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Laboratory evidence of the use of metal tools at Machu Picchu (Peru) and environs
Journal of Archaeological
Science 1985,12,3 1 l-327
Laboratory Evidence of the Use of Metal Tools at. Machu Picchu (Peru) and Environs Robert B. Gordon0 An interpretation of use-wear marks on metal artifacts is developed from the principles of metal cutting and brittle fracture and applied to surficial markings and microstructural damage on bronze tools from Machu Picchu and environs. Most of the tools have blunt edges, relatively low tin contents, and were not work hardened before use; they appear to have been designed for work that involved breaking chips from hard, brittle material. Use-wear marks on these tools are interpreted as due to sliding contacts and impacts with rock. One tool with a relatively sharp edge has a higher alloy content than those with blunt edges and has been work hardened; it appears to have been designed for cutting wood and use-wear markings suggestit was so used. A long bronze bar carries markings that suggest use by stonemasons. Many of the tools are broken and study of their microstructures shows that the bronze used has poor mechanical properties because of porosity and bands of sulphide inclusions. PERU, MACHU PICCHU, TIN-BRONZE, METALLURGY, USE-WEAR MARKS.
Keywords:
TOOLS,
Introduction The use of metals for decorative and symbolic purposes was an important stimulus for the development of metallurgical skills in the pre-Columbian culture of the Andes region but metals also served a variety of utilitarian functions there (Letchman, 1980, 1984). The 150 metal objects collected at Machu Picchu and environs by Hiram Bingham’s expedition of 1912 (Bingham, 1930) are one of the largest groups of artifacts of known provenance from pre-Columbian Peru and are a sample of the metal goods in use immediately before the arrival of Europeans. Some of the artifacts in the collection are described in papers by Foote & Buell(l912) and by Mathewson (1915), whose study was one of the first to use metallographic methods for the examination of archaeological materials. Thirteen of the artifacts seem to be intended for heavy work, these 13 items accounting for two-thirds of the weight of bronze in the collection. There is not yet agreement on the role of Machu Picchu in the Inca empire so it is not known if these artifacts are representative of the use of metals throughout the central Andes region, but, if they are workman’s tools, it is less likely that they would be unrepresentative than if they were ceremonial or military items. I report here the results of an examination of these artifacts undertaken to determine how they were used. The evidence obtained includes the size and shape of the objects, the composition and properties of the alloys of which they are made, and the wear and use marks visible on their surfaces. “Kline GeologyLaboratory, YaleUniversity,New Haven,Connecticut06511, U.S.A. 311 0305-4403/85/040311+17rs03.00/0
0 1985AcademicPressInc. (London) Limited
R. B.
312
GORDON
Table I Objects
examined
Identification Artifact
Register*
Other?
Weight
1 2
17898 17900
F&B F&B2
1
3 4
17902 17967
CHM
15
5 6 7 8 9 10
17969 17974 17899 17907 17964 17966
CHM CHM
8 10
CHM CHM
6 16
242 62 560 85 224 100
11 12 13
17975 17908 18478/9
CHM
20
65 345 1070
410 525 725 414
*Register number, tF& B=Foote& IRutledge (1984).
(g)
Where
found$
Espiritu Pampa on the Conservidayoc River. Near Rosahna in Urubamba River Valley, 90 km from Machu Picchu. No record. Machu Picchu, foot of north wall of Room at 24A at depth of 0.3 m. Near Snake Rock in the centre of Machu Picchu. Room 34A at Machu Picchu. No record. No record. Eastern terraces of Machu Picchu. Section 40A of Machu Picchu, a “kitchen midden”, 0.3 m deep. No record. No record. may not be from Peru. Reported from Cave 52 and from Section 44A of Machu Picchu. Peabody Museum of Natural Buell(l912); CHM=Mathewson
History, Yale University. (1915).
Figure 1. Outline drawing of the artifacts studied (heavy lines) showing the locations of metallographic samples (dotted lines) and the reconstructed shape where part of an artifact is missing (light lines). The dashed line at 12 shows the shape of a flat axe from Britain for comparison.
