Friday, February 12, 2010

Flintknapping Followup - Introducing Percussion

My job is generally fairly solitary. I work at home, alone most of the time, and have a pretty comfortable routine. The last two days of demos and workshops have really been a lot of fun for me. It was great to experience flintknapping from different points of view. The grade 5 kids on Wednesday were fantastic - check out this great drawing that Shauna did while I was chipping away!

We had an excellent turn out at the percussion workshop, with 11 knappers around the tarp, including myself. I have a patter down when I'm doing a knapping demo that emphasizes the predictable nature of the rock and the control that the knapper has over his or her work. The core I worked last night did everything in its power to prove me wrong, but it did provide some great opportunities to talk about all the bad kinds of flake terminations and why bifaces break in the middle when you hit them on the end. It was the right demonstration for things to go wrong and the participants did a great job of working their cores. There was some trepidation to make that first strike, but after everyone was set up and rolling, they did great - lots of good flakes, unifaces and bifaces.

There's no substitute for practice, but understanding some of the theory behind how a rock breaks - something that archaeologists call "fracture mechanics" - will really help you work your stone. The most fundamental part of the process is the Hertzian Cone.

Knappable rocks have a high silica content and a stone like obsidian is actually natural volcanic glass. If you've ever shot a BB at a window and seen the little cone of glass that pops out the other side, then you've seen a hertzian cone. The cone is a fracture in the glass that starts at a small point on the surface and expands radially outward as it travels into the surface.

Striking a core with a hammerstone in the middle will create these little cones under the surface of the rock, but they won't detach a usable flake. The secret is to move that cone to the edge of the rock and strike it at an angle so that part of the cone slices through the edge of the core. By changing the angle that you hold the rock, you can change the length of the flake. Striking at a steep angle will give you a longer flake, while striking at a shallow angle will give you a shorter flake. I like to support the core on my thigh on a big piece of leather and strike it with the hammerstone straight up and down. I don't change the angle of my hammerstone, that stays perfectly vertical, the only angle that I change is the core, by sliding it up or down the outside of my thigh, supporting the flake the whole time.

A good platform will be flat, meet a face at a 90 degree angle or less, and will have a ridge below it to guide the flake. The surface of the core that will form the back of the flake is really important. A nice ridge or two from previous flake scars create good highways for flakes to travel along. A more irregular surface, or one with fractures cutting across it will stop your flakes short. The length of your flake is determined in a large part by the angle that you strike the core, but its also influenced by the flakes thickness and you control the thickness of the flakes by how close to the edge you strike the core.

If you hit the core close to the edge you will get a narrow flake. They can still be quite long, if you have a good ridge for the flake to follow along. Check out this beauty made by one of the knappers last night. She created this nice long blade while thinning down a biface of English flint.

If you strike the platform farther back from the edge, then you'll get a thicker flake and if you position it so that it follows two or more ridges, then you'll get a wider flatter flake that will be good for further reduction. A flake that follows a single ridge will be triangular in cross-section and can be tricky to work into a biface, but a flake that follows two ridges will have a trapezoidal cross-section and be nice and flat to turn into a tool.

But that's just the theory. Doing that in real life is a little trickier and comes with the pricetag of a lot of practice and a small pharmacy worth of bandaids.

Photo Credits:
1,4-12: Tim Rast
2,3,13: Lori White

Photo Captions:
1: Tim Rast Flintkapper by Shauna
2: The Homo erectus corner
3: Flintknapping Workshop Feb 11, 2010
4: Hertzian Cone diagram
5: Hammerstone and obsidian flakes
6: Hertzian Cone diagram showing the result of hitting a core away from the edge.
7: Hertzian cone in the middle of a rock
8: You can see how the cone sliced this flake off the core
9: Change the angle of the stone and strike the core on the edge to detach a flake
10: You can make long linear blades on a core by striking a platform above a ridge
11: A very nice blade!
11: You can make wider flatter flakes by positioning you platform over two ridges
12: Practice! practice! practice!


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