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Saltpetre: The soul of Gunpowder Geoff Smith
Since the seminal works of Needham and Partington, accounts of saltpetre have beendominated by historical writers with scant attempts to verify the technical details. This paper seeks to follow the progress of saltpetre production from the earliest collection of the natural product to the large scale production demanded by the rapid growth of gunpowder weapons. The basic chemistry of the production and purification process isdescribed and the important and much misunderstood role of lime is explained, Other similar nitrates are briefly described, chiefly to eliminate them as credible substitutes for saltpetre in gunpowder.
Key words: Saltpetre, Nitre, Gunpowder, Medieval Gunpowder Chemistry.Gunpowder is compounded of three ingredients. Saltpetre, up to 75%, Charcoal about 15% and the remainder sulphur. Gunpowder can be made without sulphur, albeit somewhat more difficult to ignite, and a wide variety of carbonaceous materials such as sugar can substitute for charcoal. But saltpetre was for centuries the only practicable source of the nitrate radical that is the sole source of the oxygen and nitrogen that define the explosive nature of gunpowder. As a major component of gunpowder, saltpetre has received considerable attention from historians and medievalists. Much of their published work contains scant, and often erroneous, technical detail. They are largely omitted from the current narrative.  Al Hassan has documented its occurrence in Islamic literature and differentiates it clearly from Natron, a natural sodium carbonate occurring in Eastern regions and Needham has recorded its occurrence in early Chinese literature. Cressy has presented an historic narrative but no technical detail.Saltpetre, potassium nitrate. KNO
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 , E 252, nitre etc. has been known by a number of names. Early sources refer to ‘Chinese Snow’ suggesting both the source and, indirectly, the purity of the product. Marcus Graecus gives salis petrosi stony salt, salpetrae and saltpetre
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. Although 'salt peter' and other variants occur in the literature, the spelling saltpetre is by far the most common in early sources and will be used throughout this paper.
Etymology
Saltpetre is shown in most dictionaries as being derived from Latin Sal Petrae – 'saltof the rock' (OED etc.) but no evidence is cited for this as an early usage. However, Sal Petrae translates as rock salt whereas 'salt of the rock' should be sal petram. Nor does a translation as stone salt suffice since stone would translate as lapis and 'of stone' as lapedis
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. Rock salt has been known since antiquity as mined mineral salt – sodium
1Drayson gives ‘’sal prunella’ for nitre that has been melted. This is rare in the context of gunpowder manufacture but appears in old recipes for meat processing and sore throat remedies. 2Latin grammar requires the genitive case of the noun to indicate ‘of the’.
Journal of the Ordnance Society Vol. 27, 2020
 
chloride – as opposed to that obtained by evaporation of sea water and known simply as “salt” so the accepted derivation poses some problems. An alternative derivation can be postulated. Saltpetre was first imported into Europevia the many trading routes with the East known collectively, and often erroneously, as the Silk Road. A major centre of the caravan trade was Petra and although it was largely abandoned by about 663 AD this does not prevent the possibility of lending its name to an import from beyond that region. As precedent one might cite Stockholm tar or Stilton cheese which was never produced at Stilton but was traded through the town. Unfortunately no evidence has yet been found to either support or refute this theory although there is some evidence that saltpetre could have been produced at a site just North of the city of Petra and the raw materials were readily provided by the large herds of camels of the many passing caravans
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. Since early sources agree that the earliest saltpetre was gathered from efflorescence on walls, perhaps Sal Murum, (salt of the wall) would have been a more appropriate name.
History
Since earliest times, man has observed naturally occurring materials and sought to use them to his advantage. In some parts of the world white crystals can be observed on stable walls or on the ground where there has been significant animal excrement. For any significant spontaneous accumulation of this material climatic conditions must be appropriate. There must be sufficient moisture (see below) but a dry season for the material to crystallise rather than be washed away. These conditions are not common in the West but do occur particularly in parts of India and China. It is not surprising therefore that the first written evidence of saltpetre is from these parts and its ready availability gave rise to the development of gunpowder 
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.Saltpetre is not a naturally occurring mineral. It exists solely as a result of the decomposition of organic matter. Although it cannot therefore be mined in the conventionalsense, considerable deposits have been formed where conditions are right and have been major sources for centuries and this will be discussed later.The production of nitrates in nature is part of the well known nitrogen cycle wherebyatmospheric nitrogen is introduced into the soil, nurtures plant life and after decay or passing through the animal food chain is recycled endlessly. All fertile soil contains nitratesbut to obtain saltpetre, the aim is to optimise the process and then to extract the nitrate in a usable form. A simplified presentation of the nitrogen cycle is shown below. Most of these reactions take place in soil either as a ubiquitous natural process or enhanced by man as in a domestic compost heap. In some parts of the world, the requirements of the process are so favoured by local conditions that nitrates can be 'mined' directly from the soil. This occurs most readily in warm dry climates where there has been considerable animal activity and little rainfall to wash the nitrates away. Such conditions rarely occur in Europe but are relatively common in the East so it is not surprising that it is in these regions that
3The late Professor Al Hassan, private communication4Although China has the earliest documented evidence for proto-gunpowder, both India and Sri Lanka have a credible claim. Pre-History of Gunpowder https://www.academia.edu/10603213/The_Pre-History_of_gunpowder 
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saltpetre is first mentioned and, as a direct consequence of its availability, gunpowder was developed.
Nitrogen Cycle
 Nitrogen is circulated in the environment in the well known nitrogen cycle. . The challenge of the saltpetre maker is to enhance the process and extract the saltpetre in usable quantities.Fig.1 The nitrogen cycle.Note that nitrate is produced as the free nitrate ion NO
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not as either potassium, sodium or calcium nitrate. To produce potassium nitrate it is necessary to consider the roleof metal ions present. This is the field of soil chemistry and the following is a brief description simplified to illustrate only the present discussion.
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Soil Chemistry
The nitrogen cycle demonstrates the production of nitrate ions either naturally in thesoil or as an enhanced reaction in 'Nitrate Farms' or a domestic compost heap. To producesaltpetre it is necessary to supply potassium or other metal ions (cations). Soil and plant waste contains a wide range of metal ions that vary in concentration with the soil type. Soils
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 and organic matter form colloids with a negatively charged surface. Metallic cations have a positive electric charge and bind to this surface. The force with which they are held depends on the charge on the cation and its size. Fig. 2 Ion binding to soilsThus Al>Ca>Mg>K.>Na>H with binding force decreasing left to right with hydrogen only weakly bound.Increasing the concentration of a cation can displace a less strongly bound competitor towards the right. Since calcium is more strongly bound than potassium, the addition of lime will tend to increase the available potassium. This is an important difference between an agricultural compost heap and a saltpetre farm. Compost retains
5The Haber process is a method of synthesis and a common precursor to modern nitrate production.6More precisely clay soils. Sandy soils do not bind cations well which explains why nitrates and other nutrients are easily washed out.
Journal of the Ordnance Society Vol. 27, 2020
 