Methods Used and Artifacts Examined
The artifacts examined are listed in Table 1 and all but one (a long bar) are illustrated in Figure 1. They include all of the metal artifacts that appear to be workman’s tools among the items brought back from Peru by Hiram Bingham’s expedition and now in the collections of the Peabody Museum of Natural History, except for one item that was not available for study. The collection of the artifacts in Peru was done by a small scientific party in a short period of time under conditions that precluded recording and numbering each object in the field. The manuscript field notes of the expedition members
USE OF METAL
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313
have been searched by Rutledge (1984) to match descriptions with items in the collcction; additional information was obtained from Eaton’s (1916) account of the osteological finds. Room and location numbers at Machu Picchu used here are those shown in the manuscript map now on the Peabody Museum. Although locally pitted by corrosion, the artifacts were in good condition when found. Previous investigators cleaned (and in some cases, machined) parts of the surfaces of some of the artifacts with procedures that were not recorded. This surface alteration is localized and a thin layer of soil was found to be still in place on most of the metal surfaces. Only surficial markings that emerged from under a cover of soil after gentle washing are reported here. The metallographic sections originally made by Mathewson were repolished and etched; new samples were prepared for the artifacts which had not been previously sampled. Microhardness was measured on the metallographic specimens with a diamond pyramid indenter and a load of 0.3 kg. The profiles of the edges were measured on the metallographic specimens or, where no section was available, with the aid of a telescope and micrometer microscope stage. New determinations of chemical composition were made with an electron microprobe attached to a scanning electron microscope. A series of copper-tin alloys was prepared to serve as composition and hardness standards. Technical terms used to describe the mechanical and metallurgical characteristics of the tolls are defined in the appendix. Sources of Evidence on the Use of Metal Tools Methods are available for inferring the previous use made of stone tools (Hayden, 1979) but no corresponding set of techniques applicable to metal tools has yet been advanced. The sources of evidence used here are described below. Toolform The possible shapes that a hand-held tool intended for a particular task, or range of tasks, can assume without becoming impossibly clumsy is limited. Unless artificers have no control over the design of the tools supplied to them, it is expected that a tool form best adapted to the requirements of a particular task, set of materials, and place will be developed. When what appears to be a useful form of tool is recognized, it is not proof that the tool was actually used for the purpose envisaged, but if a tool appears to be ill-suited to a particular use, it is likely that it was not used for this task. Distortion and breakage Use of a metal tool in any kind of hard mechanical work will usually result in distortion of some part of the tool. Distortion may be detected from external evidence, as when what was intended to be a cutting edge is blunted, bent or broken. Deformation markings in the microstructure that are unrelated to any process used in the manufacture of an artifact are internal evidence of distortion. If an object is found to be broken where it would be weakest in performing a certain function, it suggests that it was used for that function. Wear, attrition, or damage caused by the use of a tool may be corrected by reforming or resharpening; evidence of repairs may be found in the microstructure or in markings on the surface. Use-wear marks Unless a tool is used very gently or is remarkably hard, evidence of its contact with the material being worked will be left on its surfaces. Metal tools employed in mechanical work will usually be marked with indentations and striations whose form depends on the class of work that has been done. In the case of edge tools, the possible classes of work
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R. B. GORDON
Clearance
:
i
Figure 2. Illustration of three modes of working with edge tools, cutting a chip from a plastic material (right), breaking a chip out of a brittle material through the formation of cracks (centre), and splitting (left). The definitions of the clearance and rake angles of a cutting tool are shown; the chip is being formed by intense local plastic deformation in the shear zone.
