Methods for Improving Groundstone Analysis Jenny Adams X

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Experiments in LithicTechnology

edited by

Daniel S. Atnick and

Raytnond P. Mauldin

BAR International Series 528

1989



METHODS FOR IMPROVING GROUND STONE ARTIFACTS ANALYSIS:

EXPERIMENTS IN MANO WEAR PATTERNS

Jenny L. Adams

Arizona State Museum

Ground stone ar t i fac ts present problems for archaeologists inthat they are often large, awkward to transport, diff icul t to store,and appear to be of l i t t l e interpretive value. Some archaeologistshave determined tha t it i s not worth the e f fo r t to haul these

ar t i fac ts back to the laboratory for intensive analysis, content,instead to field-record a few "important" at t r ibutes , usually limitedto typological placement. Yet, there is more that can be done with theground stone art i facts .

Following several recent studies of ground stone (Hayden 1987;Flenniken and Ozubun 1988; Adams 1988) , t h i s paper combinesethnograpic information, data from experiments using ground stone

tools, and descriptions of macroscopic and microscopic traces of use

wear on those tools, in a preliminary consideration of ground stone

tool function. The study demonstrates that experimentation and low

power magnification can identify some genera.L use-wear patterns onground stone tools.

ETHNOGRAPHIC IDENTIFICATION, EXPERIMENTATION, AND

TRIBOLOGY IN GROUND STONE ANALYSIS

In the southwestern United States, i: l few researchers (Bart let t1933; Eddy 1964) have addressed how specific tools were used by Native

Americans. While data on ground stone tool use can sometimes begleaned from the ethnographic record (e .g. , Hough 1918; Stephen 1936;Cushing et ale 1922), early researchers were generally not interested

in describing the form and function of tools . The most commonlyidentif ied ground stone tools are the mano and metate, but there are a~ a r i e t y of other types of ground stone tools , such as pol ishers ,

palet tes, mortars, abraders, shaft smoothers, and tools used to grindclay and sherds for temper.

A notable early attempt to combine ethnographic information onground stone tool use and a formal typology of tools i s RichardWoodbury's (1954) analys is of l i t h i c a r t i f ac t s from the Awatovi

Project (1935-1939), on the Hopi Reservation in northern Arizona.Woodbury's enduring work served as a valuable reference for theanalysis of ground stone from an archaeological project conducted at

the Hopi village of Walpi (J . Adams 1979).

The Walpi Archaeological Projec t (1975-1982) provided an

opportunity to take advantage of the knowledge of people who made andused, or at leas t saw in use, some of the tools that were found in the

archaeological record. This investigation was accomplished through a



project was working at Walpi, many Hopi looked at and commented on the

ar t i facts , either as they came out of the excavations or as they werebeing processed in the laboratory, as well as in community meetings

where artifacts were displayed for public viewing.

This opportunity to interview the Hopi called into question thevalidity of the inferences that were assumed in the application of thetypology. Interviews suggested that one-hand manos were used toremove hair from hides and soften them for moccasin making, ratherthan for grinding of grains. A microscopic and experimental researchproject distinguished wear patterns made by processing hides fromthose made by grinding corn (J . Adams 1986, 1988) and determined thatthe one-hand manos from Walpi were hide processing stones.

This ini t ia l research at Walpi suggested that a more detailedexperimental and use-wear analysis on ground stone tools would beproductive. Use-wear experimentation has been popularized by chipped

stone technologists since Semenov's (1964) illuminating publication(see also Keeley 1977, 1978; Lawrence 1979; Odell and Odell-Vereecken

1980; Vaughn 1985). In ground stone experiments, surfaces are alteredon a macroscopic scale through the leveling of areas of higherelevation, and on a microscopic scale through the leveling of the

asperities even with the interst ices. Asperities are the individual

grains of quartz or other l i thic material that are elevated above theinterstices or spaces between grains. Interstices can be empty, orfil led with cementing matrix, patination, or debris.

