The Formation of Flakes®

Brian Cotterell; Johan Kamminga

American Antiquity, Vol. 52, No.4 (Oct., 1987),675-708.

Stable URL:http://links.jstor.org/sici?sici=0002-7316%2819871O%2952%3A4%3C675%3ATFOF%3E2.0.CO %3B2-W

American Antiquity is cunently published by Society for American Archaeology.

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstoLorg/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless youhave obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, andyou may use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.j stor.org/journals/sam.html.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen orprinted page of such transmission.

JSTOR is an independent not-for-profit organization dedicated to creating and preserving a digital archive ofscholarly journals. For more information regarding JSTOR, please contact SUPPOlt@jstoLorg.

http://www.jstoLorg/Wed Sep 22 18:22: 13 2004

THE FORMATION OF FLAKES

Brian Cotterell and lohan Kamminga

An understanding of [he mechanics involved in flake [ormalion pro~ides an opporlunily for deriving morebeha~ioral information from flake and }lake scar mtJrphology. The mechanics of flake formalion lIre direcllyrelevant to [he idenllfication ofprehisloric flaking techmques and stone /00/ use. In this paper we pro~ide a mode!offlake forma/ion thai accoun!S for mw:h ojIhe variation injIake morphology. Flakes can form in a number ofways and despite popular belief rhey lIfe not all of [he conchoidal variel)! The bending flake is common in usewear though il is often misidentified as a conchoidal flake. A Ihird major type offlake, Ihe compressionjlake, isa comrrwn prtJdUCI (Jf bljJOlar impacl To accoumfor Ihe wide varialion in flake morpholOg)! we follow a Iripanitescheme offlake forma/ion comprising initiation, propagation, and termination phases, wilhin which differentmechanisms can operate.

Because flaked stone artifacts are the most enduring prehistoric cultural remains they also are themost abundant in archaeological sites, and often are the only evidence of past human activity. Thestudy of prehistory commenced with the recognition of stone artifacts which constitute a resourceon which a great deal of prehistory is based. Their physical dimensions have been measured, listed,graphed, and statistically assessed in many thousands of publications. Tool types have been definedand great "cultural" or technological sequences traced across continents. Despite the enormoussuccess of radiocarbon dating, stone artifacts still provide the most important evidence for datingunstratified archaeological sites. The contribution of stone artifacts is to be found in almost everyarea of archaeological research.

In the early years of prehistoric studies, the subject of stone technology was the concern ofantiquarians, and the general lack of understanding about how flakes were formed led to the beliefin some circles that an "Eolithic phase" had existed. The debate over the Eolithic was describedby a staunch opponent as "one of the most violently agitated of all the many storm centres ofscientific discussion" (Macalister L921: La); it was one that did not see a resolution until the 1920s.While the artifactual status of particular pieces of stone re-emerges from time to time, the currentproblems are more those of correctly identifying the smaller scars on flaked artifacts to distinguishuse-flake scarring and fracture damage resulting from intentional flaking and other processes. Webelieve that there is a general and sometimes systematic tendency for archaeologists to misunderstandthe nature of the flake scarring on stone artifacts, with the result that too many of some kinds oftools are identified, and far too few of others. For instance, it has been proposed a number of times,and in relation to stone industries in various parts of the world and of different ages, that stonecores too often are identified as tools (Kamminga L978:355-360, 1982: 85-91, LOO-IOI; Keeley1980: 116; Kleindienst and Keller 1976: L84; Toth 1985: 109-1 L2); yet many archaeologists declineto acknowledge that it is not always a straightforward matter distinguishing tools from flakingdebitage. An understanding of the fundamentals of flaking is particularly necessary in the nascentfield of lithic functional analysis, the success of which has been demonstrated, for exampLe, by theidentification of more than 200 tools in an amorphous Australian assemblage when only six tooLs

Brian COllerell, Depar/ment ofMechanical Engineering, University ofSydney, Sydney. 2006, AustraliaJohan Kamminga. Department ofPrehistory. Research School ()fPaci/lc Stuilies. AUSlralian Nalional Uni~ersity,

Canberra. A.CT., 2601, Australia

American Antiquity, 52(4), [987, pp. 675-708.Copyright © 1987 by the Society for American Archaeology

AMEAICAN ANTIQUITY [Vol. 52, No.4, 1987

were previously identified by the excavator (Fullagar 1982). The problems that occur in makingcorrect identifications of tools are more apparent when there are few formally shaped or retouchedtools in an assemblage, which is the case in many parts of the world. The difficulties of interpretingamorphous assemblages highlight the problems that are by no means hmited to them as there isevery reason to believe that simple flake tools are overlooked in analyses of collections of flakedstone that contain a high proportion of formally shaped ones,

Flaking is a large and conspicuous part of lithic use wear, and the critical need to understand itsmechanics of formation has been stressed by a number of researchers (HaYden and Kamminga1979; Lawrence 1979; Odell 1981). In the early L970s, when use-wear analysis was taking root inwestern archaeology, there was little standardization in the definitions of flake and flake scar types,and consequently there were problems in comparing the descriptions of use flaking published bydifferent researchers (d. Gould 1973; Hayden and Kamminga 1973, 1979:6; Keeley 1977; Odell1977). Compounding the difficulties of deciphering use flaking was the lack of awareness about thedifferent modes of flake initiation. This is still generally the case, even among use-wear analysts.

The purpose of this paper is to present a synthesis of our research on the mechanics of flaking.We use the mechanics that have been developed by material scientists concerned with the fractureof brittle solids to give a better understanding of flaking and to elaborate on the flake classificationthat was proposed at the Lithic Use-Wear Conference held in Vancouver in 1977 (Ho Ho CLassi­fication and Nomenclature Committee Report 1979).

TERMINOLOGY

The mechanics of flake formation in stone tool making and use are basically the same and anydifferences that occur can be attributed to scale. Our discussion encompasses fracture mechanicsand the two domains in lithic studies that each have their own conventional terminologies. As muchas possible we use nonspecific Language to describe the phenomenon of flaking, and we have foundit necessary to introduce some terminology of our own, most of which is indicated in Figure 1.

A flake is any fragment detached from a nucleus; it is not limited to the conchoidal variety_A nw:!eus is any object from which a flake is detached. A nucleus may be a core from which a

primary flake is struck, a stone artifact that is being shaped by retouch, or the edge of a tool fromwhich a f1akelet is detached during use.

An indenter is an object that applies a force to a nucleus, even if the fracture initiates away fromthe indenter, In tool making this may be a hammer, punch or pressure-fl.aking implement; in tooluse it may be the part of the worked material that is applying load to the tool's edge, or a smallhard particle acting between the worked material and the tool surface.

The initiation face is that surface of the nucleus in which the crack originates. The conventionalterm for an initiation surface on a core is "striking platform."

The side face of a nucleus is the surface behind which the fracture develops and from which thedorsal surface of the flake is derived.

The edge angle (0) of the nucleus is that made by the initiation surface and the side face of thenucleus from which the flake is formed,

The initiation angle of a flake is the angle between its initiation and ventral face. The initiationangle of the flake scar is the angLe between the initiation face and the scar surface.

The direction of the indemer motion i~ the direction in which the indenter moves during theflaking event.

The flaking angle (a) is the angle between the direction of the indenter motion and the side faceof the nucleus.

The flaking force is the force applied by the indenter in the process of flake detachment.The force angle (</J) is the angle between the flaking force and the side face of the nucleus. In

general this angle is nol the same as the flaking angle. The direction of the flaking force dependsprimarily on the stiffness of the developing flake.

The stiffness of a partially formed flake is its resistance to deformation. More precisely, it is theflaking force required to produce a unit deflection of the indenter. There are two components:

Cotterell and Kamminga] THE FORMATION OF FLAKES

ww

"o~

Sideface

Initiationface

iEdgeangle.

Flakingangle

~//

Forceangle

I~Flake'sinitiationangle

"

Scar'sinitiationangLe

,\

\

\

\

\

\

Figure L Flake terminology.

bending stiffness transverse (0 the flake's length, and compression stiffness in the direction of theflake's length.

LITHIC MATERIALS

In flaking stone to make (Ools, the lithic materials normally favored were the more homogeneoU5and isotropic varieties of siliceous stone; that is to say, those materials that have the least direction­dependent properties. The form of flakes detached from homogeneous and isotropic materials ispredictable. The most homogeneous and isotropic hthic materials are the natural glasses and it canbe assumed that all accessible outcrops of natural glass in the world were exploited heavily at sometime in the past. In Australia, where there are no sizeable exposures of volcanic glass, the Aboriginesreceived a consolation prize ofcontinent-wide distribution ofsmall glass tektites formed by meteoriteimpacts. For many thousands of years the Aborigines hving in the deserts collected these glassbullons for flaking into tiny slithers that were their sharpest cutting tools.

While natural glass was highly desired for light-duty cutting activities because of its sharpnessand flakeability, it was in general one of the scarcest hthic materials. The next most isotropic andhomogeneous lithic group is the cherts, including the variety flint. These stone types are mostlymicrocrystalline quartz and they have greater fracture toughness than glass. They are almost asamenable to controlled flaking as glass. Other lithic materials that commonly were used for toolmaking during Paleolithic times are quartz, in its mono- and polycrystalline forms, and granularbut tough siliceous stone types, such as quartzite, and microcrystalline siLcrete which has a widespreaddistribution in Australia and southern Africa.

AMERICAN ANTIOUITY [Vol. 52, No.4, 1987

Generally, lithic materials that were less amenable to controlled flaking, such as quartz, oftenwere more readily obtainable. The cleavage planes in quartz are weakly defined and therefore donot significantly affect the fracture (Hartley and Wilshaw (973). However, the usually abundantflaws in polycrystalline quartz do have a major effect. The fracture path is more unpredictable andthe flaws cause small cracks to form on either side of the main fracture, which are often visiblebeneath the surface. Isolated small flaws in lithic materials only have a local effect on the fracturepath. With rejuvenation retouch and use flaking the detached flakes often are small and the influenceof flaws can be significant.

