Buried in the lines of the accounting books of the City of Solothurn lies a mention of the purchase of saltpetre from the pope himself in 1444 (fig. 1).
The Pope in Basel… dealing saltpetre?
Pope Eugene IV (1383-1447, see fig. 2) was indeed present in Basel, several times during the longest Ecumenical Council (1431-1449) in history. This council assisted in the city’s great economic expansion and cultural development. Even the Medici opened a banking agency there between 1433 and 1444. Emperor Sigismund was responsible for the safety of the council and expected the Swiss Confederates (with the assistance of the allied towns of Fribourg and Solothurn) to ensure peace during the assemblies. This explains why the city of Solothurn was heavily involved in discussions about matters regarding defences – especially about artillery – between Basel, Bern and Fribourg. It does not explain, however, why the Pope would deal in saltpetre.
Saltpetre in the recipe… with vinegar or urine?
Gunpowder was used in Europe from the fourteenth century onwards. Its basic components are: saltpetre (potassium nitrate), sulphur and coal, but a lot of additional elements found their way into the early recipes, such as wood shavings, urine, wine, vinegar, a variety of salts, camphor or salammoniac. The anonymous author or compiler of the Firework book (Feuerwerksbuch) of ca. 1420 explains how to corn gunpowder into “lumps” (Knollen) to deal with the problem of dampness:
Moisten a mixture of saltpetre, sulphur, and charcoal with “good wine vinegar. And grind the mixture together with a wooden mortar and make it so moist with the vinegar that it will clump together, and then roll the lumps as large as you want them to be. Then take a glazed round deep saucepan or a pot or a copper bowl and press the lumps in wet just as you would force a cheese into a pot; then knock it out onto a board – it will come out easily.”.
This process is illustrated in another Firework book kept in Zürich (Fig. 3) where one can see the air-drying process in the sun. These Knollen are then prepared by the master gunner to ready the gunpowder to fire. The process of producing gunpowder in Europe is not well documented before the second part of the fifteenth century, and remains somehow obscure or even ‘magical’, mostly because its production remained to a large extent experimental.
The least stable element in the mixture is the saltpetre because the potassium nitrate is obtained through a process that involves bacterial action (discovered, in the 19th century, by the microbiologist Serge Winogradsky). Saltpetre was indeed produced in Europe, for example in Frankfurt. However, better-quality saltpetre arrived at the time from the Orient,  with northern Italy being one of its most important commercial gateway int Europe. In the same year saltpetre was purchased from the pope, the Solothurn accounting book also registers a greater amount of saltpetre bought at the fairs in Geneva, without, however, giving an indication of its original provenance.
Guns on the wall and master gunners at service
Already in the last quarter of the fourteenth century, several master gunners were hired by towns or princes for their well-needed services. In Solothurn, the first documented master gunner appears in the sources in 1442. By 1444, the accounting books mention at least two master gunners: Master Berchtold and Master Heinrich. Several lines in the accounting books of these years show these masters traveling to Bern and Basel and buying gunpowder, portable firearms and cannons to equip the town’s defences and fighting forces. Portable guns were also produced locally by the skilled metalworkers (here by the armour maker in 1444). The security of the Council of Basel was not the only concern of Solothurn. East of the town, war was waging in fits and starts (the so-called Old Zürich War of 1436-1450), and in the West, conflict arose with the “Armagnacs”, French mercenaries under the command of the daupin, the future Louis XI (Battle of Saint-Jacques sur la Birse, 1444, see fig. 4). The city of Fribourg, and old ally of Solothurn, follows the same timeline, with the first master gunners in service of the town in 1443/4. But just like in Solothurn, firearms were already in use earlier.
Papal gunners and holy gunpowder
The use of gunpowder advanced in the fifteenth century, and so did the manufacturing technology of guns and canons, gunpowder circulation and the master gunner’s trade. However, it remained unpopular in discourses and depictions, where gunners are easily associated with diabolic figures. The first printed edition of Saint Augustinus’ De Civitate Dei (printed in Basel by Johann Amerbach in 1489) for example depicts a devil shooting a gun (fig. 5). Despite this widespread association, the papacy had fully embraced the progress of the artillery since early on. Pius II (1458-1464), the third Pope in succession to Eugene VI, owned 13 guns, which he named after the 12 apostles and his mother, Vitoria.
The papacy hired a master gunner in 1450, Jörg of Nuremberg. He was captured by the Turks when casting guns in Bosnia for the Duke Stephen the Great of Moldavia in 1456. He then served the Turks until 1480 when he was sent to Alexandria from where he escaped and returned to papal services.