Understanding how wear patterns develop on surfaces is possible

through the application of tribological processes that explain theinteract ions between contacting surfaces, and the damage that i s

caused by the contacting surfaces and intermediary substances (Czichos

1978; Quinn 1971; Szeri 1980; Halling 1975; Dowson 1979). Theprocesses of abrasive wear, adhesive wear, surface fa t igue, and

tribochemical wear are relevant to ground stone analysis.

Abrasive wear is caused when the harder asperities of one surfacepress into the softer material of the opposing contact surface. Theweight and movement of the surfaces causes plastic deformation of thesofter material around the asperities (Czichos 1978:112). As theasperities move through the softer material they leave scratches,grooves, or str iat ions. Even contact surfaces that seem of equal

hardness have asperities and softer material that interact with theasperities and softer material of the opposite surface.

Adhesive wear begins when two surfaces come into contact even i fthere is no relative motion. As two surfaces approach each other

there is interfacial bonding that s tar ts with attractions betweenmolecules. Junctions are formed that have an adhesive strengthdetermined by the chemical and physical make-up of the contacting

su r f ace s . When broken, these j unc t ions leave p i t s , p las t i c

deformation of the asperit ies, remnant material from the opposing

surface, and loose wear particles from both contact surfaces (Czichos1978; Kragelsky et a l . 1982). Depending on the level and intensity atwhich these bonds occur, the damage might not be visible except athigh magnification.



Surface fatigue results from compression and repeated stresscycling of the contact surfaces. Compression causes the asperi t ies to

crush or deform plastically unti l they are capable of withstanding the

load (Bowden and Tabor 1967:16-17; Teer and Arnell 1975:95). Repeated

stress cycling is involved when surfaces move back and forth againsteach other. The alternating movement creates surf icial cracking that

loosens par t ic les which can become involved in the wear process(Czichos 1978:26). A surface worn by fatigue has cracks, pits , andloose wear part icles from one or both surfaces.

Tribochemical wear is a continuous chemical process (Czichos

1978: 126) • Fr ic t iona l energy enhances the in teract ion of thecontacting surfaces, any in termediary substances ( lubr icants intribological terms), and the environment in which the contact occurs.

The result is a buildup of reaction products in the form of oxides andfilms. Tribochemical interactions are visible as lustrous layers ofreaction products and are what is often refered to as polish (cf .

Anderson 1980:180-194; Unger-Hamilton 1984:97; Del Bene 1979; Vaughn1985). The other wear processes can breakdown the reaction products

but the chemical interactions just begin again on the fresh surfacesexposed by abrasion and surface fatigue. More than one wear processcan operate when two surfaces are working against each other; usuallythe las t process in operation will be the most visible on the surfaces

(Czichos 1978:104; Dowson 1979:500).

THE EXPERIMENTS

Experiments that replicate the known uses of ground stone tools,such as corn grinding, wild seed grinding, and the shaping of wood and

bone, in combination wi th concepts borrowed from t r i bology , shouldprovide direct evidence of the type of damage infl icted on ground

stone surfaces. With this in mind, volunteers from Earthwatch and the

University of Arizona made manos, metates, and grooved and f l a t

abraders using pecking stones and hammerstones. River cobbles of the

appropriate size and shape were used unaltered as hammerstones, or

flaked either unifacially or bifacial ly to make pecking stones. Eachvolunteer kept track of how much time was spent at each task, recorded

their observations on tool use, and noted how much of the task was

completed during their time on the part icular grinding experiment.

Eight different experiments were conducted. Each experiment, andthe damage to the ground stone surfaces, was analyzed. All of theexperimental tools were made from medium-grained quartzi t ic sandstone

collected from Shinarump Formation outcrops in the vicinity of the

Homol' ovi Ruins in northern Arizona. An unused sample remains as acontrol stone, and each hands tone retains an area that is unmodified.