In many parts of the world the flaking properties of chert were altered by heat treatment. Withnatural chert the fracture is largely intergranular whereas after heat treatment, fracture takes placein a transgranular fashion and requires less energy (Mandeville 1973). The resulting fracture surfacesalso are smoother and more lustrous. Since heat-treated stone has a reduced fracture toughness,stone tools made from it tend to wear at a faster rate and sustain larger use fractures (Olausson1983). Through transformation of minor mineral constituents of the stone and dehydration, heattreatment creates abundant microvoids (Mandeville 1973; Rick and Chappell 1983; Schindler etat. 1982). While these microvoids have little effect on the formation of large flakes they may beresponsible for the greater incidence of step and hinge-terminated scars during tool use reported byOlausson (1983).

FRACTURE MECHANICS

For brittle materials like stone, Griffith's (1920) original concept of fracture has been developedinto a coherent theory that is an accepted part of fracture mechanics. In this section we provide thebasic essentials needed to follow all of the mechanical aspects of our discussion; a more compre­hensive account is given by Lawn and Wilshaw (l97Sa).

The stresses ahead of a propagating crack in a brittle material are always tensile, even when thepredominant stress field is compressive as it is in bipolar flaking. In the more homogeneous siliceousstone used to make flaked tools, the "process zone" at the tip of a crack, where the stone deformsinelastically and fracture takes place, is small. However, if the material is highly flawed, as quartzcan be, the cracks initiated in front and to both sides ofa propagating crack can form a Larger processzone. Outside the fracture-process zone the stress (I.r) near the crack tip can be described in termsof a parameter called the stress-intensity factor (K) by

KF--

y!2;(( L)

where r is the distance from the middle of the process zone (Lawn and Wilshaw 1975a:S2). Thiseq uation is the basis oflinear elastic fracture mechanics and appLies to the behavior oflithic materials.The stress-intensity factor itself is proportional to the general stress level and is a function of thecrack length. The unit ofthe stress-intensity factor is N mm -3/2 (often in engineering, where materialsare tougher, a larger unit MN m- J / 2 is used). There are handbooks that give the values of the stress­intensity factor for various geometries common in engineering (Rooke and Cartwright L976; Tadaet aL L973), some of which are useful for modeling flaking, and we have obtained values for anumber of specific flaking situations (Cotterell et at 1985; Cotterell and Kamminga 1986).

In order for a crack to propagate, mechanical work must be done in the fracture-process zone.This work can be performed on the object being fractured or it can come from the energy storedwithin it (Griffith 1920). The energy necessary to create a unit area of fracture surface is specific tothe material being fractured. During the propagation of a crack, the potential energy available toperform the work of fracture is proportional to (L - u')K'/E where E is the elastic modulus and uis Poisson's ratio which for lithic materials is about.2 (Lawn and Wilshaw 1975a:56-57). Hence acrack will grow in a particular material when the stress-intensity factor reaches a critical value K l "

which is called the plane strain fracture toughness of the material. For glass Kk is about 25 N mm -JO

(Holloway 1973:172); the value for obsidian is similar. Unflawed monocrystalline quartz has afracture toughness of about 40 N mm -112 (Hartley and Wilshaw 1973:269), but the effective fracture

Cotterell and Kamminga] THE FORMATION OF FLAKES

toughness ofthe flawed polycrystalLine quartz used for stone tools is much less. The fracture toughnessof Bald Eagle jasper from Pennsylvania is 60 N mm-"l (Schindler et al. 1982:532). After healtreatment at 300-400°C the wughness of this swne drops w 30 N mm-"'. As far as we are aware,these values are the only ones currently available for lithic materials used to make tools. More dataon fracture toughness would be of considerable value in a number of areas of lithic studies, such asfunctional analysis.

In a brittle isotropic and homogeneous material, a crack will grow so that it maintains localsymmetry in the stress field around the crack at its tip (Gol'dstein and Salganik 1974). Inhomo­geneities in stone can cause the path of a crack to deviate from the ideal so that the stress field nolonger has true symmetry. When this occurs the stress-intensity factor at the tip of the crack hastwo components: K j , which is the symmetrical mode I stress-intensity facwr that causes the cracksurfaces w open perpendicularly to one another; and K", the antisymmetrical mode II stress-intensityfactor that causes the crack surfaces to slide over one another (Lawn and Wilshaw 1975a:52). Cracksin a brittle isotropic and homogeneous material propagate so that the mode II stress-intensity factoris zero and the stress field is symmetrical ahom the crack (Cotterell and Rice 1980). After a smalldisturbance caused by a local inhomogeneity in the material a crack either returns to propagatestably along its original path, or continues to deviate, in which case its path is unstable. How stablea crack path is depends upon the variation in stress at the tip of the crack. The stress distributiongiven by equation (I) is valid only very close to the crack tip. At larger distances the stress field hasan additional component which represents a constant-stress T parallel to the crack surface (Williams1957). This constant-stress component determines the stabiHty of a crack path (Cotterell 1966;Cotterell and Rice 1980). 1fT is negative, so that the constant-stress term is compressive, the crackpath is stable. However, a positive or tensile stress promotes instability in the crack path, causingit to curve away from its ideaL path with a radius of curvature that is inversely proportional w(T/K,)2.

THE RATE OF LOADING IN FLAKE FORMAnON

The rate at which load is applied to initiate a flake can vary considerably. Percussion flaking witha hammerstone produces a much higher rate of loading than pressure flaking with a softer materialsuch as wood. A similar wide range of Load rates can occur in the formation of use flakes; forinstance, a gentle activity like skinning has a Low load rate, while an activity like adzing wood orhammering a stone wedge has a high load rate. As we show, though the loading rate is significantlyfaster in percussion flaking, it is not fast enough to affect the stress at initiation; nor is the velocityof crack propagation sufficiently high to distort the stress field appreciably.

In the initiation phase of flake formation in tool making, the highest strain rates are achievedthrough percussion with a hard hammer. Hertzian initiation is most likely in lhis impact situationand the dynamic effects can be assessed by considering the formation of a classic cone fracture. TheHertzian stress field under impact conditions is not significantly different to one of steadily appliedpressure, unless the circular contact area between the indenter and the brittle surface expands at avelocity approaching the speed of elastic shear waves (Lawn and Wilshaw 1975b: 1053). Shear wavesin lithic materials are of the order of 3,000 m/sec (Jaeger and Cook 1969:347). Since the radius (a)of the circular area of contact between a spherical indenter of radius (R) and a flat elastic surfaceis given in terms of the distance of mutual approach (z) by

a' = 2zR (2)

(Timoshenko and Goodier 1951), the velocity of expansion of the circle of contact v~ is given by

Rv. = -Vi

a (3)

where v, is the impact velocity of the indenter. For the impact velocity to be significant, v. wouldhave to be greater than about 1,500 m/sec. A typical value of the radius of the Hertzian cone inflaking is about I mm. It is unlikely that the radius of the indenting surface would be greater than

." AMERICAN ANTIQUITY [Vol. 52, No.4, 1981

~

"-E

Za~ 1500

Test..'" 1a 0

"- -- 2 +a 3'" 1000 •"- 4 •'"u..'" 500u

~

a

>-~ 0u 0 10 30 40a 20~

w> DISTANCE FROM INITIATION FORCE Imm)

Figure. 2. Variation in experimental crack velocity with crack length.

10 mm. Therefore, equation (3) implies that the impact velocity has to be greater than about L50m/sec for dynamic effects to be significant. A hammerstone could not possibly be accelerated tosuch a high velocity_ Thus it can be safely assumed that in percussion flaking the stresses at initiationare not significantly different from those in pressure flaking. The actual magnitude of the stressescan be calculated by assuming that the kinetic energy of the hammerstone is converted into thestrain energy of the core (Langitan and Lawn 1969).

The velocity of crack propagation in flaking varies considerably and, not surprisingly, it is highestwith hard-hammer percussion. We have measured the velocity of crack propagation in flaking bytiming the breakage of painted lines of conducting siLver positioned across the crack path. The linesof silver paint were connected to an electric circuit that enabled the progress of the crack to beregistered on an oscilloscope. There was a general tendency in both the pressure and percussionexperiments for the crack veLocity to decrease as the flake developed. In a series ofpressure-flaklngexperiments on glass (Cotterell et a!. 1985), the maximum velocity recorded was 170 m/sec (theaverage being [24 m/sec). Other researchers have recorded higher velocities (200-300 m/sec) duringpressure flaking (Crabtree [968:472; Fau[kner 1972: 12 [). The crack velocity depends on the velocityof the indenter, which in pressure Raking is limited, Much higher impact velocities occur in hard­hammer percussion. We conducted four tests in which we dropped an 820 g indenter 300 mm ontothe edge of a 12 mm thick glass plate to give an impact velocity of about 2.4 m/sec, The platformangle was about 30" and the plate was damped so that the platform surface was horizontal. Thevariation in crack velocity with crack length is shown in Flgure 2. Except near a hinge termination,the crack velocity is a Linear function of crack length. Extrapolation of the linear relationship tozero crack length gives a maximum crack velocity of about I, LOO m/sec. We are confident that sucha velocity is very near the maximum achievable in flaking glass or obsidian. All the fracture surfacesof our glass flakes were smooth with the presence of only minor surface markings such as Wallnerlines (Cotterell and Kamminga [979: [08; Kerkhof and Muller-Beck 1969:447). In any material afracture surface will roughen when there is an increase in crack velocity (Cotterell 1968; Ravi­Chandar and Knauss 1984), On glass and obsidian this roughening is first observed as a misting ofthe mirror-like surface at a crack velocity of about 1,500 m/sec (Holloway [973: [85). Thus rough­ening occurs at velocities greater than those achieved during flaking. We also have measured thecrack velocity created by hard-hammer percussion of a number of Aborigina[ lithic materials that

Cotterell and Kamminga] THE FORMATION Of FLAKES '"previously have been described by one of us (Kamminga 1978, 1982). The maximum velocitiesrecorded were less than those for gLass (630 m/sec for quartzite, 670 m/sec for a chert-like voLcanictuff, and 800 m/sec for chalcedony).