are cutting, breaking, or splitting. These three modes of working are illustrated in Figure 2. Cutting is the removal of material from the workpiece as small bits ~ chips ~ formed by a combination of plastic deformation and ductile fracture of the work under the action of the tool. When a tool is used for breaking the chips are formed by brittle fractures rather than plastic deformation. Splitting is the division of the work into approximately equal pieces by the propagation of a crack through it. Each of these modes of working with an edge tool requires a different combination of tool form and material properties. The basic design characteristics of a cutting tool are the clearance and rake angles, which are defined in Figure 2. Cutting can be achieved with a rake angle that is small or even negative but the corner between the tool faces must be sharp. Making the rake angle more negative will, when a critical angle is reached, cause a transition from cutting chips to plowing, the formation of a furrow and ridges without the removal of chips (Samuels, 1982). Successful cutting of compliant materials such as wood or flesh requires a large, positive rake angle so as to avoid excessive elastic deflection of the work near the point of the tool. Stiffer work, such as bone or metal, is best cut with a tool having a smaller (or even slightly negative) rake angle. A tool used for cutting must have a sharp edge so as to localize the plastic shear zone and the edge of the tool must be harder than the work so that the sharpness can be maintained. Chips may be cut from bronze when the rake angle is as small as - 35”. Hence, grooves caused by the removal of chips may be formed on a bronze tool by rubbing it over a rough rock surface. A set of use-wear marks believed to have been so formed is shown in Figure 3. The absence of ridges adjacent to the groove and the long, continuous striations within the grooves show that the chips were being removed from the side of the tool by sliding contact with material hard enough to retain a sharp edge while moving over the bronze. Chips can be formed in a brittle material by creating tensile cracks below the area of contact between the tool and the work (Lawn & Marshall, 1979). In this case the function of the tool is to develop a high stress concentration at the point of contact. The tool need not have a sharp point but it will function better if made of material that is stiffer and tougher than the work; it need not be harder though if it is excessively soft it will become too blunt to be effective after slight use. The tool may be hammered (as with a percussion rock drill) or may be dragged over the surface of the work (as with a rotary rock drill). The essential function of a splitting tool is to wedge apart the two halves of the workpiece. If a slot has been previously cut where the split is to be developed, the tool need
USE OF METAL
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Figure 3. The side of the blade of artifact 5 showing deep striations, ing the fine grooves characteristic of the removal of chip by sliding rough rock surface. Length of scale bar = 1 mm.
many containcontact with a
not have a sharp point. Since splitting is most easily managed with an approximately equal division of the work, a splitting tool is usually made symmetrical about the splitting plane. The included angle chosen depends on the magnitude of the splitting force that is available; if it is small, a small angle will be wanted so as to gain a greater mechanical advantage. The tool need not be harder than the work but must have sufficient strength and stiffness to avoid distortion in use. A stone tool may be used to split wood or bone but use of a tool made of a tough alloy will greatly facilitate splitting rock, particularly when close control over the crack path is wanted. Tool material
Edge tools that will cut wood, bone, and flesh can be made of stone because of its high hardness and the sharp edges that can be prepared on it. Bronze offers little or no advantage in this kind of work unless the tool is to be hammered upon, in which case its superior toughness may be judged by the user to compensate for its inferior hardness. Because it can be cast or forged to convenient forms and because of its stiffness and, if of good quality, toughness, bronze offers a very great advantage over stone in tools intended for breaking and splitting rock, particularly where fine work is to be done. While bronze was a scarce and expensive material we expect that the preferred use for it in tools would be in those applications where it offered the greatest advantage over working with non-metallic tools. Where the capacity to make a range of alloys existed and the alloys chosen for individual tools enhances their capacity to perform their apparent functions, it suggests
316
R. B. GORDON Table 2. Composition
of Artifacts cu
Sn
Fe
&
s
1* 2* ::
876 93.7 95.2 94.3
12.0 5.6 50 4.0
0.08 0.0 0.0 0.3
0.0 0.65 0.4 0.0
0.35 0.08 0.4
51 7t
98.7 94.6 98.7
5.0 5.1 0.6
0.9 0.0 0.0
0.0 0.0 0.0
0.4 0.3 0.7
iI 101 llt 12t 13$
96.3 93.1 93.7 91.5 100.0 94.6
3.7 6.0 5.5 8.3 0.0 5.4
0.0 0.5 0.6 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.4 0.2 0.2 0.0 tr.
