Lithic reduction involves the use of a hard hammer percussor, such as a hammerstone, or a soft hammer fabricator made of wood or bone to detach lithic flakes from a lump of tool stone called a lithic core. As flakes are detached in sequence, the original mass of stone is reduced in size; hence the term for this process. Lithic reduction may be performed in order to obtain sharp flakes, on which a variety of tools can be made, or to rough out a blank for later refinement into a projectile point, knife, or other object. Lithic tools produced this way may be bifacial (exhibiting flaking on both sides) or unifacial (exhibiting flaking on one side only).
In prehistoric times, cryptocrystalline stones such as chert, flint, obsidian, and chalcedony, as well as some finegrained rhyolites, felsites, quartzites and a few other tool stones, were often used as a source material for stone tools. These materials are easily broken and fracture in a Hertzian cone when struck with sufficient force. In this kind of fracture, a cone of force propagates through the material from the point of impact, eventually removing a full or partial cone; this result is familiar to anyone who has seen what happens to a plate-glass window when struck by a small object, such as an airgun projectile. The partial Hertzian cones produced during lithic reduction are called flakess, and exhibit features characteristic of this sort of breakage, including striking platforms, bulbs of force, and occasionally eraillures, which are small secondary flakes detached from the flake's bulb of force. Flakes are often quite sharp, with distal edges only a few millimeters thick, and can be used directly or modified into other types of tools such as spokeshaves and scrapers. Occasionally, obsidian flakes are used for delicate surgery instead of steel blades, due to their preternatural sharpness.