It can be shown that the velocity of crack propagation (v) in blade flaking is a function of thevelocity of propagation of shear waves (v,) and a nondimensional parameter «(1) (see appendix).Hence

v/v, = f«(1)

where

(4)

and Vi is the impact velocity of the indenter, p the density of the materiaL and h the thickness ofthe detached bLade. For small velocities of impact, the crack velocity is proportional to (1. However,it can be shown that no matter how high the impact velocity is, the velocity of crack propagationcan never be greater than the velocity of Rayleigh waves (Cotterell 1964), which are surface wavesthat propagate at a speed of slightly less than that of shear waves. In practice the maximum crackvelocity under any type of load is of the order of half the velocity of shear waves (Ravi-Chandarand Knauss 1984), which in soda-lime glass propagate at about 3,500 m/sec. In flaking, our exper­iments indicate that the maximum ratio of the velocity of crack propagation to the velocity of shearwaves (v/v,) is about 0.3. Velocity has very little effect on the stress field for velocity ratios of thisorder (Cotterell 1964). Because the elastic modulus E and the density p of siliceous materials donot vary appreciably, equation (4) predicts that the crack velocity for a flake of given thickness isless in tough stone, which may account for the lower crack velocities observed by us in lithicmaterials other than glass.

The suggestion by Speth (1972, 1974, 1975) that percussion flaking is a variety of spalling causedby oblique reflection of compressive waves at a free surface can be discounted. Spalling usually isobserved onLy when small explosive charges are detonated on the surface of a solid object. Theimpact velocity in percussion flaking is not high enough to cause compressive waves of sufficientintensity that spalling occurs. Moreover, the smooth fracture surface of stone flakes detached fromhomogeneous isotropic stone is not at all like the surface of a spalling fracture, which is extremelyrough because of multiple crack initiation (Kolsky L963: L87-193).

While we conclude that the rate of loading does not have a significant effect on the way a flakeforms, in practice there often are shape differences between flakes detached by pressure and flakesdetached by percussion. Thinner and more evenly shaped flakes can be removed by pressure, thoughthe percussion-made flakes detached by a skilled stone-knapper can be neater than the novice'spressure flakes (Mewhinney 1957:57). On some obsidian or glass flakes it is possible to estimatethe velocity of crack propagation from Wallner lines that can form in the surfaces of flakes (Cotterelland Kamminga 1979: 108; Kerkhof and Muller-Beck 1969:447). Since the velocity of crack prop­agation usually is higher in percussion than in pressure flaking, it is possible in certain circumstancesto distinguish these modes of flaking. More importantly, the mode of initiation in percussion flakingoften is different from that which occurs in pressure flaking. As we will explain later, these differencesare caused by the relative hardness of the indenter and are not due primarily to the rate of loading.

MAJOR FLAKE TYPES

While there is infinite variety in the morphology of flakes, it is possible to identify certain basictypes. The standard description of a human-made flake initially was based on the observations ofthe manufacture of gun flints at Brandon, England, in the nineteenth century, which were comparedwith prehistoric stone artifacts and hafted ethnographic ones (Evans 1872: 17, 246). The only flaketype described was the conchoidal variety (Figure 3a) which, though not regarded as an outrightindicator of artifactual status, was emphasized to counter the extravagant claims of the proponentsof an "Eolithic Age." Anything that did not have characteristic conchoidal features was relegatedto the status of"chips, spalls or splinters" and effectively ignored in artifact descriptions and analyses.

1mm

Cotterell and Kamminga] THE FORMATION OF FLAKES ."

10mmI

Figure 3a. Conchoidal use-flake scars terminating in a feather (left) and a step (right) on the edge of anobsidian scraper.

Figure 3b. Bending flake scars on a woodworking scraper (top) initiation face, (bottom) side face.Figure 3c. A compression flake formed by the bipolar technique.

The acrimonious debate between the proponents of the Eolithic and the more rationaL antiquariansleft a Legacy of overemphasis of the conchoidal flake that is perpetuated in today's archaeologicaltexts. It often is assumed by archaeologists that conchoidal flakes are the only respectable ones oreven that the conchoidal type is the only one that occurs in stone flaking. Yet, the conchoidal flake(so named because the distinctive bulb of force and concentric undulations On the fracture surfacegive the inside surface of some flakes the appearance of a unionid shell), is not particularly commonin use flaking. A conchoidal flake can only be formed by a comparatively hard indenter, like ahammerstone. In tool use, where the materials being worked often are more yielding than the !lakingimplement used in tooL making, the edge of the stone tool frequently is broken by bending. Thebending scars are distinctive (Figure 3b), though they can be confused with conchoidal scars. Therehas been little awareness in lithic studies of the importance of bending-initiated fracture, which firstwas described at the Conference on Lithic USe-Wear held in Vancouver in 1977 (Cotterell and

Hertzian,,,

I,,

INITIATION

Bending,

PROPAGATION

Stiffness- controlled

-#\

\

\

Compre ssi on -controlled,

Feather;Ibl

TERMINATION

Plungmg AlCiolIIIII

InftelCedFINIAL

ReflelCed Psuedo bifurcation

,•••-

•,,,\,,

Figure 4. The phases of flake fOrmation (drawings not to scale).

Cotterell and Kamminga] THE FORMATION OF FLAKES '"Kamminga 1979; Lawrence 1979; Tsirk 1979). The recognition of the bending flake by archaeologistsin general is long overdue.

A third major flake type is the compression flake (Figure 3c), which can be difficult to identifyfrom its fracture-surface features. A compression flake is initiated by microscopic wedging and thefracture path is controlled by compression. These flakes are produced during bipoLar flaking, thoughnot all flakes detached from a bipoLar core are of this type. The bipolar technique involves pLacinga pebble or similar nucleus on a stone anvil and repeatedly striking it with a hammerstone so thatflakes are initiated from either end of the nucleus. The compression flakes that are recognized moreeasily occur when a bipolar core splits into two or three pieces of roughly equal size. Such pieces,though often chunky and extensively damaged in their initiation area by hammer impact, are stillflakes by our delinition. These compression flakes are sometimes misidentified as bipolar coresbecause they are chunky, lack prominent conchoidal features, and tend to retain distinctive fracturedamage from hammer impact.

ALthough the three flake types we have described have general utility in lithic analysis, it is notalways appropriate to classify flakes and flake scars into such broad categories. We follow a schemeof three sequential phases in describing the formation of flakes: initiation, propagation, and ter­mination (Figure 4). In the rest of the paper, we discuss the details of this scheme which draws onour experimentaL and theoretical work of the last few years. The formation of fracture-surfacemarking on flakes and flake scars, such as Wallner lines, undulations, lances, and the eraillure scar,already are well understood (Cotterell and Kamminga 1979; Crabtree 1968, L972; FauLkner L972,L974; Kerkhofand MUller-Beck 1969) and we do not deal further with them here.

INITIAnON PHASE

There are three basic modes in which a flake can be initiated. In conchoidal flaking, initiation isby the formation of a partial Hertzian cone crack around the contact zone between the flaking tooland the initiation surface on the nucleus. Flakes also can be initiated by a wedging action directlyunder the applied load, a mechanism that is common in bipolar flaking. The third mode is initiationunder bending stresses away from the point of force application.

All brittle fractures initiate at a flaw (Lawn and WiLshaw 1975a:18-27). In flaking, these flawsusually are too small to see but they sometimes can be very large; for instance, flaws in the cortexsurface ofa water-worn cobble can be more than 20 mm deep. Such large flaws often can be detectedby the presence of an oxide stain.

Hertzian Initiation

The cone fracture (Figure 5) was studied initially by English antiquarians in the mid-nineteenthcentury (Evans 1872:246-247). The German physicist Heinrich Hertz (L896) carried out the firstscientific experiments on the formation of the cone fracture and the feature thus bears his name.Hertz also determined the elastic contact stresses for spherical indenters. However, a completedescription of the mechanics of the Hertzian cone was not given until the 1960s (Frank and Lawn1967). There are two excellent reviews of Hertzian and other indentation fractures by Lawn andhis co-workers (Lawn and Marshall 1979; Lawn and Wilshaw 1975b). A classic Hertzian conefracture is formed when a hard spherical indenter is pressed perpendicularly into the flat surface ofan isotropic brittle solid. The radial stress in the surface is compressive under most of the contactarea between the indenter and the soLid, but becomes tensiLe near the edge of the contact zone wherethe stress is highest (Figure 6). If the load increases, a crack eventually will initiate in the surfaceof the nucleus at the largest flaw near the region of maximum stress, and a very shallow ring crackwill form that cannot be seen readily by the naked eye. As the load increases, the crack growsdownwards in a stable fashion until it reaches a critical size. At this moment the ring crack suddenlypropagates into a visible cone with an angle of about 1360 and a diameter of about one and a halftimes that of the ring crack (Lawn and Wilshaw 1975b). The cone crack grows in a stabLe fashionagain as the load increases. However, when the cone crack reaches a radius of about three timesthe contact area radius, its growth comes under the control of another mechanism.

'" AMERICAN ANTIQUITY (Vol. 52, NO.4, 1987

Figure. 5. A Hellzian cone formed in a nucleus of flint b.l' hard-h.:tmmer percussion.