I t is ~ x t r e m e l y useful to have a microscopic view of unaltered grainswhen formalizing concepts of use-wear patterns.

The experiments began with the shaping of the tools. As neededthe tools were pecked with a pecking stone, flaked with a hammerstone,or ground with another piece of quartzi t ic sandstone. I t took one and

a half hours to shape a mano that was comfortable to use, and just aslong to shape the hide processing stone (Table 1). The handstones

used to grind sherds and clay and the abrader used to shape shellbeads each took fifteen minutes to shape. The handstone for grinding

sunflower seeds and the f la t abrader used for shaping a wooden digging



Table 1. Summary of grinding use-wear experiments.

Tool

Corn Grinding

Mano

Sunflower SeedGrinding Mano

Clay Grinding

Mano

Sherd Grinding

Mano

Abrader - wood

Abrader - bone

Abrader - shell

Hide Processing

Stone

Manufacture

Time

1 1/2 hrs.

0

15 min.

15 min.

0

1/4 hrs.

15 min.

1 1/2 hrs.

Use Time End Product

7 1/2 hrs. • ca. 5 cups meal

2 1/4 hrs. 1 1/2 cups meal

2 hrs. 4 cups

2 hrs. 2 cups

2 hrs. digging st ick

2 1/2 hrs. 2 awls

2 hrs. 25 beads

6 hrs. 1/2 deer hide

* This figure does not ref lect an eff ic ient grinding rate . More thanone experimenter worked a t each task and some were able to grindfaster than others. The most eff ic ient experimenter ground one cup of

corn in 15 minutes. Experience might improve the grinding rate but Idon't believe i t would effect the contacting surfaces or the use-wear

patterns.

stick were unmodified. The abrader used to shape bone was a slightlymore complex tool in that i t was grooved. However, i t took only onehour to peck and grind a groove into the surface, and another 15minutes to smooth the sides of the stone.

As each experiment was conducted, the amount of time used and theobservations of the user were recorded. After the grinding tasks werecompleted, the surfaces were cleaned by soaking in water; none werescrubbed. The surfaces of the experimental tools were then described

and compared. Macroscopic descript ions considered the generaltopography of the surface before and after use. Magnification powersof 40X and aOX were used to i den t i fy the a l t e r a t i on s to themicrotopography.

Low power magnification allows the relat ionship of the grains to

the i n t e r s t i ces to be observed. Generally, use-wear patterndifferences are describable by how much of the grain 's surface has

been involved in the wear process. For example, some wear patternsinvolve only the tops of the grains while other wear patterns extend

down the gra in ' s sides and into the in te rs t i ces . Higher power

magnification focuses in on either the surface of the grain or an



in te r s t ice . I f t races of wear from two di f fe rent tasks appear

ident ical a t low power, then higher power may be the only way todifferentiate the two (examples of such situations will be presented

la ter ) .

EXPERIMENTAL RESULTS

Two types of contact si tuat ions can be described on the basis ofthe ground stone experiments. Four experiments involved the use ofone stone against another (termed stone-against-stone contact) with an

intermediary material interacting with the use surfaces. The otherfour experiments used stone against another type of surface, such aswood, bone, she l l , and hide . In the l a t t e r four experiments,intermediary substances, although accidental, could s t i l l play a role

in the wear process. This analysis considers use-wear traces only onthe hands tone and does not describe use-wear on the other contact

surfaces.

Stone-Against-Stone Contact

Corn Grinding

The technique used for grinding the corn was s imilar to one

described by Bar t l e t t (1933:13), where the surface of the manoremained f la t against the metate and was not rocked (Figure 1). Witha new mano and metate dried feed corn was processed fairly rapidly,taking only 15 or 20 minutes to grind one cup of corn. After 3 hours,

the mano and metate became dull and i t took almost half an hour to

grind a cup ofcorn.

