Heat Treatment -- An Old Indian Trick
Powers described how the Wiyot of northwestern California made long delicate arrowheads used in war:
Taking a piece of jasper, chert, obsidian, or common flint, which breaks sharp-cornered and with a conchoidal fracture, they heat it in the fire and then cool it slowly, which splits it in flakes. The arrow-maker then takes the flake and gives it an approximate rough shape by striking it with a kind of hammer. He then slips over his left hand a piece of buckskin, with a hole to fit over the thumb (this buckskin is to prevent the hand from being wounded), and in his right hand he takes a pair of buck-horn pincers, tied together at the point with a thong. Holding the piece of flint in his left hand he breaks off from the edge of it a tiny fragment with the pincers by a twisting or wrenching motion. The piece is often reversed in the hand, so that it may be worked away symmetrically. Arrowhead manufacture is a specialty, just as arrow-making, medicine, and other arts.
Detailed descriptions of Native California flint knapping are rare and the preceding by Powers, who traveled among the Wiyot in the 1870s, is interesting on three counts. First, it records the trimming of a flake by direct percussion; second, it is apparently a unique method of pressure flaking and third, it gives what may be in part a confused observation of fire cracking and fire treatment of cryptocrystalline rock (such as jasper or flint).
It is known that the flaking characteristics of such stones improve after they are heated. They become more vitreous and easier to flake, perhaps through the breakdown of the crystal structure. Ideally, a temperature around 500 degrees F (260 degrees C) must be reached and maintained for ten to twelve hours and both the increase and decrease in temperature must be very gradual to avoid breakage. Indian earth ovens would work well. Knapper Paul Hellweg suggests a bed of hot coals covered with a 2 to 3 inch layer of sand, followed by flakes and prepared blanks, a second layer of sand and a second fire that should burn down to a bed of coals before it too is capped with more sand and left for 24 hours. The sand insures a slow buildup of heat and gradual cooling. The stones should not be removed until completely cooled.
The process does not need to be so elaborate or so long, however, if both heat treatment and fracturing are desired. A quicker rise and fall of temperature would work. It may be that the Wiyot did not cool the stone so very slowly, or perhaps the stone was heated somewhat rapidly, for the breakage mentioned by Powers was clearly purposeful and occurred in the fire.
Much clearer is Shumacher's 1877 description of heat treatment and heat fracturing by the Yurok.
The rock is first exposed to fire, and, after a thorough heating, rapidly cooled off, when it flakes readily into sherds of different sizes under well directed blows at its cleavage. The fragments are assorted according to shape and size best corresponding to the weapons desired; the small ones best fit in shape and thickness, are used for arrowheads; similar sherds, but larger in size, for spear points; the long narrow pieces for borers, and so on.
In an article on the heat treatment of lithic materials in aboriginal northwestern California, R. A. Gould made no mention of Powers' or Shumacher's early observations but based solely on archaeological evidence came to the same conclusion: Indians of northwestern California fire-fractured raw lithic material also altered by the fire. the site was Point St. George, California, and prehistorically stone chipping there was a major activity.
Gould found evidence for heat treatment of common beach cobbles of agate and re or green jasper. Discoidal cortex flakes, known as "pot lid" flakes, are frequently associated with archaeological hearths and artifacts of the Point St. George region, beneath the ground and on surface sites. "A plano-convex flake leaving a concave scar" was how Don E. Crabtree, flint knapping expert who accompanied Gould on a survey of the area in 1972, described them. They lacked bulbs of percussion and the compression rings of force lines left by hammerstone technology. the differential expansion and contraction which breaks off "pot lids" was caused by heat. when these pieces (from 2.1 cm.) were tested by chipping, they also showed definite signs of heat alteration. Many of the sites contained large amounts of crazed and shattered agate and jasper, further evidence of a rapid elevation in temperature. Finally, the finished arrowheads, harpoon tips and other implements themselves exhibited signs of having been altered or "improved" by heat. Clearly, heat treatment was part of the pressure-flaked stone technology in aboriginal northwestern California, particularly along the coast.
Gould speculated that as well as rendering tough materials such as agate and jasper more susceptible to pressure-flaking through heat treatment, cobbles of agate and jasper may also have been placed in fires for lithic reduction. Most of the usable beach agate and jasper had the form of hard rounded water-worn cobbles and were very difficult to break open by direct percussion with a hammer stone. Repeated efforts by Gould found it almost impossible to form striking platform (required for orderly flake removal) by knocking off an end of one of the cobbles. But in fire without a careful effort to control the rate of heating of the cobbles, they shatter often into useless fragments frequently found in the Point St. George hearths) in some cases into a pot lid flake and its negative, a core with the bulb scar whose angular facet provided the needed striking platform. From these cores, flakes could be struck while the pot lids themselves were left where they fell, unused or retouched, thus perhaps explaining their prevalence at Point St. George. Such a method would have economically produced needed platforms from which heat-treated and therefore superior flakes might have been obtained.
There is some evidence of heat treatment from the other end of California. While no detailed analysis was performed, archaeologist Joan Schneider concluded through visual inspection that chert and jasper artifacts found at an Afton Canyon site in the Mojave Desert had been heat treated. Such treatment produces a change in color and a waxy luster.
The Afton Canyon lithic workshop overlooks the generally dry Mojave River at a point where the river has come from beneath the desert floor and flows year round. Good quarries for chert, jasper and chalcedony are found in the mountains nearby. Thanks to rugged and fairly remote terrain, even today when walking this area, I see raw material and old Indian camps and workshops everywhere.
found by Paipai near Santa Catarina:
first, likely for atlatl dart; second, for arrow.
The article is from the book, entitled "Survival Skills of Native California" (ISBN 0-87905-921-4), by Paul Douglas Campbell. Permission to use the article on the PrimitiveWays website was given by Mr. Campbell. Paul Campbell can be contacted through his publisher, Gibbs Smith, P.O. Box 667, Layton, Utah 84041.
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