The conditions for the initiation of a conchoidal flake do not correspond exactly to those requiredfor a classic Hertzian cone. Conchoidal flakes usually are initiated near a side face of the nucleus,and the initiating force usually has an outwards component, causing the tensile radiaL stress in thecontact zone furthest from the side face to be enhanced and the tensile stress near the side to bechanged to compression. Because of these compressive radial stresses near the side face, a fullHertzian cone is not usually formed. More often only a partial cone forms from that part of thecontact circle where the stresses are tensile. Hamilton and Goodman (1966) obtained a solution foran mdenter that has a tangential as well as a perpendicular component of force. The radial stresson an initiation face loaded by an indenter applied at a distance from the side face is shown inFigure 6 where the stress is given as a fraction of the mean contact pressure between the indenterand the nucleus. It can be seen that the maximum tensile stress is increased greatly even for quitesmall tangential components of the applied force. Even with these tensile-stress enhancements, thepressure over the contact area between the indenter and the nucleus must be high if a conchoidalrather than a bending flake is to be initiated. To achieve the necessary high pressure, the indentermust be hard; conchoidal flakes cannot be formed if the indenter is too soft. Conchoidal flakes canbe detached with bone or antler pressure-flaking implements. Wood is usually too soft to detach aconchoidal flake by pressure but, because its effective hardness increases with strain rate, it sometimescan produce one by percussion. In tool use, Hertzian initiation also is possible when working a softmaterial if a hard particle, such as a quartz grain bonded onto a dried skin or a broken particle ofthe tool embedded in the worked material, acts as the indenter (Kamminga 1979:152, 1982).However, by far the most commonly observed cause of conchoidal flaking is hard-hammer per­cussion.

The partial Hertzian cone determines the shape of the bulb of force that is a characteristic featureof the conchoidal flake. The crack that extends the partial cone propagates initially into the bodyof the nucleus, and in doing so increases the outward bending on the developing flake. This increase

Cotterell and Kamminga] THE FORMATION OF FLAKES '"15

. "-.:.. . .:- ,~.

. .:.- '-,. . ,...

.-'0'" ....

'"

05

10

z0

'" -05'"w<XQ.

:>:0~

-1 0 0"

"15"30

-15

zo

'"zw>-

-1.0L-.------------------'Figure 6. Hertzian CQntact stresses in the surface. of a nucleus as a function of the force. angle.

in outward bending causes the crack to curve back toward tne surface of the nudeus to completetne bulb afforce. No other mode ofinitiation produces a bulb afforce, wnich is the most characteristicfeature ofa conchoidal flake. Hertzian initiation also can be identitled by the initiation angle whichis greater than 90e on the flake and Less tnan 900 on the scar. A conchoidal flake forming on a flatside face of a nucleus spreads laterally, and the platform takes on a characteristic inverted V shape(Figure 7).

A blow with a hard hammer often can cause a number of concentric partiaL Hertzian cone cracksto form in the initiation face of the nudeus (Figure 8). One of these parallel cracks will dominateand grow to form the flake_ The others often form flakeLets that terminate in a step or, less frequently,a hinge. The size range of these secondary detachments is wide-in length they can be less than Lto more than 10 mm in length. As a discrete unit, or together with other small detachments thatresult from hammer contact during a primary flake detachment, tnese flakeLets can appear as acascade of step scars (Figure 9). The extent of secondary detachments during percussion flaking canbe seen when the primary flake is refitted to its scar on the nucleus.

'" AMERICAN ANTIQUITY [Vol. 52, No.4, 1987

Figure 7. The ideal shape of a conchoidal flake formed on a flat side face of a nucleus (cross-sections indicatedby hatching).

Wedging Initiation

There are two wedging mechanisms that can initiate a crack during flaking. Detrital particles canbe wedged into a pre-existing flaw on the surface of a nucleus, thereby initiating a crack at tne tipof the flaw. An alternative indentation wedging mechanism that may occur in homogeneous siliceousstone has been described by Lawn and Swain (1975). Under a very hard sharp indenter, such as aprominence on a hammerstone or a particle broken from the edge of a stone tooL, the surface ofthe nucleus can deform plastically. This plastic deformation has a wedging action that can causecracks to be initiated at its tip. With blunt indenters the force is distributed over a larger area, anda partial or complete cone fracture forms. However, if the force on a blunt indenter is increasedafter the cone crack has grown about three to four times the diameter of the original ring crack, thematerial immediately under the indenter will flow plastically and initiate a median crack at themiddle of the cone. At this time, as the median crack predominates, the growth of the cone crackvirtually ceases. In other words, bLunt indenters under nigh loads behave as if they were sharp. LikeHertzian initiation, wedging requires high pressures and usually only occurs with a hard indenter.There is competition between the two modes and wedging is much more likely to happen if theindentation takes place well away from a side face, or if the edge angle is greater than 90°. A highlyflawed nucleus is more likely to sustain a wedging initiation than a Hertzian one.

In bipolar flaking all modes of initiation may occur, but wedging is the predominant one. The

Cotterell and Kamminga] THE FORMATION OF FLAKES '"

Figure 8. Concentric partial Hertzian cone cracks formed on an initiation face by hard-hammer percussion.

bipolar core normally is struck a number of times before a large flake is removed and, during theseunsuccessful blows, cracks are initiated which can then be wedged with detritus (Figure 10). Sincemore than one crack is initiated, small flake1ets usually are detached from the area around theprimary crack and often from the bottom end of the core, forming a cascade of small step scarssimilar to that which can occur during Hertzian initiation. Flake initiation by wedging is possibLewith techniques other than bipoLar flaking, and also during tool use.

The initiation angle in wedging is about 90°, but the initiation site frequently is damaged bysecondary detachments, often making its measurement impossible. There is no bulb of force andthe fracture surface is usually flattish near the point of initiation.

Bending Initiation

With soft indenters the contact stresses are small and the fracture initiates under bending stressesaway from the indenter. In bending, the stress field has less of a three-dimensional character andthe effect of force angle (4)) and edge angle (1:/) on the bending stress can be assessed from a two­dimensional model (Tsirk 1979). Assuming that the initiation force acts at a unit distance from theside face of the idealized nucleus, the maximum stress on the initiation face will occur away fromthe point of application of force, except for large edge and force angles. In Figure II the contoursof the non-dimensionalized maximum stress (o/F) are shown on a pLot of force angle against edgeangle. The distance r at which these maxima occur also is indicated. The highest bending stressesoccur on edges that have small angles. In this case even hard-hammer percussion can cause bending

690 AMERICAN ANTIQUITY

10mm1--------.,1

[Val. 52, NO.4, 1981

Figure 9. Cascade of initiation step scan caused by hard-hammer percussion_

initiation. However, with a soft indenter bending initiations can occur even when the edge angle islarge, though the probability of bending initiation is Less for large edge angles.

Flakes initiated by bending often have characteristic features. The initiation face of the nucleusthat fmms the proximal end of the bending flake is segment shaped and a neat concave scar is Lefton the initiation face. In the formation of a bending initiation, the crack begins at a flaw in or nearthe initiation face and grows downward at an angle of 90°, lfthe edge angle of the nucleus is rdativelyLarge-say, greater than about 45°-the crack will curve out so that it runs approximately parallelto the side face of the nucleus. A flake detached from a flat side face has a characteristic waistedappearance in plan view because of the reduction in flake width in the initiation-propagationtransition (Figure 12). Bending initiations do not have a bulb of force, though the flake surfacecreated during the transition from initiation to propagation can look superficially like a diffuse bulband has been mistaken as such by archaeologists. As Tsirk (1979:85) has pointed out, what Crabtree(1972:74) describes as a conchoidal flake showing a pronounced lip is a typical waisted bendingflake. Similarly, the flake made by indirect percussion that is illustrated by Bordes and Crabtree(1969:20) dearly has a bending initiation-propagation transition area that they have misidentifiedas a bulb of force. There are very few or no secondary detachments associated with a bendinginitiation and therefore the resultant flake can be refitted to its scar without there being any appre­ciable gap between them on the initiation surface of the nucleus. Undulations usually are notpronounced on the fracture surface in the initiation region.

Bending is a common mode of flake initiation in preforming and in rejuvenating worn edges bysoft-hammer percussion and pressure flaking with wood, antler, or bone, Newcomer (1976) has

Cotterell and Kamminga] THE FORMATION OF FLAKES '"

Figure 10. A c{ack (arrowed) caused by the wedging of detritus in the battered end of a bipola{ core.

reported that in tooL making, small flakes can form on the margin of a flake as it rotates off thenucleus. We have observed this type of flaking damage in the form of small bending-initiated scars.In use flahng, bending initiations are even more common (cf. Cotterell and Kamminga 1979:102;Kamminga 1982:65; Keeley 1980:44; Lawrence 1979: I 19; Odell 1981: 199). While this form ofinitiation is not in itself diagnostic of particular tool-use activities, it is an essentiaL or commonfeature in a number of use-wear patterns, such as those from adzing wood, sawing bone, and anycutting activities that require thin-edged tools.

On a very acute edge a crack initiated by bending propagates straight down from one face to theother to create a fracture that is nearly at right angles to both faces (Figures 13, 14). These transversebreaks, termed snap fractures by the Ho Ho Committee (L 9 79), have virtually no propagation phase.Because many stone tools have acute cutting edges, snap fractures are common in use wear. InFigure L4 we show a heaviLy snap-fractured working edge of a flake tool used experimentally to sawbone, typical damage from this type of activity.

When a flake detached from a core hits the ground it may snap transversely in a bending fracturenear its middle. A flake aLso can be broken across its middLe by bending if it is supported at its endsand loaded in the middle (in engineering called three-point bending). In this case the fracture initiatesopposite to and almost directly under the force applied to the middle (Figure 15a). Such fracturescan occur if a flake accidentally is trodden on, or when a flake intentionally is snapped into twopieces by placing it on a hard surface and striking it with a hammer In the Latter situation the typeof fracture depends upon the support given to the flake by the surface it rests on. If it is supportedat its ends, a bending fracture occurs, but if it is supported immediately under the point of impactit will break in a compression fracture propagating from the striking surface downwards (Figure15b).

