The Flintlock - Theoretical Considerations and Experiments
Flintlock Pistol "AN IX de Gendarmerie"
The above photo shows a replica of a flintlock pistol from the Napoleonic era.
This compact model was primarily issued to French police forces but it was also
popular among military officers. Like many smoothbore guns of its time, it has
no sights. Nevertheless, it is an effective close-combat weapon due to its large
caliber (15.2 mm). Although it is not exactly what you would call a target
pistol, shooting it is MUCH fun. I use a patched .58 cal round ball and 40
grains of Swiss Powder #1.
The flintlock was the prevalent ignition system for muzzleloaders for more than two hundred years. After 1820, it was gradually superseded by the more reliable percussion lock which utilizes a small quantity of an impact-sensitive explosive as ignition source. The invention of the flintlock was inspired by the ancient method of starting a fire with flint and steel. When a sharp-edged flint strikes a steel surface at an acute angle, it shaves off tiny steel particles which are heated to their ignition temperature by the intense friction in the contact area. The burning metal particles, visible as sparks, are able to ignite a suitable solid fuel, e.g., tinder or, in our case, black powder.
The outside of a flintlock shows five main parts: the hammer (holding the flint in its jaws), the hinged pan cover (protecting the priming powder) with the attached frizzen (striking surface), the V-shaped frizzen spring (keeping the pan cover either open or closed), the flash pan (holding the priming powder), and the touch hole. The latter, also called "vent", is a small hole in the side wall of the barrel forming a connection between flash pan and powder chamber. The internal parts of a flintlock are more or less identical with those of a percussion lock.
The following photos show the four basic positions of a flintlock:
This is how we shoot a flintlock gun: after loading the barrel in the usual
manner, we set the lock at position II and pour a small quantity of priming powder
(typically 3 grains) into the flash pan. Next, we pull the frizzen back until the
pan cover snaps shut (position III)). Now the gun can be moved around without
losing the priming powder. The powder is further protected from being blown away
by an air draft.
Immediately before shooting, we pull the hammer all the way back into the full-cock
position. The lock is in position IV now and ready to fire.
As soon as we pull the trigger, the following things happen: The hammer gets
impelled forward by the tension of the main spring, and the flint strikes the
frizzen surface at an acute angle. The blow pushes the frizzen forward and thus
opens the pan cover while the flint simultaneously scrapes the curved frizzen
surface. The sparks thus created fall into the (now uncovered) flash pan and
ignite the priming powder which produces a fireball and ignites the main
charge through the touch hole. After the shot, we find the lock in the resting
position (I) again.
Shooting a flintlock gun is undoubtedly the highest art of muzzleloader shooting. A flintlock requires much more care and maintenance than a percussion lock does. The edge of the flint facing the frizzen needs frequent resharpening. It further has to be aligned parallel with the frizzen surface. Flint and frizzen have to be clean and dry, otherwise no or not enough sparks will be produced. The touch hole has to be free from obstructions. A flintlock is unforgiving. If you neglect it, you will soon experience misfires and hangfires. When shooting, you have to keep a steady hand not only during but also after pulling the trigger because of the time lapse which is (slightly) longer than with a percussion lock. Particularly unexperienced shooters tend to flinch because of the flash produced by the priming powder immediately after pulling the trigger.
It should be mentioned that a flintlock works best (fastest) when the touch hole is empty. A touch hole entirely filled with powder works like a fuse, delaying the ignition of the main charge (hangfire). This is why experienced flintlock shooters temporarily insert a piece of wire or a toothpick into the touch hole while loading the barrel. A well-designed and properly handled flintlock is almost as fast as a percussion lock. The following picture shows a schematic cross-section of barrel and flash pan at the moment of ignition:
Schematic Cross-Section of Barrel and Flash Pan
These are the known facts. One question, however, remains unanswered in my opinion:
how exactly does the priming powder ignite the main powder charge? In other words,
how is the energy required to ignite the powder charge transferred through the
touch hole? The literature is rather vague in this respect. Most authors talk
about a "flame dashing through the touch hole" or use similar nebulous
In my opinion, there are three possible mechanisms of energy transfer:
1. thermal radiation emitted by the fireball of the deflagrating priming powder
2. a flame or a jet of hot combustion gases passing through the touch hole
3. hot particles (sparks) flying through the touch hole
Here is a summary of what I did to learn more about the ignition mechanism:
1. "Radiation Theory"
My first attempts were of purely theoretical nature. Remembering what I had learned about thermodynamics many years ago, I calculated the radiation power per unit surface area emitted by the deflagrating priming powder using the Stefan-Boltzmann Law (assuming a fireball temperature of 2000 K and an emission coefficient of 0.8). Then I calculated the energy of the heat pulse (assuming a duration of 0.05 s) passing through the touch hole and hitting the surface area of the powder defined by the cross-sectional area of the touch hole. Since I had no information about the thermal conductivity of black powder (porous material), I assumed a certain volume of powder, defined by the cross section of the touch hole and the assumed thickness of a surface layer, to be heated uniformly by the heat pulse. Using material constants obtained from the literature (density of black powder, specific heat capacities of its components (solid and molten), heat of fusion of sulfur, heat of fusion of potassium nitrate), I was able to calculate the maximum temperature of the powder layer thus heated. To make it short, you can produce any result you want by changing those parameters which are not exactly known, particularly fireball temperature and duration, emissivity, absorptivity, layer thickness, etc. Since my calculations didn't give me the clear answer I had hoped for, I abandoned the theoretical approach (although it was a good exercise) and tried to get more insights by experiments.
