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 predominant ignition system for muzzleloaders for more than
two hundred years. After 1820, it was gradually superseded by the more reliable
and easier to maintain percussion lock (caplock) which uses a small quantity of an
impact-sensitive explosive as the 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 this case, black powder.
Designing a reliable and reasonably fast flintlock is a challenging task. Critical
factors are the overall geometry, the characteristics of main and frizzen spring,
the hardness of the frizzen material, etc., etc. Reportedly, the most sophisticated
flintlocks were made by British gunsmith Joseph Manton in the early 19th century,
shortly before the advent of the caplock.
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 caplock.
The following photos show the four basic positions of a flintlock:
I |
II |
III |
IV |
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
terms.
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. Experiments with thin sheets of silicon and potassium bromide (both
highly transparent to infrared light) brought no success either. Therefore, I
am not inclined to believe that thermal radiation is the dominating factor for
ignition. It may, however, promote it to some extent.
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 photo 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 surface of the adhesive tape.
Powder Residues
Conclusion:
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
radiated heat and a shower of hot particles emitted by the deflagrating powder
in the flash pan. It may be possible that the topmost layer of powder gets
preheated by a pulse of infrared radiation and is thus more susceptible to being
ignited 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
is available to ignite the powder. The known fact that touch holes with a larger
diameter are more reliable than smaller ones (fewer flashes in the pan) may be
a consequence 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 radiated heat but also reduce the fuse effect (hangfire) since
the bottleneck of the ignition channel is shorter than with a cylindrical
touch hole.