Understanding Flake Mechanics
An Unifying Theory

Tony Baker    November 18, 2001

For the last four years I have been using Finite Element Analysis to model flake creation. This work began in association with Dr. Andrew Pelcin with the intent of understanding Folsom fluting. Several years ago I began conferring with Bob Patten, who is an engineer and accomplished knapper. Our common technical backgrounds, his knapping experience and skills, and my computer modeling have proved to be an extremely powerful team. Today, we have come a considerable distance in developing a new paradigm for flake mechanics and we want to begin to share it with the reader. To do this I have created one image that conveys the various flake types that can be created from a particular Folsom preform. This image fits the old adage "a picture is worth a thousand words." To view this image, click map and then return with the "back button" for more discussion.

The image you just viewed is divided into two frames. The left frame contains a map (graph) with various dots. Clicking on a dot will display an image in the right frame that represents the flake that can be made for the associated angle of blow (AOB) and vibrating frequency. Click map, to try a few dots and then return.

The map contains different regions of flakes (dots) defined by colors and lines. White regions contain flakes made in the energy-poor mode and the colored regions indicate flakes made in the energy-rich mode. Energy-poor flakes are only a function of AOB, so all the dots in the white region with the same AOB will produce the same flake. Energy-rich flakes are a function of AOB and the natural frequency of the preform and impactor during impact. Therefore, each dot in the colored area will produce a different flake.

For this Folsom preform and support configuration, energy-poor flakes exit out the dorsal face of the preform with the exception of a few full-length flakes that occur very near the AOB of zero. The energy-rich flakes exit both the dorsal and ventral faces, in addition to running the full length of the preform. The pink color designates the region that produces full-length flakes. The green region produces flakes, often called overshots, that exit the ventral face. The yellow region contains two types of flakes that exit the dorsal face, feather flakes located above the separating line and step flakes located below the separating line. The step flake is the only energy-rich flake that stops propagating in the energy-rich mode and converts to the energy-poor mode in order to finish. A final observation is made about the overshot flakes. These flakes exit below the upper support. It is physically impossible to exit above the upper support.

Energy-poor flakes are pressure flakes (manual or levered), most indirect percussion flakes, and some direct percussion flakes. Energy-rich flakes can only be created by direct percussion and are faster than the energy-poor ones. They have straight ventral faces and work best with preforms (cores) with straight faces. Energy-poor flakes appear at this time to be more compatible with curved faces than energy-rich flakes.

To create energy-rich flakes, the impactor's natural vibrating frequency needs to be equal or somewhat greater than the core's natural frequency. During impact (or the time of contact) the impactor and the core unite and vibrate at a frequency that lies between their individual, natural frequencies. Individual natural frequency is a function of mass, shape, and material and this is explained in the following principles.

These principles can be seen operating as a knapper reduces a large globular core with hard-hammer percussion. As the core gets smaller its natural frequency increases. Remember, to operate in the energy-rich mode, the impactor must have a frequency equal to or greater than the core. So the knapper is forced to change to a smaller stone hammer to match the increasing frequency of the core. If the knapper decides to create a biface, then the frequency of the core begins to drop, instead of increasing, as the biface develops. The dropping frequency of the biface causes the knapper to choose between increasing the size of his hammerstone or changing to a billet. Since the work is becoming more precise and delicate, the knapper always chooses the billet.

I know there are knappers who use antler billets on large globular cores. This is a technique to make large curved blades. These are made in the energy-poor mode because the impactor has a lower frequency than the core. If straight blades are desired, the knapper must move to the energy-rich mode by increasing the frequency of the impactor.

Most of this document has focused on the frequencies of the impactor and the core because this variable is the least understood. However, there are three other variables that are equally or more important than frequency. One is obviously AOB, which is the vertical axis on the Folsom map. The knapper can manipulate AOB while reducing a core and offset much of the changing frequency caused by removing flakes.

The two remaining variables not mentioned as yet are the most important. These are core morphology, which includes load location, and support(s). On the Folsom map these two variables are held constant because I have not devised a method of presenting the effects of changing them on a single graph. I am developing an identical map for a globular core that is four inches long and three inches thick. This will then be added to this web page for comparison to the Folsom preform map.

Finally, I want to declare that the numerical values on the Folsom map are not real world. However, they are relative to real world. There are two reasons for this. First, the computer model has rigid supports, which means that the preform is firmly attached via supports to the earth instead of being more loosely supported. Second, it is almost impossible to calculate accurate natural frequencies of globular masses. Therefore, I have created relative values based on boundary conditions of various known flake velocities.


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