The mano wasused

forseven

and a halfhours

with traces of wear visible macroscopically after less than one hour.

Figure 1. Experimental mane and metate grinding dried feed corn.



Prior to use, the topography of the contact surface of the manowas variable. Contact with the metate was most direct on the higher

elevations, and this is where the f i r s t signs of wear were visible asstr iat ions from abrasive wear. As grinding proceeded the topography

became more uniformly leveled. Frictional energy could be fe l t as

warmth that quickly dissipated when grinding stopped. The edge of themano nearest the grinder received the most downward pressure and the

grains were worn level with the in terst ices. Across these leveled

grains there was a lustrous sheen. While being ground, corn meal wasan intermediary substance that became involved in the wear process.The fine meal coated the gra ins , f i l led up the in te rs t ices , and

interfered with the stone-against-stone contact.

Under 40 and 80 power magnificat ion, ef fec ts of the wear

processes were visible on individual grains (Figure 2). The tops ofthe grains had a frosted appearance created by scratches and tiny

chips. Abrasive wear caused scratches, and surface fatigue caused thechips by crushing the grain tops unti l they could withstand the load

of the two contacting stone surfaces ( J . Adams 1988). None of thisdamage extended down the sides of the grains. In some areas entire

grains were loosened by surface fatigue and had been plucked out byadhesive wear. The damage from abrasive wear and surface fatigue did

not extend into the remaining holes.

Figure 2. Surface of an experimental corn grinding mano magnified

40X. The l ighter areas are the frosted grain tops.

Along the edge of the mano the leveled grains provided a broadersurface upon which t r ibochemical wear could build up react ionproducts. These reaction products were created by the interaction ofthe o il from the corn, fr ict ional energy, and the chemical composition

of the stone. Under magnification this surface did not appear so

shiny. Abrasive scratches extended uninterrupted across the surfacesof several grains.



Sunflower Seed Grinding

Raw, unsalted sunflower seeds were crushed and ground between twostone surfaces. After the f i r s t hal f hour, grinding became very

diff icul t . The fine sunflower seed meal had f i l led up the interst ices

and coated the grains, making the surface too smooth to do anymoregrinding. A soft paintbrush was used to clean the seed meal off the

surface so grinding could resume.

After two hours and 15 minutes the surface of the handstone wascleaned and examined. Unmagnified the topography of the used surfacedid not seem to be altered except for a few small patches where the

grains and interst ices are level . The most striking characterist icwas the sheen that covered the contact surface. The shininess of the

surface is reminiscent of a hide processing stone.

Magnifications at 40X and 80x of the microtopography revealed a

wear pattern that was remarkably dist inct from that of corn processing(Figure 3). Instead of having a f rosted appearance, the grainsmaintained more angularity l ike an unused area. Where the meal waspushed away or cleaned o ff allowing a t rue s tone-against - s tonecontact, abrasive wear and surface fatigue occurred. This was visiblein the small patches where the grains and interst ices were level .However, the sheen tha t was vis ible macroscopically remainedreflective and unmarred by abrasive scratches. The grains in higherelevations were encompassed by the sheen, while the grains in lower

elevat ions were shiny only on the tops. Oil may be an importantelement in the involvement of tribochemical wear. Sunflower seeds areoi l ier than corn, and the buildup of lustrous reaction products is

visibly greater on the hands tone used to grind sunflower seeds than onthe one used to grind corn. This suggestion wil l be consideredfurther as use-wear patterns are described for hands tones used to

process other materials.

Figure 3. Surface of an experimental hands tone used to grind

sunflower seeds (magnification 40X). Note sheen on tops of grains.