PROPAGATiON PHASE

Cracks in any solid can grow either in a stahle fashion under mechanicaL work done by the externalforces or unstably under the release of elastic strain energy. We illustrate schematically in Figure

16 the stability of fracture propagation under different types of stresses. Cracks are unstable in atensile stress field and once initiated they quickly will accelerate to high velocities of propagationand branch to form muLti-crack paths (Cotterell 1965; Ravi-Chandar and Knauss 1984). Undercompressive stresses, cracks are completely stable and grow only as the load is increased (Cotterell1972). Under bending stresses, however, cracks are quasi-stable. If the crack is wedged open grad­ually, the force drops as the fracture develops and the propagation is stable. However, if such anopening force is applied suddenLy and held constant, the propagation of the crack is unstable. Rakesdevelop under a combination of bending and compression forces and therefore they form in a moreor less stable manner.

There are two modes of crack propagation in flaking. A fracture that has created a relativeLy thinflake will have propagated under a combination of bending and compressive forces. The crack pathof such a flake has been controlled largeLy by the stiffness of the flake as it forms (Cotterell et at.1985). The most significant parameter for this mode of propagation in nuclei of large edge angLe(greater than about 45°) is the force angLe. The edge angle of the nucleus is unimportant to thepropagation phase. In the other mode, the crack propagates under secondary tensile stresses createdby an essentiaLLy compressive stress field well away from the edge of a nucLeus. Compression­controlled fractures are a characteristic of bipolar flaking.

Colterell and Kammingal THE FORMATION OF FLAKES '"

Figure 12. The ideal shape of a bending flake formed on a flat side face of a nucleus (cross-sections indicatedby hatching).

Stiffness-Controlled Propagation

The propagation phase in flakes formed nearer the side of a nucleus can be understood in termsof a two-dimensional model (Faulkner 1972:96). In blade flahng a longitudinal ridge is used toprevent the crack from spreading laterally. The tip of the propagating crack is relatively straightand the two-dimensional nature of the fracture process is readily apparent. However, even if a flakeis detached from a flattish side face on a nucleus, the crack front usually does not have muchcurvature and the stress parallel to it plays little part in the fracture process. In these circumstances

Figure 13. A snap fracture (with an inflexed finial) on an acute edge.

'" AMERICAN ANTIQUITY [Vol. 51, No, 4, 1981

Figure 14. Snap fractures on an edge used experimentally to saw bone. Top and boltom: ~iew of side facesof the saw; center: view of the cutting edge.

tne fracture also is essentially two dimensionaL Hence the propagation of such fractures can beexplained from a two-dimensional model.

Conchoidal and other flakes formed along the side face ofa nucleus are usually long relative totheir thickness because the crack forming them essentially runs parallel to the side face. A crackcan propagate parallel to the side face of the nucleus only if the stress-intensity factor at its tip ismode [so that there is local symmetry in the stress field. However, the stress field is very sensitiveto the direction of the flaking force applied to the nucleus (Cotterell et al. 1985). To maintain amode I stress field and local symmetry, the angle of force applied to the nucleus must decrease asthe flake develops (Figure 17). When a microflake is detached during tool use there cannot be anyconsciously imposed variation in the force angle. Even in tool making, the time taken to detach aflake is so short (less than a millisecond) that it is impossible for a person to vary the angle afforcein any controlled way during the flaking event. While the fingertips may be held against the side ofthe nucleus to exert an inward force on the developing flake, thereby reducing the bending component(Crabtree 1968:472), there can be no deliberate variation in the force angle during the formationof the flake. The major control during the time the crack is propagating is over the direction inwhich the indenter moves. It is impossible for this direction to change significantly in this extremelyshort time. The actual angle of force during the propagation phase is determined by the flaking angleand by the developing flake's stiffness (Cotterell et at. 1985). As the crack propagates, the outward­bending component of the flaking force decreases relative to the direct-compressive component

Cotterell and Kamminga] THE FORMATION OF FLAKES

(a)

'"

(b)

Figure 15. (4) A flake broken into two pieces by bending; (b) a flake broken into two pieces by compression.

because a flake is less stiff in bending than it is in compression. Hence, the angle of force mustdecrease with crack growth (Figure L7). Rather surprisingly, the direction of indenter motion haslittle effect on the angle of force during the propagation of the crack, and there is little difference inthe force angle for flaking angles in the range of 0_600 (Figure 17). If local symmetry is maintained,a crack will continue to propagate parallel to the side face of the nucleus. In Figure 17 we indicatewith a line the force angle required to maintain local symmetry at the tip of a crack propagatingparallel to the side face. This force angle requirement for local symmetry at the crack tip agreesclosely with the force angle obtained from the usual range of flaking angles. Hence, a crack initiatednear a side face of a nucleus propagates parallel to it. [f the stiffness of the developing flake did not

...TE:NSION

UNSTABLE:

BENDING

...

STIFFNE:SSCONTROLLEDFLAKE:

,---~\-

\

STABLE

Figure 16. The stability of crack propagation.

696 AMERICAN ANTIQUITY [Vol. 52, No.4, 1987

40,lr------------------,

30

~

~

~~

~~

0

20~

~

'"z...~

u

'"a~

10

o·

1hI--

Force angle requiredfor crack growthparallel to t~l! Sideof thl.' nucleus

.....,.. :.....• ...:.... ;'.-." ....... -.-.-.., , ,.,.

oo 2 6 e 10 12 14 16

NONDIMENSIONAL FLAKE LENGTH ('Ih )Figure 17. Stiffness-controlled cu.ck propagation.

cause the outward component of force to decay, the crack would hinge out to the side face, and,conversely, if it caused the outward component to decay too quickly the crack would plunge intothe body of the nucleus.

To demonstrate that force angle control is essential for the successful detachment of a thin flakefrom the side ora nucleus, we devised a simple loading rig that artifidally could maintain a constantforce angle (Cotterell et at. 1985). A slit was machined parallel to the side of a rectangular plate of

Cotterell and Kamminga]

55hI-

THE FORMATION OF FLAKES

•5 15'

M!~:::;;::: F# orceManglef~

h-

5'

Figure 18. Crack paths obtained from controlled force angles.

glass to model a partially formed flake. By varying the angle at which the force was applied to thismodel flake through only J00 we made the crack either hinge to the side face or plunge into thebody of the plate (Figure 18). In real flaking the force angle closely is controlled by the bending andcompression stiffnesses of the developing flake so that the crack propagates parallel to the side faceof the nucleus.

69. AMERICAN ANTIQUITY [Val. 52, No.4, 1987

It is possible to detach a relatively long flake from a side face ofa nucleus because of the opportunecontrol afforded by the bending and compression stiffnesses of the developing flake. Without thisstiffness-control mechanism, flaking as we know it would be impossible, and most attempts wouldresult in short stubby flakes of little or no value as too1s_ Similarly, flaking to shape or rejuvenatea stone tool depends on the stiffness-control mechanism. Raked-stone technology developed becausebasic flaking requires little skilL Anyone easily can detach a flake suitable for tool use from the sideof a nucleus by pressure or percussion. The flake detached by a novice probably will be irregular;it requires experience to produce a flake of predetermined shape. The skill in flaking is in thepreparation ofthe core and preform and in the positioning of the indenter rather than in the variationof the flaking force during the flake detachment

Compression-Controlled Propagation

Flakes formed away from a side face of a nucleus essentially propagate under compression in thedirection of the force. Such flakes often are initiated by wedging, and this mechanism can continueto influence the crack's growth. Secondary tensile stresses also can aid the propagation of a crackthat is under an essentially compressive force. These tensile stresses are developed when a nucleusthat is thicker at its middle than at its ends is loaded in compression. For ex-ample, in rock mechanicsthe Brazilian test (an indirect tension test) makes use of the secondary tension stresses that developwhen a cylindrical specimen is compressed across its diameter (Jaeger and Cook 1969: 160). Underan essentially compressive stress field, the constant stress term T in the expression for the stress atthe tip of a crack is highly compressive. Hence a compression-controlled crack has a highly stablepath. The stabilizing effect of compression is utilized in the double cantilever beam test which isused in engineering to measure the fracture toughness of a material. In this test a slit is introducedalong the center line of a long rectangular plate and the plate is fractured by applying opening forces.For isotropic materials the fracture path is unstable in this load configuration and the specimenbreaks by the crack hinging to one side or the other. To stabilize the fracture path, so that the crackruns down the center of the specimen, a controlling compressive stress often is introduced parallelto the crack line (Benbow and Roesler 1957; Streit and Finnie 1980).

Gramberg (1965:47-48) also has demonstrated the path stability conferred by compression withexperiments on single crystaLs of rock salt which have highly preferred crystallographic cleavageplanes. When a cylindrical specimen, cut so that the cleavage plane is inclined at an angle of 360 tothe axis, IS compressed axially the fracture does not follow the cleavage plane but forms undercompression control on an axiaL plane.

Compression-controlled crack propagation occurs in bipolar flaking and is analogous to axialclea~age which can occur in tests on rock cylinders loaded axially (Gramberg 1965). Compressionfracture can happen when soft inserts of lead or plastic are placed between the rock specimen andthe loading platens of the testing machine. These inserts, which reduce the friction between thespecimen and the platen, can intrude into microscopic surface flaws and produce initiation bywedging similar to that which can occur in bipolar flaking (Holzhausen and Johnson L979). In acompression-controlled fracture the nucleus usually is spLit into two or three fragments of roughlyequal size (Figure L9), which is what occurs during axial cleavage in compression tests on rocks. Itsometimes is difficult to identify a surface that has been formed by a compression-controlled fracturebecause in practice there is considerable variation in the degree ofcurvature. Compression-controlledpropagation is probably a rare occurrence eXCept when the bipoLar technique is used in primaryflaking or in retouching a thick "back" on a ftake.

