Flint Physics

Some of the Physics of Flintknapping

Flint is not the best material to flintknap. Glass is the best. Flint is only good in so far as it mimics a few key qualities of glass.

Man-made glass is a relatively recent invention, but fortunately nature provides us with Obsidian, which is a naturally forming volcanic glass. It possesses all the same properties of modern glass, such as being non-crystalline, having a similar degree of plasticity and brittleness, and breaking with a conchoidal fracture. Non-crystalline means that it is a single coherent mass which is not composed of clumps of crystals or particles, like most rocks. Think of the difference between a smooth sheet of glass and a lump of sandstone. The sandstone is composed of lots of little grains, while the glass is completely smooth. Glass and obsidian, are technically cooled fluids

The elasticity of glass is also an important characteristic. The elasticity is a measure of how flexible the material is and how easily it breaks apart. It ranges from Brittle to Plastic. A brittle material is totally rigid and will break rather than bend. Plastic material, on the other hand will bend or stretch rather than break. Think of the difference between peanut brittle and plastecine. Where a rock falls on this continuum determines its flaking qualities. A certain amount of brittleness is required for the rock to be flakable, while it must also possess some plasticity to keep the material from shattering. Glass has these properties in just the right amounts to make it ideal for flintknapping.

These properties of glass; its amorphous non-crystalline structure and its balance of plasticity and brittleness produce a very characteristic and predictible fracture pattern when glass is broken. This fracture pattern is called conchoidal fracture. Because glass has no internal crystalline structure a force applied by striking it or pushing it will travel equally in all directions, in the same way that a pebble dropped in a pond will send ripples out in all directions around the spot where is landed. As these ripples spread out inthe glass they also move forward, creating a cone, with the point at the point on the surface where the impact occured. And because glass is brittle this force will spread very quickly through it. That little cone is what does the work of making the flake, all the flintknapper has to do is decide how to make the cone and where to position it so that it produces the desired effect. These fracture pattern is also present in the crypto-crystalline silicates like chert, flint, and jasper.

Knappers Anonymous was prepared by Tim Rast and is being updated by Mike Melbourne