Pottery Clay Grinding

Pottery clay was collected from sources in the Moenkopi Formation

near Winslow, Arizona. Hard chunks of this clay were ground to a finepowder for two hours. The clay powder was similar to corn meal in the

way that i t coated the grains and fi l led the interst ices. The surfaceof the hands tone was also similar to the mano with wear along the edgenearest th e grinder result ing in the leveling of th e grains and the

i n t e r s t i c e s . In t h i s area abras ive sc ra t ches could be seenmacroscopically. Unlike the contact surface of the mano, however,the re was no lus t rous sheen across the leve led gra in s . This

strengthens the suggestion made ear l ier that the o il in the corn andsunflower seeds is contr ibut ing to the t r ibochemical buildup ofreact ion products . The clay, which contains no oi l s , could notcontribute in the same way to a chemical interact ion.

Under 40 and 80 power magnification it was diff icult to define

exac t ly what the c lay cont r ibu ted to the wear process . The

indicat ions of abrasive wear and surface fatigue (e .g . , frostedappearance, scratches, etc.) were not any more intense that they wereon the corn grinding mano. The wear was only on the tops of thegrains and did not extend down the sides or into the interst ices as

was the case for the corn grinding mano. Thus, the most distinctiveuse-wear pattern produced by grinding clay i s an absence of obvious

tribochemical wear caused by the buildup of reaction products andresidues.

Sherd Grinding

Sherd fragments were quickly reduced to medium- and fine-grained

temper. Indeed it was hard to keep from grinding them into dust.After use, the surface topography of the hands tone was uniformly levelbut individual grains remained elevated above the interst ices. Theleveling of the topography was accomplished by the removal of entiregrains. This may have been a resul t of a weaker bonding in the matrix

of the stone but there seemed to be no difference between this stone

and the bonding of the other experimental handstones. I t is possiblethat abrasive wear and surface fatigue loosened the bonding so thatthrough adhesive contact between the sherds and the stone ent i re

grains were plucked out.

Under 40 and 80 power magnif icat ion the individual grainsappeared to have been damaged by abrasive wear and surface fatigueleaving the characteristic frosted grain tops (Figure 4). As with the

handstone used to grind clay it is diff icul t to determine how the

sherds are involved in the wear process • At this point, i t is not

possi'ble to differentiate the use-wear produced by grinding clay fromthat of grinding sherds. Yet, the two are different from the use-wear

of grinding corn and sunflower seeds by the lack of a tribochemical

sheen. Analysis at higher power magnification might be able to focus

in on more minute damage patterns that could help distinguish between

clay and sherd grinding.



Figure 4. Surface of an experimental hands tone used to grind sherds

(magnification 40X). Note the holes where the grains were removed.

Other Materials Against Stone Contacts

Wood-Against-Stone

A greasewood branch was shaped into a digging stick by using oneedge and one f la t surface of an abrader. I t took two hours to removethe bark and shape a f la t blade on one end of the st ick. The stone

was used in two different ways and resulted in two types of use-wear

patterns. A broad edge of the abrader was pressed against the stick to

scrape away the bark. A f l a t surface was used to sand away any

remaining bark and to shape a broad blade (Figure 5). Scrapings of

wood and bark clung to the grains and f i l led the in terst ices on both

the edge and the surface. A broad area along the edge was damagedfrom use and was very rounded, but only th e highest elevations on the

surface were worn. This damage difference is directly attr ibutable tothe greater amount of pressure applied against a small area along theedge as opposed to the more dispersed pressure against the surface.

Under 40 and 80 power magnification, the individual grains on the

flat surface had a more rounded appearance than those in unused areas.

This rounding extended down the sides of the grains only a l i t t l e .

Rounding could be a resul t of adhesive bonding pulling off minute

particles from the grains ins tead of the grains being chipped andcrushed by abrasive wear and surface fatigue (Adams 1986, 1988). In

contrast , the grains on the edge of the abrader were more heavily

damaged by abrasive wear and surface fatigue. The use-wear patterns

from making a digging stick are much different than those of any ofthe stone-against-stone contact si tuat ions. The softer wood involved

more of the g r a i n s ' sur face in the wear process , yet did notcompletely envelop the grain nor did i t extend into the inters t ices .