TERMINAnON PHASE

The mechanics involved in the final detachment of a flake from a nucleus can be different fromthose operating in the propagation phase. We recognize five basic endings, or terminations as theyare sometimes called in use-wear analysis-feather, axial, step, hinge, and plunging (Figure 4). Thefirst two of these endings do not have special termination-phase mechanics but simply involve acontinuation of those in their propagation phase

Cotterell and Kamminga] THE FORMATION OF FLAKES '"

I

tEGG-SHAPED

PEBBLE

l

tFLATTISH

PEBBLEFigure 19. Ideal compression fractures in symmetrical nuclei (fracture planes indicated by hatching).

Feather Termination

A feather termination occurs when the crack forming the flake that has been propagating parallelto a side face of the nucleus turns slightly to meet it at a very acute angle. Our experiments indicatethat feather terminations occur for a wide range of initiation force angles (Cotterell et al. 1985). Thefeather termination can be viewed as the natural termination for the stiffness-controlled propagationphase of a flake formed along the side face of a nucleus.

Axiat Termination

In an axial termination, the crack forming the flake continues straight through a nucleus to meetthe surface opposite the initiation face approximately at right angles. This termination is the natural

'" AMERICAN ANTIOUITY [Vol. 52., No.4, 1987

ending for the compression-controlled phase of a flake formed near the center of a nucleus andoccurs in bipolar flaking when a nucleus is spljt into two or tltree equally sized fragments. It alsocan occur wilen a very thick flake is detached from the side of a nucleus so that tile crack propagatesstraight through to the bottom of tile nucleus. Axial terminations aLso occur on snap fractures onvery acute edges, or wilen a flake is broken transversely (Figure 15).

Slep Termination

In the formation of a step termination there is an abrupt c1tange in the direction of the crack.Two varieties, termed (a) and (b), are known to occur. In the rrrst variety the flake detaches com­pletely. Part orthe partially formed flake remains attacned to the nucleus in a type (b) step termi­nation. Witn use scars on semi-transLucent stone like obsidian or flint, tne continuation ofthe crackin a step (b) termination often can be seen hignlighted by tne reflection oflight. Sometimes, if thetwo fracture surfaces are not appreciably wedged apart by detritus, interference will cause the colorsoftne spectrum to be visible. It was our original view that these two varieties of flake terminationswere caused by different mechanisms (Cotterell and Kamminga 1979: 105-106). WhiLe we stilL thinktnat there are two mechanisms responsible for the formation of step terminations, we now believetnat one of tnese mechanisms can cause both varieties.

Step terminations are caused by crack arrest. The crack can arrest either because there is insufficientenergy availabLe to complete a fracture or because tne crack intersects a significant flaw that effectiveLyblunts it (Cook and Gordon 1964). Cores ofhighly flawed lithic materials such as quartzite frequentlybecome exhausted because of tne formation of step terminations. Haw-induced arrest is more likelyto occur with smalLer !lakes. A feather scar sometimes can have an extremely shallow step causedby the crack intersecting a surface flaw. For tne purposes of identifying scar types in use-wear analysissuch fine terminations on feather scars probably are not important.

Flake formation requires a continuing driving force to maintain the crack propagation, and arrestof the crack wilL be almost instantaneous if this driving force falLs below a critical value. The unalcrack that completes the step fracture is caused by bending and can occur either almost instanta­neously after the arrest of the initial crack or at any time later. If the crack comes completely to

the side face of the nucleus along the lateral margins of the developing flake, the step will occurright at the tip of the propagating crack. However, if the crack has not come completely to the sideface at the lateral margins, the flake will break olfbehind the crack front at the junction of the crackand the surface of the nucleus. It often is very difficult to detach the thin piece of stone that is leftattached to the nucleus.

Step type (b) terminations also can be caused by flake buckling. In a series of excellent high-speedphotographs Crabtree (L968:475) has shown a thin obsidian blade buckling and snapping as itdetaches from the core. Although in this case the flake had completely detached (ending in a feathertermination) before it buckled, the same mechanism can occur before complete flake detachment.Step formation by buckling is likely only if the developing flake is very thin relative to its length.

Hinge Termination

On flakes formed near the surface of a nucleus, the crack can turn to approach the side face ofthe nucleus roughly at right angles to form a hinge termination. The resuLtant flake has a blunt endrounded in cross section. Crabtree (1968:466) observed from his experiments on blade flaking inobsidian that hinge terminations were formed when there was more outward pressure than wasneeded to produce a blade with a feather termination. Our calculations for the mode I and II stress­intensity factors and our experiments have shown that an increase in the bending component ofthe force will cause the crack path to be deflected toward the side face of the nucleus (Cotterell etal. 1985). Although the increase in the outward component need not be large, it is unlikely to happenwhile the crack is propagating at an essentialLy constant velocity. We have observed that there is ash'Up drop in the velocity of crack propagation immediately prior to the formation of a hingetermination, which can provide the necessary time for the development of an outward componentof force.

Cotterell and Kamminga] THE FORMATION OF FLAKES '"Hinge terminations frequently occur on flakes removed from a flattish surface of a nucleus. The

reason for this is that the width of the developing flake increases, thereby requiring more energy tokeep the crack propagating. If this energy is not available, the velocity ofcrack propagation decreases,giving time for a hinge termination to form. The crack front immediately prior to hinging has tobe fairly straight, because the outward component of force causes the direction of principaL stressto rotate around a single axis paralLel to it and alL of the crack front has to curve out almostsimultaneously.

It is possibLe for a succession of hinge scars to form on a nucleus of fine-grained siliceous stoneand we have observed such a phenomenon on exhausted platform cores. When a flake is beingdetached from a nucleus that already has a hinge scar on its side face, the thickness of a flake beingformed must increase suddenly as the crack tip passes the end of the scar. The crack can propagatepast the hinge termination ofthe scar on the surface only ifthe flaking force is increased substantially_Since the flaking force can only increase with the displacement of the indenter, an increase in forcecannot occur instantaneously. Hence, the crack propagation must decelerate as it approaches theend of the scar, thus creating the condition for the formation of another hinge termination.

In tool making, hinged flakes usually are undesirabLe. One notable exception is the pirri graver,an ethnographic stone-tool type from the arid region of Australia used for cutting flutes on woodenartifacts such as shields and boomerangs (Kamminga 1985). The prominent design feature of thetool is the transverse convexity of its underside surface. What provides this necessary convexity isoften the hinged end ofa large flake; in fact, not uncommonLy the flake was so reduced in preformingthat only its hinged end was retained.

Plunging, or Outrepasse Terminmlon

With a plunging termination, a crack running near an edge of a nucleus intraflexes, and plungesinto the distal end of the nucleus and detaches it as part of the flake (Crabtree L968:466; Tixier1974: L9). This mode of termination is most common in tool making, especially during bLade flaking,but it also can occur occasionally in use wear. We occasionally have observed use scars with pLungingterminations on hafted flakes used experimentally to scrape wood.

Plunging terminations are caused by the end of the nucleus. The effect is intensified if the end isa sharp corner, and can be observed most easily when a flake is formed on the side of a rectangularplate. This type of plunging termination was first illustrated by Faulkner (L972:LI8), and wascommon in our own experiments (Cotterell et at. 1985). In a two-dimensional situation, the stressat the tip ofa corner is zero (Timoshenko and Goodier 1951:28). Hence a crack cannot propagateinto a sharp corner and will veer away to produce a plunging termination. However, if a very thickflake is detached from a rectangular-shaped glass plate (or stone core), the crack can propagatestraight through to finish as an axiaL termination.

When a blade is detached from a typical blade core, the situation is slightly different. If a thinflake is detached, the crack does not come under the influence of the end of the flake. However, asCrabtree (1968 :466) has observed, a plunging termination will result from positioning the tip of thepressure-flaking implement too far from a side face of a core so that the crack propagates towardthe tip of the core (Figure 4). Blade cores are shaped with a gentle curvature at their base so thatthin bLades are most likeLy to end in a feather termination. A sharp corner can cause even thinblades to pLunge inward.

Finials

The crack path in a feather termination is stable because there is a compressive stress parallel tothe crack at all times. However, in the formation of hinge and step terminations, the crack propagatesapproximately at right angles to the side face of the nucleus under a bending force that causes thestress term T parallel to the crack to be tensile and large compared with K, (Cotterell and Kamminga1986). The crack is unstable and curves sharply away to run parallel to the side face of the nucleus.The crack can either retroflex back toward the initiation face, or inflex so that it propagates awayfrom the initiation face to create a thin and often fragile extension (Figure 4), which we call afinial.

AMERICAN ANTIQUITY [Vol. 52, No.4, 1987

Figufe 20. Finials on an Acheulian "hand-axe" fmm Swanscombe: (a) is an inflexion, and (b) is a felmflexionon a hinge termination.

to the flake ending (Cotterell and Kamminga 1986). The retroflection on a hinge fracture has beenrecognized by many archaeologists (e.g., Crabtree 1972; Tixier 1974), but inflections, which alsoare common, generally have not been recognized. Inflexed extensions on the end of a flake aresometimes very long; and there are exceptional ones that can be as much as half the length of theflake. To demonstrate just how ubiquitous finials are, we offer two examples, one inflexed and theother retroflexed, on an Acheulian «hand axe" (Figure 20).

Cotterell and Kamminga] THE FORMATION OF FLAKES '"Every time the crack forming a flake turns to approach the side face of a nucleus at a high angle

during the termination phase, its path is unstable and may tum to propagate parallel to the sideface. On a retroflexed hinge flake, a second instability may occur as the crack path turns once againto approach the side face and can Lead to the formation ofa hook which Tixier (1974) regarded asa characteristic feature of that flake type.