the potassium strongly bound to the soil and is not easily washed out by rain whereas, in the production of saltpetre, the requirement is to wash out the nitrate as efficiently as possible. The addition of lime facilitates this. Far from producing mainly calcium nitrate, theaddition of lime actually increases the yield of available potassium nitrate.Calcium will also displace hydrogen, increasing acidity but if it is added as either quick limeor slaked lime this will react with hydrogen ions (protons) to produce neutral water. This is important in the nitrogen cycle as the essential bacteria are only active in a limited range of acidity.It is assumed by many authors that the primary product of the saltpetre process is calcium nitrate that must be converted to the potassium salt by the addition of wood ash, potassium carbonate. However, while it is true that calcium nitrate is very soluble, the calcium in the nitrate bed described in early sources
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 is added as lime or gypsum
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 which have very low solubility. Thus the concentration of
available
 calcium ions is limited and potassium nitrate will always be present although, obviously, the final yield of saltpetre will be considerably enhanced by the addition of more potassium ions. Some procedures add calcium in the form of quicklime, calcium oxide. In the presence of water, this immediately reacts to form calcium hydroxide, slaked lime, which itself further reacts with carbon dioxide from the air to form calcium carbonate
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. Any calcium (or magnesium) bicarbonate is destroyed by the boiling process and precipitates the carbonates as is commonly seen in furred up kettles. Although calcium containing materials may be present in large quantities, the limitedpotential availability of calcium is demonstrated by the low solubility of the materials used.Solubilities (at 20
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C)Calcium sulphate 2.6g/l (gypsum, main constituent of old mortar)Calcium hydroxide 1.7g/l (slaked lime)Calcium carbonate c. 0.007 g/l (limestone and lime mortar)  All potassium salts are highly soluble and the solubility for potassium nitrate at the same temperature is 330g/l  Although the presence of nitrate ions will to some extent solubilise part of the calcium, it is clear that, as there is very limited calcium present in solution, this is from a low starting point. Any calcium nitrate present would be removed during the extraction process; see below.The first written record of the use of wood ash in this context is The 13th Century  Arabic text of Hassan al-Rammah who gives a detailed description of the purification of saltpetre using wood ash8. However it should be noted that the FWB records the addition of other sources of potassium; alum, (potassium aluminium sulphate), cream of tartar (potassium bitartarate) and, most interestingly, although it does not specify wood ash,
Sal Sylvestris
, wood salt i.e. pure potassium carbonate extracted from wood ash. These sources of potassium in the Firework book were completely overlooked by the translators and gave rise to the erroneous belief that calcium nitrate was the dominant species. It is unfortunate that following authors have seen fit to repeat the error without questioning the
7FWB etc.8Calcium sulphate added as 'old mortar'.9An elementary school experiment to demonstrate carbon dioxide in breath.
Journal of the Ordnance Society Vol. 27, 2020

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