Figure 5. Experimental f la t abrader used to shape a digging s t ick .

Bone-Against-Stone

Two bone awls were made by spli t t ing the shafts of fresh sheepmetapodials. The bones had been scraped clean but were s t i l l greasy

when shaping f i r s t started. Several quick strokes against the abrader

removed the greasy surface down to dry bone. It took less than onehour to make a good sharp point on each awl using the grooved abrader,but grinding continued for two and a half hours (Figure 6). Bone dust

f i l led the i n t e r s t i ces but in i t i a l ly th is was easi ly removed byblowing. By the end of the two and a half hours, however, there wasan accumulation of bone residue that had to be soaked out. The boneawl was not always drawn the entire length of the groove but insteadworked back and forth in the third of the groove farthest away from

the experimenter. This process resulted in an enlargement of thegroove at the distal edge.

Under 40 and 80 power magnifications, grains in the middle of the

groove appeared more angular, similar to those in an unused portion ofthe stone. Some grains near the distal edge of the stone were worn byabras i wear and surface fatigue resul t ing in t iny chips and

scratches.. Most, however, were rounded on the tops and part way downthe s ides , s imi lar to damage produced by adhesive wear in s toneagainst-wood contact s i tua t ions . I t may require higher power

magnification to distinguish more minute pattern differences betweenwood wear and bone wear. The use-wear patterns created by making the

bone awls are, nonetheless, quite dis t inct from any of those createdby stone-against-stone contact. The most distinguishing attr ibutesare the amount of grain surface involved in the wear, and the apparentdominance of adhesive wear over surface fatigue or abrasive wear.



Figure 6. Experimental grooved abrader used to shape a bone awl.

Nowhere was there any sheen that would indica te a buildup oftribochemical reaction products. This furthers the suggestion thato i l i s creat ing reaction products . Two poss ib i l i t i e s need to be

considered: 1) the greasiness of only two bones was not suffic ient tobui ld up enough r eac t ion product s to be seen a t lower powermagnification; or 2) animal fats interact differently than vegetal

fats in the tribochemical process.

Hide-Against-Stone

A deer hide was rubbed for s ix hours with a handstone across the

hairless side. No meat remained, but there was a layer of greasy

connective t i ssue that was worked off the hide to produce a sof t

supple texture (Adams 1986, 1988). On the hands tone the topography of

the contacting surface was unaltered, yet overall i t appeared smoother

than the unused surface. The smoothness was a resul t of the softerhide pushing up, around the individual grains, into the interst ices.The angularity of the grains had been smoothed and the interst iceswere free of debris.

Magnifications of 40 and 80 power clearly showed that each grain

had been rounded on top and completely down the s ides . Even theinterst ices had been smoothed. An occasional str ia t ion did indicate,however, tha t there was some abrasive wear, probably by a grainloosened through surface fatigue, or by a particle introduced from the

environment in which the hide i s worked. Nothing in the make-up of

the hide is hard enough to cause abrasive wear, and the surface of thestone has an occasional pit where a grain has been plucked out (Adams1986, 1988).

Another d i s t i nc t i ve use-wear pa t t e rn was the sheen t ha t

surrounded the individual grains and extended into the interst ices



(Figure 7). Undoubtedly th is was the resu l t of the buildup ofreaction products and residues created by tribochemical interactions.

Figure 7 . Surface of an exper imenta l hide process ing s tone(magnification 40X). Focus i s on an interst ice showing the extent of

the sheen.

Shell-Against-Stone

The ends of over 25 Olivella shel ls were rubbed against a f la t

abrader. After about 10 minutes, grinding exposed the natural cavityi ns ide the sh e l l , producing a bead s imi l a r to those found in

prehistoric contexts. Grinding continued for two hours. Most of thegrinding was done in the center of the f la t surface. A fine dust

fi l led the interst ices turning the surface white, but it did not clingto the grains nor interfere with grinding.