A further possible finiaL morphology, that combines a finial with a second crack, is the pseudo­bifurcation (Figure 4). In this case the crack fonning the flake retroflexes, but the thin fragile lip lefton the nucleus is broken off almost immediately so that it appears as if the crack has branched.When the pseudobifurcation flakelet scars are examined, the '"retroflexed" scar shows a smoothcrack path, while the "inflexed" scar is angular at the place where the second crack has initiated.The pseudobifurcation is not considered significant for lithic studies because the second fracture isincidental to the true finial.

Inflexed finials often are present on snap fractures that occur during tool use. Snap fractures withfinials were first included in a classification of utilization scars by Hayden and Kamminga (1973)who named them "invasive-break" and "stepped-break" scars. They were described subsequentlyby Lawrence (1979: I 15) who noted that they were bending initiated. Finials that are retroflexed canoccur on snap fractures, but in our experience they tend to be very smaLl. Finials also occur on axialterminations when flakes are broken into two parts by bending (Figure 15a). Faulkner (1984:328)has called these particular finials "hangnails."

DISCUSSION

The shape of a flake and the scar it leaves on the nucleus are the result of a complex interactionof many variables. In this paper we have attempted to explain in mechanical terms how fractureoccurs and why flakes have different morphologies. An appreciation of the mechanics of flakeformation can lead directly to behavioraL implications. For instance, with glassy stone like obsidianit is possible to distinguish flakes produced by pressure from those produced by percussion bycalculating the crack velocity registered by Wallner lines. Similarly, the form of flake initiation canreveal the hardness of the indenter in tool making and tool use. To exploit the potential of flakemechanics will require continuing effort by many researchers. The relationship between tool-useactivities and the frequencies of different flake-scar initiations and terminations is as yet llttleunderstood, though broad correlations are evident. Similarly, the importance of finials in definingmore specific use-wear patterns has yet to be thoroughly investigated. What is required is a newgeneration of use-wear experiments that take into account the potentially significant attributes ofuse fractures.

Our research into flake formation is built on the foundations laid by other researchers (Bordesand Crabtree 1969; Crabtree 1968; Ho Ho Committee 1979; Lawrence L979; Kerkhof and Muller­Beck 1969; Faulkner 1972, 1974) as well as on our own earlier work (Cotterell and Kamminga1979). We believe that the tripartite scheme of flake formation, based as it is on mainstream fracturemechanics, offers a rational basis for understanding the phenomena of flaking. Some of the me­chanical modes we have described often are found in association, while others cannot occur together.For example, because bending initiation only can happen near a side face of a nucleus, its associatedpropagation phase cannot be one of compression control. Because the phases of flake formationare, to some degree, defined arbitrarily, not all need to be represented in the formation ofa particularflake. Obviously, every flake must have an initiation phase, but the propagation phase may be sosmall (e.g., in a snap fracture) that it can be considered absent for any practical purpose.

In a sense the most important phase in a flake's formation is its initiation because it oftendetermines the mechanisms that will operate in the subsequent phases. If a flake is initiated nearthe side face of a nucleus, its propagation probably will be stiffness controlled. Initiation by wedgingrequires a hard indenter and it is unlikely to occur near the side face of a nucleus where Hertzianor bending initiation is more probable. Hence, a wedging initiation is likely to lead to a compression­controlled propagation and axial termination. Therefore, in flake formation the most importantcompetition between mechanisms occurs at initiation. Generally in use scarring, bending initiations

'" AMERICAN ANTIQUiTY [Vol. 52., NO.4, 1987

-<•

\"'-.0

30·

1} 60·

;1-

~ hI-

>­>­ua~

w>

~

0<Za

'"zw>:azaz

10

08

06

0.4

0.1

oo 5 10 15 10

NONOIMENSIONAL FLA:(E LENGTH t;h

Figure 21. Theoretical \'eiocity of crack pwpagation as a function of uack length.

are most common, because bending stresses are high on acute edges and most of the materialsworked with stone tools were not particularly hard. When a nucleus is struck near its edge with ahammerstone and that edge does not have too acute an angle, the most likely flake initiation toresult is Hertzian. Since the study of tool making preceded that of use flaking, it is understandablethat attention was focused on the Hertzian-initiated conchoidal flake and that the bending-initiatedflake, which is radically different, was not recognized for a very long time.

We have given the criteria for identifying the various features of flakes and their scars, such asthe waisted shape and llpped profile of an ideal bending initiation, and the mechanisms responsiblefor them, However, some formations are difficult to identify even when a fracture surface is pristine,such as for instance, the surface created by compression-controlled propagation which has fewdistinguishing features. While determining how large flakes or scars were formed sometimes isdifficult, such problems are more common with small flakes or scars, because the surface texture ofmost lithic materials becomes rougher as scale decreases. Finials on use scars, for instance, may bedifficult to detect on granular hthic materials. The details of small flake scars can be difficult toobserve at the magnifications needed for identifying, say, a bending from a Hertzian initiation, ora step from a hinge termination. While a hard indenter can leave its mark in the form of concentriccone cracks, these cracks often can propagate at the time the flake detaches from the nucleus andremove the flake's initiation area, sometimes completely replacing it with tiers of small scars. Itmust be accepted that the mechanisms responsible for flake formation will not always be apparent.

Probably the most Important application of flaking mechanics is in use-wear analysis. The amountof experimentally derived data on the incidence and morphology of use-flake scars is still relativelylittle and undoubtedly there is much to be derived from them about the mode of tool use and theproperties of worked materials. We recommend that whenever possible in tool-use experiments theflake scars are recorded in the sequence of their appearance rather than at the completion of theactivity. We hope that more use-wear analysts will acknowledge the potential that use-flake scarringoffers for defining more precise use-wear patterns. Our paper describes only the basic mechanics

Canerell and Kamminga] THE FORMATION OF FLAKES '"responsible for flake formation. Further experimental studies of specific aspects of flaking are re­quired. For example, more needs to be known about such issues as the causes of hinge terminationand the relation among the indenter, edge angle, and mode of initiation. We hope that the nextgeneration of experiments addresses issues such as these.

Acknowledgmel1lS. This research was funded by the Australian Research Grants Scheme. We thank BarryCundy, Brian Hayden, and Peter Hiscock for their comments on this paper, and E P. Dickson for his helpfuldiscussions. The phOlographs were laken by Trevor Shearing and Dragi Markovic.

APPENDIX

Velocity o/Crack Propagation in Stiffness-Controlled Flake Formation

For the purpose ofstudying the propagation phase in blade flaking, we have modeled the formationof a flake as a crack propagating parallel to the surface of a nucleus (Figure 21). Although the forcethat initiates a flake may be considerable, the energy absorbed by the nucleus and flake is relativelysmall. Therefore, for percussion flaking we have assumed that in both magnitude and directionthere is little change in the hammerstone's velocity. We have placed the point of contact betweenthe hammerstone and the top of the flake close to the corner of the idealized flake and made theassumption that this inside comer moves with a velocity v; equal to the hammerstone in the directionof the blow. using simple engineering theory of elastic bending, it can be shown that the energystored (U) in the developing flake is given by

U = E7B Vi2t2(~)l2 sin'a + 3 (~) sin a cos a + 2(~)'cos2a]

where \! is the length of the crack forming the flake, h its thickness, and B its width; t is the timethat has elapsed from the initiation of the flake. The rate of release of the energy stored aufa\! mustbe equal to the energy K,/IE)B required to produce the flake, provided the kinetic energy of theflake is small (that is v, is small). Hence

P:i2

t2 = 2(~)'[sinl a + (')sin a cos a + !(,)2cos1a]K'oh 6h h 3h

and the crack velocity v is given by

[ ( ') 1(')' ]'"~ = 4"\ fit3

(h) sin2a + h sin a cos a +"3 h cos'a

v, V 7 '[ (') 2(')']4 sln'a + 3 h sina cos a +"3 h cosJa

where v, is the velocity of shear waves,

t3 = [E(l

and 11 is Poisson's ratio. This expression for the crack velocity is shown in Figure 21. It is onlyaccurate for fJ «: I. The ratio v/v, is limited because at high velocities of crack propagation muchof the energy released is absorbed as kinetic energy by the developing flake.

REFERENCES CITED

Benbow, J J., and E C. Roesler1957 Experiments on Controlled Fractures. Proceedings Q( the Physical SocielY Ser. B 70;201-211.

Bordes, E, and D. E. Crabtree1969 The Corbiac Blade Technique and Other E"periments. Tebiwa 12:1-22.

,,, AMERICAN ANTIQUITY [Vol. 52, No.4, 1981

Cook, J., and J. E. Gordon1964 A Mechanism for the Control of Crack Propagation in All-briule Systems. Proceedings oj the Royal

Society A 282:508-520Colterell, B.

1964 On the Nature of Moving Cracks. Journal ofApplied Mechanics 31: [2-1 7_1965 Velocity Elfects in Fracture Propagation. Applied Maurials Resenrch 4:227-232.1966 Notes on the Paths and Stability of Cracks. InternatiOlla! Journal of Fracture Mechanics 2: 526-533.1968 Fracture Propagatlon in Organic Glasses. lmernalional Journal of Fracture Mechanics 4:209-21 7_1972 Brinle Fracture in Compression. !nlernal1onal Journal of Fracture 6: 195-108.

Cotterell, B., and L Kamminga1979 The Mechanics of Aaklng. In Lilhic Use-wear AnafJ'sis, edited by E Hayden, pp. 97-[ 12 Academic

Press, New York,1986 Finals on Stone Rakes Journal ofArchaeological Science, 13:451-461.

Cotterell, E, 1. Kamminga, and F. P Dickson1985 The Essential Mechanics of Conchoidal Flaking. International Journal of Fracture 20:205-221.

