First used for battle in China in about 900 A.D., the promise of gunpowder spread to the rest of the world by the early 13th century, eventually revolutionizing warfare as a propellant in firearms and artillery. Throughout this time, master gunners tinkered with gunpowder formulas, trying to find the ideal concoction.
Now, researchers have recreated medieval gunpowder recipes and analyzed the energies released during combustion, revealing that the evolution of the perfect powder was a slow, trial-and-error process—like almost all science is.
For the study, published in ACS Omega, the researchers identified 16 gunpowder recipes from medieval texts dated 1336 to 1449 A.D. They prepared the powders and measured the energies released just before and during combustion using differential scanning calorimetry and bomb calorimetry.
Beside medieval gunners adding somewhat random ingredients, gunpowder is now and has always been a combination of potassium nitrate (KNO3, also called saltpeter), sulfur (S8, octasulfur) and charcoal. What has changed throughout the years is the ratio of each of those ingredients.
Whereas today’s gunpowder composition ratios are typically 75:10:15—KNO3: S8: charcoal— medieval recipes were generally lower in potassium nitrate and higher in sulfur. For example, during the mid-1300s to early 1400s, KNO3: S8 varied from 2:1 all the way to 16:1; while KNO3: charcoal ratios varied from 1:1 to 8:1.
According to the bomb calorimetry data reported in the study, increasing the percent of charcoal leads to higher heats of combustion. The research team recorded the two highest combustion values from 1336 recipes where KNO3: charcoal ratios were 1:1.
“It has been suggested that one reason gunpowder recipes changed over time is the need for safer recipes that did not put medieval gunners at risk or cause damage to cannons,” the authors write in their paper. “This idea is supported by the fact that the two oldest recipes had two of the highest heats of combustion. Gunners may have stopped using these recipes because they had such high levels of thermodynamic activity.”
The researchers found that, as time went on and sulfur content in the recipes decreased from approximately 20% to 5%, the heat of combustion decreased by about 3.5%.
After 1400, gunners began corning their recipes and introducing somewhat random additives. Corning, or the practice of drying gunpowder into small clumps, was thought to improve combustion and consistency. To that point, recipes from this time period sometimes included water, varnish, vinegar and brandy.
When the research team recreated the recipes, they found that corning with water lowered the heat of combustion but the impact of vinegar, varnish and brandy was inconclusive.
Although they tested a few of the recipes at a West Point firing range using a replica of an early 15th-century stone-throwing cannon, the researchers say more field work should be done to evaluate which formulation would perform the best in historical contexts.
In future studies, the team wants to use porosimetry and scanning electron microscopy to compare the surface areas and spacing between the ingredients of the recipes to better understand the impact of corning.
“[We want to] analyze gunpowder recipes to aid historians in their interpretation of medieval texts and to determine whether there was intent in the creation of these recipes by master gunners,” writes the research team from West Point Military Academy and Stevens Institute of Technology. “Additionally, understanding the energetics of the recipes provides important technical information on the early manufacturing of gunpowder.”
Photo: Cannon from the 15th century at Šibenik city walls. Credit: Silve Rije