NATURAL ALTERATION


Lithic material can altered by natural processes so that they sometimes resemble tools. Such lithics are known as eoliths. Also some lithic material can appear to be polished by agencies such as wind and sand, often referred to as desert polish. Also debitage can be altered by natural processes such as heat, frost
and patination.

 


heat fracture
"Pot lids are plano-convex flakes that leave a concave scar. These are the result of differential expansion and contraction of isotropic material but are minus the compression rings of force lines usually associated with these conditions. Generally they are a natural occurrence rather than intentional results of man-made flakes" (Crabtree 1982., 49).

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frost fracture
Frost fracture is caused by water within the flint (or in cracks in the flint surface) freezing and so expanding. When the water melts pieces of flint break off the nodule. This freeze-thaw action can continue through the depositional history of artifacts. Also natural nodules of flint may be 'flaked' by frost action to give the appearance of being deliberately knapped, e.g.starch fractures


patination
"Many cherts and flints will patinate, developing a weathered surface as water and sometimes chemical stains work their way into the flint and as silica and other materials are leached out, producing a thin patina or rind of a different color" Whittaker 1994. , 70

desert polish
Desert polish is created by a combination of wind and sand. The movement of sand across exposed flint surfaces polishes the surface to a high gloss that can be seen by the naked eye. It can have the appearance of sickle gloss except that the polish is all over the surface.

Sickle gloss is a use wear polish created by using tools for cutting cereals and therefore is a use-wear polish. It is visible to the naked eye, but only occurs on the used edge, not all over like desert varnish.
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starch fractures
Starch fractures occur from freeze- thaw action along planes in the flint sometimes appearing as if blades have been deliberately removed and sometimes leading to such natural objects being classified as blade cores.

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fire-cracked rocks
Fire-cracked rocks are produced by the heating of water in the flint which then forces off pieces. The same process as Heat fracture often resulting in pot lid fractures. The same process as Heat fracture often resulting in pot lid fractures. Futher heating will cause severe cracking of the flint surface and changes in colour first blue, and then as the temperature increses, white.

ballast
Flint that may appear to be humanly struck can sometimes be ballast, that is flint that was used as ballast on ships which has subsequently been discarded.

edge damage
When flakes have been removed from the edge of a blank by natural processes this can (and often is) confused with retouch. There are various sources of edge damage:

Trampling during occupation of the site;
Post depositional movement in the sediment;
Rough handling during excavation, e.g.. trowel damage;
Storage conditions, e.g.. 'box damage' (if flints are kept loose in a box they can rub together often causing fractures along the edges.

"Edge damage: The removal of material from edges by natural processes, spontaneous retouch, soil movement, trampling etc." as opposed to, "Edge wear: The removal of material from edges by flaking and/or rounding by use." (Grace 1989,, 114) It is impossible to reliably tell the difference between edge damage and edge wear without using special techniques such as use-wear analysis.(see Grace 1989, Grace1990a, Grace 1993., Kamminga 1982 , Keeley 1980., Semenov 1964)

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