bottle below the level of the cold water. Others objected to any agitation of the wine in the bottles and preferred that the water be swirled around the stationary bottles with a large spoon or paddle. Either of these methods will somewhat speed the cooling of the wine. However, some cold will be lost since the lid will not be on the cooling tub. If the wine bottles were allowed to remain stationary in the cold water, with the lid on the tub, for at least an hour, there was no need to agitate the wine or the water, yet the wine would be well chilled, and would remain so for at least another hour or more.
Saltpetre cools water by producing an endothermic reaction. This is a chemical reaction whereby, as it dissolves, the saltpetre literally pulls the heat out of the water as part of that process, thus lowering the temperature of the water. For this reason, there is a limit to how cool the water can become. Once it has become fully saturated with saltpetre, the water is not able to absorb any more. Therefore, assuming a cooling tub containing five gallons of water into which six or seven pounds of saltpetre has already been dissolved, a second addition of five to seven pounds of finely powdered saltpetre could be added to the water to extend the cooling period for perhaps an hour. It is, however, very unlikely that any further additions of powdered saltpetre will prolong the lower temperatures of the chilled water, since the water is already fully saturated. It is only the process of the saltpetre dissolving which chills the water, so if the water cannot absorb any more saltpetre, no further cooling will occur.
The bulk of the saltpetre used in this way could be recovered by either boiling off the water later, or simply allowing the water to evaporate. In either case, the saltpetre would be left behind as coarse white crystals. It could be pulverized into a fine powder once again, and reused repeatedly for the same purpose. Records show that during the course of some experiments in the early nineteenth century, saltpetre used to cool water was recovered and reused more than a dozen times, and could have continued to be recovered indefinitely. Saltpetre was not prohibitively expensive. During the Regency, it ran about four pence per pound, and since it could be recovered after use, it was a very economical form of artificial refrigeration, particularly if ice was not available. Saltpetre was much less expensive in India that it was in England, and since there was seldom any snow or ice available in such a warm climate, saltpetre was routinely used to cool wine and other bottled beverages by the English living on the sub-continent.
It was safe to cool wine and other beverages which were inside sealed containers in a saltpetre solution. However, it would have been somewhat dangerous to use such a solution for the cooling of anything in open or unsealed containers. In very small doses, such as a few grains dissolved in a large glass of water, saltpetre was a very effective diuretic and very low doses were also used to treat asthma and hypertension, among other medical applications. But in large doses, it could cause severe intestinal distress, and in rare cases, even death. Therefore, any food or drink which was to be cooled in a saltpetre solution must be sealed in a water-tight container to ensure it was safe to eat or drink once it attained the desired temperature. Ideally, the container should be rinsed, or at the very least, wiped down before it was opened, in order to keep any saltpetre contamination to a minimum. Saltpetre in solution is not irritating to the skin, since it is PH neutral. Nevertheless, it should be rinsed away as soon as possible to ensure it is not ingested.
During the Regency, saltpetre was most likely to be used as a coolant in England as a scientific novelty, or for cooling when no stored ice or snow was available. An experienced and well-prepared Regency butler might keep ten to twenty pounds of saltpetre on hand in order to ensure he could cool enough wine or other bottled beverages for a family meal in the absence of ice, perhaps after an unusually warm and snow-less winter. Modern day experiments with saltpetre have shown that a half pound of saltpetre can cool one to one and a half gallons of water by at least thirty degrees within fifteen minutes, and retain that temperature for twenty to thirty minutes. Long enough to cool a bottle of wine. Might an officer of Wellington’s army on the Peninsula carry a pound or so of saltpetre with him, for use in cooling a bottle of wine or other beverage when there is no ice to be had? Or, might some soldiers help themselves to a few pounds of gunpowder, having heard a rumor that it could be used to cool bottles of wine, probably procured illegally? The saltpetre in the gunpowder might be enough to cool the water and thus the wine, but the other components of the gunpowder are also suspended in the solution. Perhaps the drunken soldiers tip over their cooling tub after their drinking binge and the water quickly evaporates in the hot Iberian sun. These same soldiers, or others, unaware of what has happened on that spot, lay a campfire. Will there be enough gunpowder residue on the ground to cause a fire or an explosion? Or will they be lucky, and see only a flash as the thin layer of dried gunpowder burns off? Englishmen who traveled to India would soon learn the value of saltpetre for cooling their beverages. Since saltpetre was very cheap there, and ice difficult to get, they would have used saltpetre regularly to ensure their wines and other beverages were cool and refreshing in the hot climate.
Saltpetre was one of the few chemicals available in the Regency, when used alone, which could drop the temperature of water by thirty to forty degrees in a fairly short time and maintain that low temperature for a substantial period. When used in combination with other chemicals, with or without snow or ice, cooling solutions could be made even colder. Some could be made cold enough to make ice, or better still, ice cream. Next week, other chemical cooling options available during the Regency and the tale of how an Oxford don saved a confectioner’s ice cream business during an ice shortage.