Posted by Ava Sheftik
An article titled “This Chemist’s Pandemic Hobby? Firing Medieval Cannonballs” found in the New York Times discusses an activity of Dr. Dawn Riegner, a professor of chemistry at the United States Military Academy West Point, early quarantine activity. She convinced her colleagues and daughter into beginning a study on how well different kinds of gunpowder recipes from the Middle Ages performed.
Their report on their gunpowder analysis was featured in Omega which is a peer reviewed journal of the American Chemical Society. The purpose of their experiment was to determine if the creators in the medieval time period actually understand the chemical characteristics of their materials and processes that they were completing. They analyzed gunpowder recipes which consisted of potassium nitrate (KNO3), charcoal (C), and sulfur (S8) by bomb calorimetry. They then used this to determine their enthalpies of combustion and differential scanning calorimetry to determine their pre-ignition and propagative ignition enthalpies.
The cannon used for the range tests was a reproduction of a Steinbüchse, which is a stone throwing cannon, copied from an extant gun from the 15th century. For safety reasons the gun had some alterations but nothing that should have affected the shot.
Figure 1. Heats of Combustion for Each recipe in Chronological order
This figure represents the thermodynamic potential of gunpowder recipes in chronological order. The suggestion for the change in these recipes is because they needed to have safer ones that did not put the gunners at risk or damage the cannons. The trend observed is that the closer the KNO3:C ratio is, the heat of combustion will be higher and reaction rate will be faster.
The bomb calorimetry data shows that increasing the percent of charcoal will lead to a higher heat of combustion. Furthermore, most samples that were pressed produced a slightly lower thermodynamic potential. This may be due to the lack of oxygen ability to enter to materials due to the lack of spacing between the particles in the pressed sample. Therefore, incomplete combustion would be the result and is the explanation for the smaller enthalpies observed for the pressed samples.
There were several additives also tested: water, varnish, vinegar, and brandy.
It was determined that the addition of water and its effect on enthalpy was specific to each different recipe. However, the addition of varnish to a recipe with high charcoal content and relatively low sulfur content decreases the potential energy measured by the bomb calorimeter. The varnish can provide clumping of the ingredients and prevent sulfur from mixing and which then decreases the surface area of the charcoal.
The addition of brandy as a corning agent did not show a significant increase in the heat of combustion. The suggested reason for this is that it provided missing organic compounds for better quality burning
Figure 2. Addition of varnish
Figure 3. Addition of Brandy
The addition of vinegar requires further studies to come to an ultimate conclusion. It was added to enhance mixing of dry ingredients so that they didn’t separate during transport. Some recipes produced a result that was similar to water, some yielded a lower enthalpy, and some yielded higher.
Figure 4. Pre 1400 vs. Post 1400 ingredient ratios and their impact on the heat of combustion
As time progressed throughout 1300s to1400s the recipe creators were creating a formula that would provide them with a lower heat of combustion which involved increasing the amount of potassium nitrate. After the 1400s the heat of combustion rose again and the potassium nitrate decreased.
These advancements completely changed the nature of warfare during the time period. Sieges that used to take years and months began to only take weeks and days. The guns were now safer for the gunners; however, they became larger and more effective in their use.
References
Broad, William J. “This Chemist's Pandemic Hobby? Firing Medieval Cannonballs.” The New York Times, The New York Times, 7 Oct. 2021, https://www.nytimes.com/2021/10/07/science/gunpowder-medieval-cannons.html.
Ritchie, T. S.; Riegner, K. E.; Seals, R. J.; Rogers, C. J.; Riegner, D. E. Evolution of Medieval Gunpowder: Thermodynamic and Combustion Analysis. ACS Omega 2021, 6 (35), 22848– 22856.
Good title. The opening paragraph succinctly describes the study. A general reader would probably find it immediately engaging. The explanation of the experiments is quite clear. A general reader might find the figures from the original paper hard to interpret, but your explanations of the general trends produced by the various additives are quite good. You might actually have gotten away with omitting the figures and relying on your summaries. A figure with people or cannons might have been eye-catching. Incidentally, did they actually fire the cannon? Overall, however, the reader is likely to come away from the post with a sense that chemists do interesting things and that chemistry has in fact driven historical developments for a long time.
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