Saltpetre in medieval gunpowder: Calcium or Potassium Nitrate?

Saltpetre in medieval gunpowder: Calcium or Potassium Nitrate?

By Geoff Smith

medieval cannon

Until recently, it has been accepted that the formulation of gunpowder has always been based on variable mixtures of charcoal, sulphur and potassium nitrate. This has recently been challenged. It has been asserted that early gunpowder was based on lime saltpetre that is calcium nitrate. This paper examines that claim.

The earliest documentary details of gunpowder manufacture known in western literature are contained in Das Feuerwerkbuch written in about 1400. The language is Middle High German and we are indebted to Prof. Gerhard Kramer for a modern translation. The introduction to the translation claims that the text “ …refers exclusively to calcium nitrate” and that “… potassium nitrate was introduced into powder making towards the middle of the sixteenth century.” It is further stated that… “This surprising inference can now be considered as well established.” The translation was reported in Proceedings of The International Committee for the History of Technology (ICOTECH) and the editor of the compiled papers commented that this could require revision of our current understanding of early gunpowder. The idea has been repeated in more recent publications and appears to be taking root in the literature with authors quoting it as established fact.

The proposition that all early gunpowder was based on calcium nitrate represents such a significant shift in our understanding of the history of this important mixture that it should reasonably be expected to withstand rigorous testing before it is universally accepted. In particular, any theory should pass the test of Occam’s Razor; that it should be supported by the available evidence and that no simpler explanation is available.

First, it must be recognised that the terms saltpetre or nitre were loosely defined at the time and of little help. Chemicals in the 14/15th centuries could have different names in different localities and until about the 17th century, they can only be reliably identified by their associated characteristics of appearance, taste, smell or how they react with other materials.

Saltpetre was historically either collected from naturally occurring deposits in very limited geographic locations or, more usually, extracted from rotting organic material. Dung, urine and vegetable matter were stacked and allowed to ferment. Ammonia in the mix was converted to nitrite by Nitrosomas and this was converted to nitrate by Nitrobacter. Both of these organisms require a neutral or slightly alkaline environment to operate, whereas the decaying matter is naturally acidic. Lime was added, as in agriculture, to reduce the acidity of the pile to a more suitable level. There was therefore a large amount of calcium present in the mix. However calcium carbonate and sulphate are both essentially insoluble in water and it cannot be assumed that calcium ions were proportionately present in the subsequent leachate. Saltpetre was manufactured by this method until well after WW1 and was well-documented using modern terminology. The liquor is rich in nitrate and contains variable proportions of sodium, calcium and potassium ions. Conventionally, the potassium salt is enhanced by the addition of excess potassium in the form of potash derived from wood ash. This operation is first described in western literature by Biringuccio in his Pyrotechnica dated at about 1538. Since this post-dates Das Feuerwerkbuch (1432) which does not mention the procedure, the translators draw the conclusion that the chemical conversion did not take place and that only calcium nitrate was formed.

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