If heat radiated from the fireball of the priming powder were the source of ignition, the powder charge would ignite even if it were behind a window of a transparent material, provided the transparency in the infrared range is high enough.
At first, I made some experiments with transparent 0.1 mm polymer foils (adhesive tape, polyester foil) which I inserted between flash pan and touch hole. I loaded the barrel with a small quantity of loose powder only (no wad) to avoid any significant pressure build-up which would destroy the foil. The results were inconclusive. In some cases, the powder charge ignited, but in all cases of ignition, I found the foil destroyed. When I used a double layer of foil, it remained intact but now there was no ignition any more. I then replaced the foil with a thin (0.2 mm) cover glass and afterwards with an even thinner (0.1 mm) sheet of mica. These inorganic materials resisted the heat, but there was no ignition. After a while, I managed to acquire a thin (0.2 mm) silicon wafer of the type that is used for the manufacture of solar cells. Although silicon has a much higher infrared transparency than the materials previously used (up to 10 µm wavelength), there was still no ignition. For my last experiment, I used a potassium bromide (KBr) window of 0.4 mm thickness. The transparency of KBr ranges from visible light to a wavelength of approx. 20 µm. Even with this highly transparent material, no ignition was observed. These experiments do NOT prove that the heat emitted by the burning ignition powder is insufficient to ignite the main charge, but they make it seem less likely.
2. "Flame Theory"
I had my doubts about energy transfer through a flame or jet of hot gas from the beginning because any jet of gas would probably be stopped by a cushion of entrapped air in the touch hole before reaching the powder charge (remember, the barrel is plugged by powder and ball). Moreover, any hot gas entering the touch hole would rapidly lose its heat to the cool metal walls because of the relatively high specific surface area (this is how Humphry Dayvy's miners' safety lamp works). To check this, I tried to ignite the powder charge inside the barrel by pointing the flame of a propane torch straight at the touch hole. Even after several seconds, the powder did not ignite. I only stopped the experiment when the barrel got so warm I could barely touch it. I repeated the procedure several times, but in no case did the powder charge ignite.
3. "Hot Particle Theory"
When experimenting with transparent foils, I inspected them after each "shot" and found them covered with soot and ash, as expected. When I examined these residues under the microscope, I noticed small particles which looked like droplets of molten material which had solidified when hitting the cooler surface of the window material. Apart from the soot, these droplets are composed of inorganic reaction products, predominantly various potassium salts. Due to the high melting points of inorganic salts (usually several hundred degrees), the droplets should be hot enough to ignite black powder (ignition temperature approx. 200-460°C / 392-867°F according to MSDS) upon contact. To check if the droplets are able to pass through a touch hole and hit the powder, I took a piece of sheet metal (thickness 5 mm) and drilled a 2 mm touch hole through it. After covering one side of the hole with adhesive tape, I attached an improvised flash pan to the sheet at the opposite side of the hole and secured the assembly in a vise. After putting 3 grains of priming powder into the pan and igniting it, I removed the piece of adhesive tape from the touch hole and examined it under a microscope.
The microscope image below reveals that several particles of different size (the diameter the biggest one is about 0.25 mm) passed through the touch hole and embedded themselves in the adhesive layer of the tape.
Although I am not able to present rock-solid scientific proof, I tend to believe that the powder charge of a flintlock gun is ignited by a combination of radiant heat and a shower of hot particles emitted by the deflagrating powder in the flash pan. It may be possible that the outer layer of powder gets preheated by a pulse of infrared radiation and thus ignites more easily when hit by small hot particles. The occasional "flash in the pan" (misfire in spite of ignited priming powder) may be explained by the fact that number and size of hot particles hitting the main charge are random-controlled. In other words, there is always the risk that no hot particle of sufficient size passes through the ignition channel. Accordingly, the known fact that large-diameter touch holes have a smaller percentage of misfires than smaller ones may be a result of the greater probability of hot particles hitting the powder charge. Since there are limits to the size of a touch hole (pressure loss), biconical touch holes are the preferred design. They not only collect more particles and radiant heat but also reduce the fuse effect (hangfire) since the bottleneck of the ignition channel is shorter than with a cylindrical touch hole.