Artifact
6:
*Wet chemical analysis reported by Foote & Buell(1912). tElectron microprobe analysis. $Wet chemical analysis reported by Mathewson (1915). All analyses are in weight percent. Arsenic was not detected in any of the microprobe analyses nor reported for any of the wet chemical analyses.
that the tools were designed for those functions. For example, if a tool with an edge suitable for cutting wood were made of hard, high-tin bronze and another with a blunt point suitable for striking blows were made with tougher, low-tin alloy, the match of material properties to apparent tasks suggests that the tools were made to perform those tasks. Use Made of the Peruvian Tools Material and manufacture Alloys. All of the artifacts studied are made of arsenic-free tin bronze and most contain about 5% tin, the composition of bronze commonly found at Machu Picchu (Rutledge, 1984). The analyses, Table 2, show that the principal impurities are iron and sulphur. Annealed, 5% tin bronze of good quality would have a strength of 350 MPa and 55% elongation (Brandes, 1983) but a tensile test of artifact 13 gave an ultimate strength of 192 MPa and an elongation of 6% in a gauge length of 50 mm (Mathewson, 1915). The poor strength and ductility of the bronze in this and most of the other artifacts studied is due to porosity and/or bands of sulphide inclusions (Gordon, 1984). Artifacts 1 and 11 have unusually high tin contents. In artifact 1 the eutectoid structure which would ordinarily be present in a bronze casting of this composition is not found, showing that it must have been annealed at a temperature between about 600 and 800°C. This anneal, by dissolving the brittle delta phase, would have made subsequent forging of the blade easier. The high tin content may be responsible for the relatively good surface finish that has been preserved, since the composition is near the optimum for resistance to corrosion in soil (Tylecote, 1979). Artifact 11 is the only one in which there is large-scale tin segregation and its microhardness ranges from 163 on one side of the blade to 84 in a copper-coloured region on the other side; the eutectoid structure is visible in the harder regions. The tin content of artifact 7 is unusually low and its microstructure consists of recrystallized grains of copper-rich solid solution with a few sulphide inclusions.
USE OF METAL
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317
Manufacture. Evidence of plastic deformation in the microstructures of the artifacts is confined to localized areas; this and the absence of forging marks on most of their surfaces shows that the artifacts were cast nearly to their final forms. Most have bilateral symmetry that could not be attained in open moulds but have no parting lines such as are often found on bronze objects cast in bivalve moulds. Annealing twins are common in the microstructures and may have originated in thermal strains developed during cooling of the castings. Only artifacts 1, 6, 9 and 10 show evidence of having been further shaped by forging after they were cast. The most satisfactory evidence of this is the elongation of the sulphide inclusions, which persists even when the deformed metal has been recrystallized (Gordon, 1984). Mathewson (1915) used a substantial part of artifact 9 for a hotworking experiment in which he duplicated the microstructure found near the edge, thereby showing that the pointed end was formed by hot forging. In artifact 10 the sulphides are elongated for a distance of about 4 mm back from the working edge while in a region extending about 15 mm back from the edge, and in another reaching 30 mm down from the fracture, deformed and undeformed grains are adjacent to one another in a structure similar to, but finer than, that formed in artifact 9 by hot working. These two parts of the blade were probably forged while the object was cooling from its last anneal. The elongation of sulphide inclusions shows that the metal near the edge of artifact 1, which is one of the sharpest found in the collection, has been forged and, since deformation marks and the distorted grains are present in the microstructure, some of the forging must have been done after the last anneal. The final cold forging is responsible for the hardness gradient, shown in Figure 4, which extends about 20 mm back from the edge. Since elongated sulphide particles intersect the surface of the point for a distance of 5 mm back from the edge, at least some of the final shaping of the blade must have been done by the removal of