Grinding the shel l a l tered the topography of the stone byremoving grains. Under 40 and 80 power magnification the remaining

holes were visible where grains had been plucked out (Figure 8). Thehard shell , as i t rubbed across the surface, created enough surface

fatigue to loosen the bonds between the grains and the matrix. Thenadhesion between the shel l and the grains worked the grains out. Thegrains that remained were frosted by abrasive wear. The interst icesappeared to have been uninvolved in any contact with the shel l . Noneof the grains are worn f la t and there was no sheen from tr ibologicalreaction products. This wear pattern i s most similar to that of the

hands tones used to grind clay and sherd t e ~ p e r . Differences in these

wear pat terns would become more obvious, however, as the stoneagainst-stone contact of the clay and sherd hands tones continued tolevel the grains even with the in ters t ices .



Figure 8. Surface of an experimental f la t abrader used to shape shellbeads (magnification 40X). Note the holes where grains were removed.

SUMMARY OF EXPERIMENTAL RESULTS

The eight experiments described above can be best summarized by

consideration of the contact si tuat ions. The contact si tuations thatproduced th e most dist inctive use-wear patterns were stone-againststone and hide-against-stone, a resul t of the relat ive hardness (or

softness) of one of the contact surfaces. When two hard surfaces comeinto contact, abrasive wear and surface fatigue are the most dominantwear processes. When a hard and a sof t surface come into contact,tribochemical and adhesive wear processes are the most dominant. Whena hard surface contacts a rigid surface ( i . e . , bone and wood) then the

dist inctions are less clear cut. The generalizations about stone

against-stone contact have to be tempered by whatever involvement the

intermediary substance has in · the wear process. Corn, sunflower

seeds, pottery clay,and

sherds eachhave

unique physicaland

chemical

~ r o p e r t i e s that contribute to wear in different ways. The analysis ofthe experimental tools had varying success with identifying thedifferences.

The most sal ient features of the use-wear patterns seem to be the

type of destruction to the surface of the grains and th e extent to

which the sides of the grains and the interst ices are involved in thewear process. Where two contacting surfaces of equal hardness comeinto contact, abrasive wear will occur only where a harder asperity of

one surface can penetrate into a softer area of the other surfacecausing the plastic flow of the softer material around th e harder

asperity (Czichos 1978: 112) • As weight is brought to bear on thesurfaces, the asperi t ies will crush unti l they can withstand the load.This surface fatigue mechanism and the abrasive wear mechanism leave

minute chips and scratches that give th e surfaces of the grains afrosted appearance. Intermediary substances interfere with these



processes by providing cushioning and lubr ica t ion , but they alsobecome involved in the wear process through the adhesive andtribochemical wear processes which cause the buildup of residues andreaction products. These shiny residues and reaction products can bebroken down, however, by abrasive wear and the process begins again on

a fresh surface. When the abrasive wear and surface fatigue processhave worn the grains level with the in te rs t i ces , there i s less

opportuni ty for these two mechanisms to work and t r ibochemicalreaction products are allowed to build up without being abraded away.

With contact situations that involve one softer surface (e .g. ,bone, wood, hide), abrasive wear can be caused by grains that are

knocked loose from the hands tone through surface fatigue and adhesive

wear. Indeed, several of the experimental handstones have empty

pockets where the grains had been plucked out. Analysis of chipped

stone tool use-wear recognize a similar process, termed "autoabrasion"

(Kamminga 1979:154).

When the harder, more aspirate surface of the stone comes into

contact with the softer surface, the softer surface will be displacedaround the asperi t ies and into the interst ices. The grains that comeinto contact with bone and wood had some use-wear damage part way downthe sides but not in the interst ices. This is contrasted with the

stone-against-stone contact where only the tops of the grains werefrosted, and with the hide-against-stone contact where the entiregrain as well as the interstices were smoothed.

