I want to begin this section by considering two almost identical rods. Both are 24 inches long and have the diameter of a wooden pencil. The only difference between them is one is made of glass and the other is made of steel. Consider what happens when you grasp the ends of the rods and try to bend them. Both will bend a little and then the glass rod will break. If you continue to bend the steel rod, it will deform and be permanently bent when you release your grip. The difference in the behavior of the two rods is a result of their material composition. The steel rod is made of ductile material and the glass rod is made of non-ductile material. If the forces and corresponding deflections applied to the rods are measured, they will plot as depicted in Figure 16. Of interest to us is the point at which the glass breaks or the steel starts deforming. This point is called the elastic limit. If a force that is less than the elastic limit is applied to the ends of each rod, each rod will bend. If the force is removed, each rod will return to it original straight shape. The rods are behaving elastically. If the applied force is greater than the elastic limit, the steel rod is permanently bent and is behaving plastically. The glass rod just breaks. It has no plastic behavior.
The force application location (FAL) is the location on a core where the knapper applies the force that creates the flake. Often this location is called a platform, but platform is too general for my purposes here and, therefore, I have chosen to use FAL to refer to this location.
Similar to the glass and steel rods, FALs have an elastic limit. For example, if a pressure knapper does not apply sufficient force to the FAL, nothing will happen. If a percussion knapper does not swing the impactor fast enough, the impactor just bounces of the FAL. In each case, the cores are deflected, but the applied forces are less than the elastic limit of the FAL. To exceed the elastic limit of the FAL the pressure knapper has to push harder and the percussion knapper has to swing faster. And, as every knapper knows, one must exceed the elastic limit to make a flake.
The strength of the FAL or its elastic limit usually is out of the knapper's control. The best example of this is the first flake removal from a nodule. All surfaces are natural and therefore Mother Nature dictates the strength of all possible FALs. The knapper can select a given FAL from several that Mother Nature offers, but the knapper can not alter the strength of any of the FALs. Not being able to manipulate the strength of the FAL continues to be a reality as long as the knapper chooses to do off-margin striking. Consider Figure 17 with the three FALs; A, B, and C. Location A is on-(the)-margin and, B and C are off-margin locations. The strength of these three locations is depicted in Figure 18 (graph with FAL strength of vertical and distance from margin of horiz.). As is evident, FALs B and C are extremely strong in relation to FAL A. Their strength is the result of being off-margin locations. Usually off-margin locations are so strong that only percussion flaking can remove flakes from these locations. The only way the strength of these two locations can be reduced is by reducing the margin or moving the edge toward them. Their strength can not be increased above that indicated in Figure 18 (graph with FAL strength of vertical and distance from margin of horiz.) and in reality, the knapper has no reason to increase their strength.
In contrast, FAL A is extremely weak. It is so weak that flakes created from this location are unacceptable. The reason A is so weak is because it is on the margin. Knappers do not pressure flake an un-altered margin. Instead they grind, crush, or whatever, the margin to flatten it. This moves the FAL's location slightly away from the margin and the FALs strength is greatly increased. See Figure 18 (graph with FAL strength of vertical and distance from margin of horiz.). This increase in strength will permit acceptable flakes to be removed from location A.
In summary to this section and as a lead into the section on potential energy, FAL strength partly controls the size of the flake that can be produced. A weak FAL can only yield small flakes. A strong FAL has the potential to produce large or small flakes, depending on how the knapper manipulates the other variables.