metal. In artifact 6 the sulphide inclusions within 5 mm of the edge are elongated. Deformation markings are present in a zone extending 5 mm inwards from both surfaces, except at the edge, where there are no markings. The longitudinal hardness profile (Figure 4) confirms that the edge has not been work hardened but there is hardening at the sides of the blade associated with the bands of deformation marks. The sides were probably hammered lightly after the last anneal but it is possible that the sides may have been deformed by the process that produced the striations discussed below. Interpretation. Two of the objects have an unusually high tin content, two are nearly pure copper, and the rest contain approximately 5% tin. If a bronze tool.with a sharp edge is to retain its sharpness, it must be made hard by a high tin content or work hardening or both. All three characteristics are found together in artifact 1, which suggests that it was designed for use in cutting wood or other organic matter. No positive evidence of intentional alloying to particular compositions can be adduced for the other objects but it is interesting to note that the 5% tin alloy would, in the absence of porosity and sulphides, have a combintion of toughness and hardness that is appropriate for tools to be used for heavy breaking and splitting work and that work hardening the edges of such tools would be undesirable. Artifact 7, which has only a trace of tin, also has a very blunt edge. Form Shape. All of the objects shown in Figure 1 can be described as consisting of a blade with
a more-or-less sharp edge; some have a cross-bar at the opposite end from the edge. Artifacts l-7 have an approximately common form and size such that any of them will fit easily into one’s hand in an attitude convenient for making chips on a work surface by hammer blows on the top of the cross-bar. The bar then protects the user’s hand against
318
R. B. GORDON
d km)
Figure 4. Hardness gradients measured and 9. The distance from the edge is d. fully annealed copper-tin alloys having data for artifact 2 are taken on the most d; they represent an upper bound to the
along the central axes of artifacts I, 2, 6 The lines marked “a” are the hardnes’s of the same tin content as the artifacts. The heavily deformed grains at the indicated hardness attained along the section.
badly directed blows. Artifacts 8811 are also edge tools and the absence of cross-bars and would be make them suitable for insertion into the workpiece. A long bar, artifact 13, is of a size and shape to be conveniently manipulated as a hand-held tool. None of the artifacts is decorated. The shape of artifact 12 is different from all the others in the collection but is very similar to the form of, for example, flat axes of the British Early Bronze Age; a comparison with a flat axe found near Bristol (Parker, 1982) is made in Figure 1. Because of this, and our inability to find any identification of artifact 12 in the expedition records, it is not discussed further here. Edge projile. The profiles of the working edges of the artifacts, shown in Figure 5, range from quite sharp to blunt. The two badly damaged edges, 2 and 9, appear, on the basis of microstructural evidence, to have been originally blunt as well. The blade profiles of artifacts 4, 5 and 6 are of intermediate sharpness (the slight asymmetry of artifact 4 is part of the original casting). The similarity of these profiles suggest a more-or-less standardized tool design; this is the basis of the reconstruction of the forms of artifacts 5 and 6 shown in Figure 1. Only two artifacts, 1 and 10, are sharp enough to be useful in cutting wood or other organic matter. They are compared with what is now considered to be the optimum point profile of an axe for chopping wood in Figure 6; both are slightly blunter than the optimum and they would only be effective in cutting under blows from a hammer - they are not sharp enough to be useful in carving wood, for example. The artifacts with the blunt points would be useless for cutting but are suitable for breaking or, with the aid of a starting notch, splitting. The other working edges are of intermediate shape; they are too blunt for cutting and could be used for breaking or splitting.
USE OF METAL
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\_---’
Figure
5. Measured
cross-sections
Figure 6. Comparison of the profiles with the optimum shape of a modern
5mm
4
of the edges of the tools.
of the tips of artifacts 1 and 10 (heavy axe for chopping wood (light lines).
