When you find an arrowhead, or someone shows you an arrowhead, how can you tell how old it is?
A lot of the answer for how old an arrowhead is depends upon the type of stone the point in question is made from. Old flint tends to get coarser and turn lighter colors, towards white even, if it is exposed to sunlight and air and rain and freezing over time. However, if it is buried, sometimes there is very little apparent aging, unless the soil itself is chemically acting on the surface of the flint. I have found buried points and blades, which, after I washed them, were almost as smooth and unchanged as new materials. So, you then look for deposits of minerals from the buried condition. Also, if flint has been exposed to the atmosphere, often the exposed side of a flint point is quite discolored relative to the unexposed side. This is a good indication of age.
If the point is obsidian, especially in a desert area, and it has been exposed on the surface of the ground, the exposed side many times is somewhat polished by wind and blowing dust or sand. The underside is more original in feel, or it is often coated with chemical deposits from the soil. In addition, in desert or normal weather pattern areas, obsidian is affected by rain and other moisture over time, and gets what is called a “hydration rind” on the outer surface. This tends to dull the originally shiny, slick appearance of the flaked stone. It makes an outer layer of changed material, due to the absorption of minute amounts of water, that is thicker depending on the length of time of the exposure to the elements. Archaeologists have even taken to calibrating the age of obsidian points or tools based on the thickness of the hydration rind in a known area. This is done by comparing a sequence of rind measurements to ages determined by other accepted dating methods, C14, tree rings, etc., to set up a scale to get a rough age for a piece based on the thickness of the rind. The rind thickness can be observed by cutting a slice through an expendable obsidian tool.
Yet additional indicators of possible age are the style, size and source location of a point. Many styles of points were unique to certain time periods, which enables the observer to position the point in a time sequence. Also, many specific styles were used in specific regions, which also helps to identify the cultural identity of a particular piece. And, the material from which a particular regional style of point is made is generally known, at least to experienced collectors. So, when a point made of an abnormal, atypical material for a particular style is exhibited, the likelihood of modern production increases.
These concepts are just the beginning of the discussion of determining the age of an arrowhead.
Artifact authenticators also examine the edges of the points under high magnification to observe natural aging and “use wear” patterns vs. chemical aging or mechanical wear and tear with metal or other modern materials.
Mixed, confused or varying chipping and flaking styles indicate possible reworking of a point, either in ancient times to refurbish the edge or tip of the point for re-use, or in modern times to reshape it into an apparently complete tool.
Additional analytical methods come into play after an examination of the consistency and style of flaking patterns on a point or tool.
Authenticators can observe knapped stone surfaces under special light to see the potential difference in the age of the visible surfaces. Old surfaces and new surfaces react differently to certain kinds of light, and are visibly different to the eye under that special light. This difference between old and new stone surfaces also enables the detection of potentially modern rechipping of old points to reshape them.
This information can help collectors to make sure of what they are purchasing or acquiring by trade. And it will help identify the native culture and time period of the pieces which they find, in the field or through the various connections which are available, such as the internet and regional artifact shows.