I t i s in te res t ing to compare the use-wear pat terns of hide

processing stones with those produced by corn and sunflower seedgrinding, and contrast i t with bone wear. The tribochemical reactionproducts on the hide processing stone were as lustrous as those on the

handstone used to grind sunflower seeds. The main difference was in

the distribution of the sheen over the grains. On the hide processing

stone the sheen extended into the interstices; on the sunflower seed

hands tone i t did not. Such a distribution can not be compared with

the corn grinding mano because the reaction products built-up only onthe gra ins t ha t were leve led to the po in t t ha t the re are no

in te rs t i ces . In contrast bone-against-s tone contact produced no"visible" reaction products, even though the bone was greasy when

shaping began. I t seems most l ike lytha t

t r ibochemical wear wascreating reaction products, there just were not enough of them to beseen at 40 or 80 power. Tribochemical wear accounts for more rapiduse-wear than any other wear process, and is always in operation aslong as there is fr ictional energy available to enhance the chemical

interactions between the contacting surfaces and any intermediary

substances (Czichos 1978:123).

IMPLICATIONS AND DIRECTIONS FOR FUTURE RESEARCH

Much work s t i l l needs to be done before use-wear analysis can

confidently be extended to ground stone tools from preh is tor ic

contexts. More experimentation should allow us to determine i f boneand wood-working surfaces produce dist inct ive pat terns of use-wear.

They are both firm yet resi l ient surfaces that may require longer

per iods of use before di f fe rences become apparen t . The hideprocessing stone, the corn grinding mano, and the hands tone used to

grind sunflower seeds each have very dist inct ive use-wear patterns;



however, further experimentation must be conducted to determine i f

these patterns are specific only to these contact situations.

Although the ultimate goal is to determine whether the identifieduse-wear patterns are unique to each type of contact situation, thisseries of preliminary experiments has raised many questions. Theseinclude questions associated with dif fe ren t material types, seedprocessing situations, and other cultural and natural processes thatmight complicate recognition of use-wear on ground stone tools.

The next step is to build several models that predict the type ofdamage that would be expected in a particular contact situation usingtr ibological concepts that seem to have explanatory power. Higher

power magnification should be used to try to identify tribochemical

and adhesive mechanisms that might not be visible at lower power. I t

might also be productive to begin experimenting with residue analysis

and the conditions under which residues are destroyed.

Although many questions remain, progress has been made with these

in i t ia l experiments. Use-wear analysis can strengthen and diversifythe interpretations of the activit ies in which ground stone tools wereinvolved. This new interpretive power can then be incorporated into astronger reconstruction of activit ies at the s i te level. Additional

evidence for how a r t i fac t s were used can help identify behaviorpatterns concerning with food and nonfood processing.

This study has shown t ha t exper imenta t ion and low-powermagnification are successful methods for identifying some general use

wearpatterns.

These methodsshould

beapplied to archaeological

assemblages through research tailored specifically to each assemblage.The concepts of t r ibology allow us to predic t under what contactsituations surface damage might occur, and this alone increases our

understanding of ground stone tools.

ACKNOWLEDGMENTS

I would f i rs t like to thank Raymond Mauldin and Daniel Amick for

inviting me to write this paper and for editing an earl ier version.There have been so many people involved in helping make this paper

possible and i t is impossible to name each one. As a group, I thank

the Earthwatch volunteers who enthusiastically helped make and use theexperimental tools. Lee Fratt helped design, set up, and monitor theexperiments. Lisa Young and Jim Vint made pecking stones and workedon some of the experiments. Cathy Cameron and Mike Schiffer read andmade helpful comments on an earl ier version of this paper. Mike alsomade available the microscopic equipment of the Laboratory ofTradi t ional Technology and is a constant source of encouragement.

Finally I thank my husband Chuck Adams for reading manuscripts,grinding corn, and generally being supportive.

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Methods for Improving Groundstone Analysis Jenny Adams X