lines)
Breakage
All but two of the artifacts have been fractured; four (5, 6, 10 and 11) have been broken bodily with fractures that show very little ductility and four (2, 3, 4 and 13) contain smaller breaks. Incomplete fractures or small cracks are present in artifacts 1, 2, 4, 8, 9, 11 and 13. The fractures are all brittle or nearly brittle, although in the case of artifact 10 there was enough ductility in the metal to permit the object to be twisted through about 12” before it fractured. Sometimes a trace of necking (localized plastic deformation preceding fracture) can be seen, as in artifacts 10 and 11, where the small necks on one side show the presence of tension, from which we infer that breakage was by application of lateral force. The fracture surfaces are rough and the fracture paths usually follow
320
R. B. GORDON
bands of sulphide particles. The high degree of porosity in artifact 13 is the cause of the brittleness of this artifact. While the items fractured bodily might have been broken by their owners to prevent their being used by others, it is more likely that they were damaged in use; the other breakages must have been failures in service. Because of the limited ductility of the bronze, nearly brittle fracture is expected to be the most common type of failure encountered in the use of these artifacts. Microstructural
evidence of use
It would be nearly impossible to use a bronze tool in any sort of heavy work without producing deformation markings in the microstructure; where the damage is more extensive, the resultant work hardening can be detected with microhardness measurements. These features will be preserved unless the tool has been subsequently annealed at high temperature. Most of the artifacts examined contain evidence of deformation that cannot be accounted for by manufacturing procedures. Working edges. Artifacts 2,3,5,9 and 1 I show evidence of deformation near their working edges that is unrelated to the manufacture and in some of them the working edges are severely distorted. In artifact 2 sulphide inclusions near the edge of the blade have not been elongated and the coring in the as-cast microstructure is retained, showing that the tip of the blade has not been forged or subjected to prolonged annealing and that any deformation markings found in the microstructure must be due to damage incurred during use. Heavy plastic deformation is visible in the microstructure to about 3 mm back from the edge; at the edge the grains are elongated perpendicular to the centre plane of the blade and two cracks have developed as the metal at the tip has spread laterally. These features are visible in the section of the edge of the blade shown in Figure 7. There is strong work hardening within a few millimeters of the tip (Figure 4) but back from the edge the hardness (HV 115) is only slightly greater than that of annealed bronze with a comparable amount of tin but no silver (HV 97). The edge has been upset and in some areas turned over and partially detached; one corner of the blade has been broken Off.
The working edge of artifact 9 has been deeply indented, upset, and turned over. The metal near the working edge has split along lines of sulphide particles that were elongated during the hot forging of the pointed end. This has widened the edge, as shown by the dotted line in Figure 5, but the original working edge was almost certainly blunt rather than sharp. The hardness gradient (Figure 4) along the axis of the blade extends well back from the edge that has been damaged by use and so must have resulted from a final cold forging during preparation of the artifact. Since the working edge shows such heavy damage, it is surprising that the surviving part of the top shows no evidence of deformation in its microstructure. The microstructure of artifact 5 taken near the fracture shows residual coring, no annealing twins, and abundant sulphide inclusions. This appears to be the as-cast structure unaltered by subsequent forging or annealing. The metal 5 mm back from the tip along the centre line of the blade is recrystallized but the sulphide inclusions are not elongated and there are few deformation marks. Thus, the blade has neither been forged nor work hardened. But, deformation markings are found in the grains along the sides of the blade and these become more intense as the working edge is approached. The metal in a zone about 0.25 mm thick at the tip is so heavily deformed that the grains are elongated perpendicularly to the blade axis. A structure such as this would result if the edge had been flattened against a hard object and then resharpened. The presence of undistorted sulphide inclusions in artifact 11 shows that it has not been forged but deformation marks are found near the fracture surface, along the
USE OF METAL TOOLS AT MACHU PICCHU
Figure 7. Micrograph of a polished and etched section of the edge of artifact showing the flattening and splitting of the edge and the presence of silver-rich areas (dark) with traces of a second phase. Length of scale bar =0.5 mm.