Cotterell, B., and J. R. Rice1980 Slightly Curved or Kinked Cracks [merna/ional Journal of FraC/ure 16: 155-169.

Crabtree, D. E,1968 Mesoamerican Polyhedral Cores and Prismatic Blades. American Anliqui/y 33:446-478.1972 An InlroduC/ion to Flimworking Occasional Papers of the [daho State Museum No. 28, Pocatello.

Evans, J.1872 The Ancient Stone [mplements, l-VeaponI, and Orna/tU!nlS of Great 8n/ain Longmans, London

Faulkner, A.1972 Mechanical Principles of Flint J,j,'Olking. Ph.D. dissertation, Washington State University. University

Microftlms, Ann Arbor, Michigan.1974 Mechanics of Eraillure formation. News/mer ofLithic Technology, 3:4-11,1984 Examining Chippe.d Stone Tools. Wisconsin ArcheolOffisl 65:507-525.

Frank, F. C, and B. R Lawn1967 On the Theory of Hertzian Fracture. Proceedings ofrhe Royal Societv 299:291-306.

Fullagar, R, L. K.1982 rVhl1l's /he lise? All Anal.nis qfAiri' She!/{'r II, Glenaire, ViC/orla. Master's (qualifying) thesis, La Trobe

University, Melbourne. Australia,Gol'dstein, R. V" and R. L. Salganik

1974 Brittle Fracture of Solids with Arbitrary Cracks. [Mema/lonal Joul'flal of Fraclure 10:507-525Gould. R, A.

1973 Use-wear on Western Desert Aboriginal Stone Tools: A Reply to Messrs Hayden and Kamminga.Ne\tsll'lIer qf Lilhic TI'chllologr 2( 1-2):9-13

Gramberg, J1965 Axial Cleavage, A Signiftcant Process in Mining and Geology. Engineering Geology 1:31-72,

Griffith, A A.1910 The Phenomena of Rupture and Flow in Solids, Philosophical Traflsacrtons q( Ihe Royal SocielJ.' qf

LOildon A221 ' 163-198.Hamilton, G M. and L. E. Goodman

1966 The Stress Field Created by a Circular Sliding Contact Journal ofApplied Mechaflics 33:371-376,nartlev, N, E. W" and R. Wilshaw

In] Deformation and Fracture of Synthetic a-quartz. JoulIlal of Matl'l'ia/s Science 8:265-278.Hayden. B., and 1. Kamminga

1973 Gould, Koster and Sontz on 'Microwear': A Critical Review, Ne>t'sleuer ql Lllhic Techllologl' 2(1-2):3-8. 13-14.

1979 The First CLUW: An Introduction to Use Wear. In Llthl( C'w-wear AlJalysis. edjted by B. Hayden,pp. 1-13 Academic Press, New York.

Hertz, H.1896 Hl!rI:'s Miscl'lIaill'OIlS Papas. Reprinted. Macmillan, London, Originally published 1882, Verhand­

lungen des Vt-reins zur BefOrderung des Gewerbefleisses 61 :449.Ho Ho Classification and Nomenclature Commiuee Report

1979 In Lithic L'sc-\l'('4/' Analnl.", edited by B. Hayden, pp. 133-137. Academic Press. New York.

Holloway, P G.1973 The Ph.l'slca! PropI'meJ oj" Glass, Wykenhan Publications, London,

Holzhausen, GR., and A. M Johnson1979 Analyses of Longitudinal Splitting ofU niaxially Compressed Rock Cylinde.rs Rock Mechanics, Miniflg

Scil"lce and Geomechanical Ahstroas 16: 163-177.Jaeger. J. C, and N G. W. Cook

1969 FlIl1damenla!s of Rock Mi'(hallics. Methuen, London

Cotterell and Kamminga] THE FORMATION OF FLAKES '"Kamminga, J.

1978 Journey Into the Microcosms A Functional Analysis ofCertain Classes ofPrehistoric Australian SloneTools Unpublished Ph.D dissertation, Department of Anthropology, University of Sydney, Australia.

1979 The Nature of Use-pollsh and Abraslve Smoothing on SlOne Tools. In Lithic Use-wear Allalysis, editedby B. Hayden, pp. 143-157. Academic Press, New York.

1982 Over [he Edge. FUllaiollal Analysis ofAuslralian Slone Tools. Occasional Papers in Anthropology No.12 Anthropology Museum, Queensland University, Brisbane, Australia.

1985 The Pirri Graver, AUSlralian Aboriginal Studies 1985/[1:2-25.Keeley, L H.

1977 An Experimenllll Study ofMicrowear Traces on Selected British Palaeolithic Implements, UnpublishedPh.D. dissertation, Oxford University, Oxford, England.

1980 Experimental Determination ofSlOne Tool Uses. A Microwear Analysis, University of Chicago Press.Kerkhof, von, F., and H. Muller-Beck

1969 Zur bruchmechanischen Deutung der Schlagmarken au Steingediten. Glostechnische Ber/chte 42A39­448,

Kleindienst, M. R., and C. M. Keller1976 Towards a Functional Analysis of Hand-axes and Cleavers: The EVldence from Eastern Africa. Mtln

(n.8-) 11:176-187,Kolsky, H.

1963 Stress Wtlves in Solids. Reprinted. Dover Publications, New York. Originally published 1953, Clar­endon Press, Oxford.

Langitan, F. B., and B. R, Lawn1969 Hertzian Fracture Experiments on Abraded Glass Surfaces as Definitive Evidence for an Energy Balance

Explanation of Auerbach's Law. Joumt1l ofApplied Physics 40:4009-4017,Lawn, B. R., and D B. Marshall

1979 Mechanics of Microcontact Fracture in Brittle Solids. In Lithic Use- wear Alltllysis, edited by B. Hayden,pp. 63-82. Academic Press, New York.

Lawn, B. R., and M. V. Swain1975 Microfracture Beneath Point Indentation in Brittle Solids Journal ofMaterials Science 10: I 13-122.

Lawn, 6., and T. R. Wilshaw1975a Frtlcture of Briale Solids Cambridge University Press, Cambridge, England.1975b Indentation FraclUre: Principles and Applications. Journal of MtlIerials Science 10:1049-1081.

Lawrence, R. A.1979 Experimental Evidence for the Significance of Attributes Used in Edge-damage Analysis, [n Lilhic

Use- wear Analysis, edited by B. Hayden, pp. I 13-12 I. Academic Press, New York.Macalister, R. A. S.

1921 A Text Book of European Archaeology, vol. IL Cambridge University Press, Cambridge, England.Mandeville, M. D.

1973 A Consideration of the Thermal Treatment of Chert Plains Anthropologisl 18: I 77-202.Mewhinney, H.

1957 A ManUllI for Neanderthals University of Texas Press, Austin.Newcomer, M, H.

1976 Spontaneous Retouch. Second Imernational Symposium 011 Flim. Staringia 3:62-64.Odell, G. H.

1977 The Applicalion of Micro-wear AnaljlSls to the Lilhic Component of an Elllire Prehistoric SeU!emelll.­Methods, Problems tlnd Functional Reconslruaions. Unpublished Ph.D. dissertation, Department of An­thropology, Harvard University, Cambridge, Massachusetts,

198 I The Mechanics of Use-breakage of Stone Tools: Some Testable Hypotheses. Journal of Field Archae­ology 8:197-209

Olausson, D S.1983 Experiments to [nVestlgate the Effects of Heat Treatment on Use-wear on Flint Tools. Proceedings of

[he Prehistoric Sociely 49: 1-13.Ravi-Chandar, K., and W. G. Knauss

1984 An Experimental Investigation into Dynamlc FraclUre. [lL On Steady State Crack Propagation andCrack Branchlng. Imernalional Journal of Fracture 26, 141-154.

Rick, J. W., and S, Chappell1983 Thermal Alteration of Silica Materials in Technological and Functional Perspective. Lithic Technology

11:69-80.Rooke, P P., and D, V. Cartwright

1976 Compendium of Stress Illlellsily FaclOrs. H. M, Stationery Office, London.Schindler, D. L, J. W Hatch, C A. Hay, and R. C. Bradt

1982 Aboriginal Thermal Alteration of a Central Pennsylvanian Jasper Analytical and Behavioral [mpli­cations, Americtln Amiquity 47:516-544.

Speth, J D.1972 Mechanical Basis of Percussion Flaking, AmeriCtln Antiquity 37:34--60.

". AMERICAN ANTIQUITY [Vol. 52, No.4, 1981

1974 Experimenlallnvesligatlons Into Hard Hammer Percussion Flaking. Tewiba [7:7-36.1975 Miscellaneous Studles in Hard Hammer Percllssion Flaking: The Effecls of Oblique Impact. American

Amiquilj! 40:203-207.Streit, R" and L Finnie

1980 An Experimental Investigation of Crack Path Directional Stability. Experimental Mechanics 20: [7­23.

Tada, H., P. C Paris, and G. R. Irwin1973 The Stress Analysis of Crncks Handbook, Del Research, Hellertown, Pennsylvania.

Timoshenko, S" and 1. N. Goodier1951 Theory of Elaslicirj!. 2nd ed. McGraw-Hill, New York.

Ti:<-ier, J.1974 Glossary for the Description of Stone Tools. Translated by M. H. Newcomer Newsleller of Ulhic

Technology, Special Publication. The Center for Archaeological Research, University ofTex:as, San Antonio.Toth, N.

1985 The Oldowan Reassessed: A Close Look at Early Stone Artifacts. Journal of Archaeological Sciencel2:I01-120,

Tsirk, A.1979 Regarding Fracture [nitlatlons, In Lilhic Use-wear Analysis. edited by B. Hayden, pp. 83-96. Academic

Press, New York.Williams, M. L

1957 On the Stress Distribution at the Base of a Stationary Crack Journal of Applied Mechanics 24: 105­114,