The following was written by Rich Pierce (who knaps some really good flint) and is used with his permission.
Common questions about flints
Are English or French or cut American or knapped American flints “best”? There is no simple answer to this question. Lumping all French flints or all English flints or all American flints together and discussing which are best does not recognize the variability you will find among these different types. But as you decide which gunflints may be best for you, consider what you want from your flint. Generally the 3 qualities shooters care about are performance, price or value, and authenticity. Let’s start with the easiest first. If you are interested in authenticity, then get French flints if your persona would be in French trade areas, and English flints if you are in English trade areas, and do not worry about price or performance. Performance? Performance can mean “flint life” or how many shots before a misfire, and how many shots total before a flint needs replacing. Or “performance” can mean, “I want this flint to throw SHOWERS of hot sparks into the pan when I am deer hunting and if I get 10 shots out of it and never a misfire at deer, then this is a great flint.” Performance does not depend so much on the TYPE of flint (English, French or various American cherts or agates) used to make gunflints. Performance depends on material, shape (thickness and angle of the cutting edge), and most of all, on the lock and the user. Price or value may mean “sparks per dollar” or “shots per dollar”, and you may find that flints from some suppliers give excellent value. But if you got 100 flints for 50 cents and each was only good for three shots, you might get so frustrated that you would toss them in the weeds. So cheaper is not always better.
What size flint do I need? How do I know if it will work better with the bevel up or the bevel down? First, your lock or gun maker may have a recommendation for the size of flint you need, and that is a good place to start. But even with common production locks there is some variability in lock timing, etc and so it’s worthwhile to figure out what size flint works the best in your lock. A half hour of your time tinkering with your flintlock is never wasted
To test this, make a “fake flint” of hardwood. First measure the frizzen at its widest point. Get or cut a piece of 1/4″ thick hardwood slat and cut it to that width. If your lock is tiny, use thinner stock. Make a chisel edge at one end of the hardwood slat. Not a knife edge, but a chisel edge like on a gunflint, so there’s a “bevel up/bevel down” side to
the “fake flint”. Now cut the piece off at a length 1/4″ longer than the width measurement.
Mount this in your lock with a folded leather piece just as you would a gunflint, bevel up. Now test it.
1) Does the flint clear the frizzen at half‐cock by more than a smidgeon? If not, take some off the back until you have at least 1/16″ clearance.
2) Take it to full cock and fire it. Hopefully, no matter how hard the wood, you won’t get sparks. But did the frizzen fly open easily?
3) Go ahead and turn the flint so the bevel is down (flat surface on top). Now it should strike higher on the frizzen. How’s your clearance to the frizzen? How did the frizzen fly open? Keep trimming the backside of the fake flint until you have a measurement that works, then trace around the fake flint on a piece of cardboard. This is you “max size template”. Make 3 copies and write which rifle they belong to. Stick one in the patchbox now.
4) Now keep shortening the back of the “fake flint” until it starts to fail to flip the frizzen open and note whether it fails first in the bevel up or bevel down position. Note when the flint is so short that the top jaw hits first (been there!).
Now you have the correct maximum width and the minimum and maximum length of flints that will work in your flintlock. Use this when ordering or better yet when picking out flints at a shop or vendor.
How do I mount the flint to get the best performance? Always mount the flint so that it will strike the frizzen 2/3 to at most 3/4 of the way up from the bottom of the frizzen. Make adjustments as needed if bevel up or bevel down. Do not trust the adage that the flint should just about touch the frizzen at half cock. That’s hooey and just depends on how the tumbler notches are cut. To pad the flint in the jaws of the cock, use a piece of folded leather thick enough to deform some if the flint is “peaked” but supple and thin enough to easily fold at the rear of the jaws. Whenever possible anchor the rear of the flint back at the jaw screw. If it is short, put a piece of matchstick or a square of hardwood behind the flint so it has support in the back against the top jaw screw while you extend the front edge the right amount. If the flint is the same width as, or narrower than the frizzen, mount it in the center of the jaws. If it is wider than the frizzen, mount it offset a little to the outside of the jaws so the flint will not strike the barrel. When you have it mounted so the striking edge is square to the frizzen and so it will strike 2/3 or slightly more above the base of the frizzen, tighten the dickens out of it and re‐check to see it will strike the frizzen squarely. Now snap the lock in your unloaded gun 2x. Look for sparks and double check to see that the flint did not chip or get loose.
Now you should get 10‐15 shots with lots and lots of sparks with most any lock and flint. When you have your gun emptied after 10‐15 shots, dry fire once and see how things are going. Make adjustments as needed. This may involve flaking to sharpen the flint (some call it knapping the edge), flipping it so it is bevel down after some wear, or moving it forward in the jaws and placing support behind it. Go back to the steps outlined above after adjustment: tighten the dickens out of it, check to see it is square, and dry fire it 2x to see it is sparking. Go shoot another 10‐15 times or until you experience ignition problems.
How do I knap or sharpen my flint when it dulls? Flints can become dull or develop a projecting shiny round spot that prevents the rest of the edge from striking the frizzen. The solution is to knap a fresh edge on the flint. There are at least 3 basic techniques for knapping the flint. Safety first: NEVER, no matter what precautions you take, knap the flint on a loaded gun. No matter what technique you choose, you must make sure the gun is unloaded. If the gun won’t fire with the flint as it is, replace the flint. Now, to get down to business, flip the frizzen open and put the lock on half cock. Always keep the muzzle pointed in a safe direction and for good measure you may want to plug the touchhole with a feather or toothpick. Being right handed I cradle the gun in the crook of my left arm with the muzzle pointing to my right and me facing the flint head on.
The simplest way to knap the flint is to strike downward across the face of the flint with a piece of coarse threaded rod about 2” long and 5/16 in diameter. The threads give repeated small blows to the edge and give a very controlled result. I angle a sweeping blow forward from behind the edge of the flint toward the pan. Little chips should fall in the pan. Always blow or brush the flint chips out. Flint dust will cause wear or pitting of your frizzen spring and frizzen pivot and affect the fit between pan and the frizzen pan cover.
The second method is to strike the front edge of the flint with a copper, brass or soft iron tool shaped like a miniature hammer. Place your left forefinger below the edge of the flint and press upwards with a good bit of pressure as you give little blows along the front edge of the flint. Little chips will fall off the underside.
The third method uses no tools. With the lock in the fired position, bring the frizzen back to lie atop the flint. With your right hand draw the cock back as you press the frizzen down on the flint. When the very edge of the flint is just barely below the base of the frizzen, press down mightily on the frizzen while drawing the cock back. This should shear a fresh edge on the flint. But it feels like scratching your fingernails on a chalkboard, so it is not for everyone.
Written By: Rich Pierce