321
2
adjoining sides, and throughout the metal near the edge. These are interpreted as deformation caused by use of the tool. The edge, which now appears to be undamaged, has been reformed by grinding, as is shown by the striations discussed below. Other surfaces. The microstructures of five of the artifacts (1, 2, 3, 7 and 8) show that they have been deformed on their tops or sides in a way that is unrelated to any manufacturing procedure. The microstructure at the long end of the cross-bar of artifact 1 consists of recrystallized grains with undeformed sulphide particles and much porosity; deformation marks are found adjacent to the surface at the end of the cross-bar. Both the end and the centre part of the top of the cross-bar have been indented by blows from a blunt object harder than bronze (as described in the following section) and these blows caused the deformation markings in the metal near the surface. Deformation markings are also found in the metal near the end of the cross-bars of artifacts 1 and 3 and the indentations on the ends and tops of the bars suggest that they have been used as hammers and have been hammered upon. In artifact 7 there are a few indentations on the top of the cross-bar near its centre and deformation markings are found in a band about 2 mm thick at the end of the cross-bar that was sectioned. This artifact appears to be in nearly the as-cast condition and to have had very little hard use. Although no indentations are visible on the top surface of artifact 8, the metal below the top to a depth of 1.7 mm is heavily deformed. Below this zone of deformation the microstructure consists of undeformed, recrystallized grains, spherical pores, and sulphide inclusions. Summary. Microstructural evidence shows that two of the artifacts, 2 and 9, have been used in such a way as to cause heavy damage to the working edges; it is likely that the
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R. B. GORDON
Figure 8. Indentations bar = 1 mtn.
and striations
in the flat edge of artifacts
3. Length
of scale
same would be found for artifact 3 (as discussed below). Two others, 5 and 11, also appear to have suffered internal deformation from use. Five of the artifacts show microstructural evidence of having been hammered upon or hammered with; the surficial evidence of this use is discussed in the following section. Use- wear marks Three types of use-wear marks have been found on the surfaces of the artifacts. Two types, the indentations found on the tops and on the edges, are associated with internal microstructural damage but the evidence of this association for the third type, the striations found primarily on the side of some of the artifacts, is weak. The tops and ends of the cross-bars of artifacts 1, 2, and 3 are heavily indented, in some places to the extent of turning the edge over. Artifact 7 is lightly indented on its top surface. The presence of the deformation markings in the metal below the surface shows that these indentations were made after the metal was last annealed. These markings have no discernible relation to the fabrication of the artifacts and are interpreted as the marks left by hammer blows. Those on the tops were probably made with hammer stones, such as the one illustrated by Bingham (1930), while the tool was being driven into a workpiece by blows. Those on the ends of the arms could be due to the use of the cross-bar as a hammer. The presence of these markings shows that these objects were not hafted and so should not be called axes. The second type of wear or use marks present are the striations and indentations on, or adjacent to, the working edges. The working edge of artifact 3 has been flattened, indented, and upset, as shown in Figure 8, and some of the indentations on the edge contain striations that suggest sliding motion against a hard material. No microstructure
USE OF METAL
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Figure 9. Side view of the point of artifact at the edge. Length of scale bar = 1 mm.
PICCHU
2 showing
the striated
323
burr turned
over
sample of this edge was available but the external evidence makes it quite likely that deformation markings would be found near the working edge in an etched section. In artifact 2 the edge has been turned over and the upset edge is deeply striated, as shown in Figure 9. The turned-over end of artifact 9 is also deeply grooved with longitudinal striations. The markings on all of these artifacts are interpreted as due to contact with material that is harder than bronze. Since the edges have been deformed, one possible interpretation is that they are due to work done that involved impact with rock, such as breaking out chips in dressing stone. Since there are no use marks on the sides of artifacts 2 and 3, the work was probably done on large, free stone surfaces where insertion of the tool into slots or cracks was not required. Striations on the sides of the artifacts are the third type of use-wear mark found. These are illustrated by the well-preserved grooves along the length of both sides of the upper half of artifact 10 (Figure 10). The edge is undamaged but the marks on the sides show that the blade has been inserted to a depth of at least 70 mm in a crack or slot. Similar grooves are found on the side of artifact 9. The fine striations within the grooves show that they were formed by cutting a chip from the bronze by the motion of hard material over the metal surface. Sliding contact with the rough surface of split granite, for example, would produce such marks because of the relatively large negative rake angle at which chips can be cut from bronze. These striations could arise in a number of ways; two possibilities are sharpening or dressing the bronze tool on stone or the insertion of the tool into a slot in the stone, as would be done in splitting a stone block. The presence of deep grooves on the sides of artifacts 10 and 11 suggests that they were deeply inserted into a slot. With the tool so placed, a badly directed hammer blow could easily break the bronze, which has poor ductility because of its high sulphide content and porosity, so
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R. B. GORDON
Figure 10. Deep striations on the side of artifact IO formed chips. These were probably cut in the bronze by sliding contact face of hard rock. Length of scale bar = 1 mm.