But what about the pope as dealer of saltpetre in 1444? It remains unclear if this was a regular papal side-business or if some cunning sellers of gunpowder benefited from the presence and fame of the pope in Basel to sell their product to the Solothurn master gunner. With Basel being at the centre of widespread trade at the time, and in view of the fact that a good-quality product from the Orient might have easily found its way to the upper Rhine region, one call well imagine how a “trademark” of holy gunpowder could have impressed a town involved in war at the time.
Credit for the main image: Frontpiece of the De Civitate Dei of Aurelius Augustinus (woodcut from Meister des Haintz Narr), printed by Johann Amerbach (Basel, 1489), detail. Münchener Digitalisierungszentrum (BSB – Ink A-863 – GW 2888) urn:nbn:de:bvb:12-bsb00025317-8
 The 17th Council had 25 sessions in Basel, Florence and Ferrara between the year 1431 and 1449.
 Translation of Bert S. Hall, Weapons and Warfare in Renaissance Europe (Baltimore and London: John Hopkins University Press, 1997), p. 71. The digitized manuscript and bibliographical information about it are available at the University Library of Heidelberg (online: https://digi.ub.uni-heidelberg.de/diglit/cpg585, accessed 20.05.2020).
 Bert S. Hall, Weapons and Warfare in Renaissance Europe (Baltimore and London: John Hopkins University Press, 1997), p. 58.
 Wilfried Tittmann, ‘China, Europa und die Entwicklung der Feuerwaffen’ in Europaïsche Technick im Mittelalter: 800-1400. Tradition und Innovation, edited by Uta Lindgreen (Berlin: Gebr. Mann, 1996), pp. 317-32.
 Item fazin hatt bracht von Ienf Salpetter und zentner 18lib Ienf gewicht tut an Solott’ gewicht 10 zentner 61lib da hort ein zentner nach Ienfer 12 guld und 12gross. tut in einer / summa 29 gulden und 1gross SS. Solothurn, Staatsarchiv, BB25, vol. 4 (1444), p. 82. Transcribed by the author.
 Leng, Rainer, ‘Zum Verhältnis von Kunst und Krieg in den illustrierten Kriegslehren des 15. und 16. Jahrunderts’, in ‘Mars und die Musen’: das Wechselspiel von Militär, Krieg und Kunst in der Frühen Neuzeit, ed. by Jutta Nowosadtko and Matthias Rogg, Herrschaft und soziale Systeme in der frühen Neuzeit, 5 (Berlin: Lit-Verl, 2008), pp. 33–58, here p. 40.
 Item ein Büchsenmeister der die büchsen beschiessen sollte verzart an Clewin lopsinger 10SS. Staatsarchiv, BB25, vol. 2 (1442). Transcribed by the author.
 Louis Jäggi (500 Jahre Schützengesellschaft Der Stadt Solothurn, 1462-1962 : Die Entwicklung Des Schützenwesens Der Stadt Solothurn von Ihren Anfängen Bis Zur Gegenwart (Solothurn: Union Druck, 1962)) mentions the two masters in 1444, but ommitted the master mentioned in 1442.
 Item dem Harnascher im Hensli peters hus vmb siben handbüchsen 7guldn. Staatsarchiv, BB25, vol. 4 (1444). Transcribed by the author.
 In a document of 1431, large and medium cannons equiped already the fortification walls. For references and details about the master gunner, see Willy Schulze, ‘Freiburgs Krieg Gegen Savoyen 1447-1448 : Kann Sich Eine Mittelalterliche Stadt Überhaupt Noch Einen Krieg Leisten?’, Freiburger Geschichtsblätter, 79 (2002), 7–54 <https://doi.org/10.5169/SEALS-391870>.
 Bernard Rathgen, ‘Feuer- und fernwaffen beim päpstlichen Heere im 14. Jahrhundert’, Zeitschrift für historisches Waffenkunde 7 (1915-17), 1-15.
 According to the writings of Enea Silvio Piccolomini (1405-1464). I would like to thank Kelly DeVries for this reference.
 Based on the master gunner own account (printed in the 1480s), see A. Vasiliev, ‘Jörg of Nurember: a writer contemporary with the fall of Constantinople’, Byzantion, 10 (1935), 205-9, quoted and commented in Peter Purton, The Medieval Military Engineer (Woodbridge: Boydell, 2018), p. 260.