Evolution of Medieval Gunpowder: Thermodynamic and Combustion Analysis

Medieval gunpowder recipes of potassium nitrate (KNO₃), charcoal (C), and sulfur (S₈) were investigated by bomb calorimetry to determine their enthalpies of combustion and by differential scanning calorimetry (DSC) to determine their pre-ignition and propagative ignition enthalpies. Various sample preparation methods and several additional ingredients were also tested to determine any effects on the thermodynamic values. Gunpowder recipes were prepared and used in a replica cannon that was manufactured and operated according to medieval records. Post-firing residues were collected from the bomb calorimeter and the cannon in efforts to further characterize recipe energetics using DSC. In general, during the period of 1338-1400, the %KNO₃ increased, and heats of combustion decreased, while between 1400 and 1460, the %KNO₃ decreased, and heats of combustion increased. However, since KNO₃ was usually found in the post-bomb calorimetry and post-cannon firing residues, it was not the limiting reactant. The highest pre-ignition and propagative ignition energies occurred when the KNO₃:S₈ ratio was 3:1 as determined by DSC, and the highest enthalpies of combustion were measured for recipes where the KNO₃:C ratio was 1:1 as determined by bomb calorimetry.