by the removal of with a rough sur-
causing the observed fractures. The striations found at the edges of artifacts 5,6 and 11 are interpreted as marks left from sharpening or dressing rather than use. For example, there is no evidence of use of artifact 5 on the outside of the tip of the blade but the adjoining sides are covered with striations that are generally parallel to the edge and frequently cross one another, as shown in Figure 3. In artifact 11 they are associated with the bevel, which was probably put on the edge by sharpening against a stone, and in artifacts 5 and 6, with the removal of metal in forming the profile of the point. Artifact 13, a long, tapered bar, has not been sharpened, subjected to impacts, or used for splitting, but it has short grooves cut in the bronze by sliding contact with stone. Such marks would arise from levering stone blocks about in the construction work. Discussion
I interpret the accumulated evidence above as showing that most, if not all, of the artifacts described were designed as workman’s tools, that most have been used for their intended purpose, and that several of them have had hard use. Only one artifact, 7, would be of little use in utilitarian tasks; it is too ill-formed, soft, and blunt to accomplish much with. One of the tools, artifact 1, seems to have been designed and used for cutting wood or other organic material with heavy hammer blows while hand held. It has a unique combination of characteristics - a sharp point hardened by alloying with 12% tin and cold work, a form massive enough to sustain hammer blows, and a size and shape convenient to hold in one’s hand. It could be used either for splitting or for removing chips from the workpiece.
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At least three of the tools, artifacts 2, 3 and 9, were made with blunt edges and of an alloy having a low tin content which would be tough in good quality castings. They appear to have been intended for work such as breaking chips out of stone (as illustrated in the centre diagram of Figure 2) and they carry evidence of having been used in work that involved impact with stone. Artifacts 10 and 11 appear to have been in sliding contact with rough, hard surfaces, such as would occur in splitting stone while artifact 8 seems to have been made for splitting but not much used. The edges of artifacts 4,5 and 6 are not sufficiently sharp to be useful in cutting wood but would deform more easily than the blunt edges of artifacts 2, 3 and 9 in uses which involve impact with stone. The heavy deformation of the metal at the edge of artifact 5 shows that it has had hard use although the damage which we infer from the deformation markings to have been present has been repaired. These tools may have been intended for a finer class of work on stone than could be done conveniently with the blunt edges found in artifacts 2, 3 and 9. It seems likely that artifacts 5 and 6 received substantial blade damage in use, were reformed by stoning (leaving the sharpening marks now visible on their sides) and were then found to be cracked and unfit for further work. Tool 4 was not used, perhaps because a corner of the blade broke off when it was first placed in service. Because of the deficient ductility of the bronze used in the tools found at Machu Picchu, breakage in service must have been frequent. It is likely that the broken tools were remelted and used to cast new tools, which would, of course, reduce the number of broken tools to be found at the site. The actual work done with the tools that have been in contact with stone cannot be deduced from the evidence that they carry. It could have been the preparation of ashlars or other large stone blocks having recesses or sharp internal corners, work in which long, thin tools that could only be made of metal are needed to reach the working surfaces. These tools would also be useful in cutting decorations or figures on stone blocks but they are too large to be of much service in making small stone items, such as might be used for personal adornment. Field studies are needed now to identify the nature of the tasks that were being performed with metal tools at Machu Picchu and environs. Most of the tools examined thus appear to have been designed, manufactured, and used for work on stone and one seems to have been specifically designed for woodworking. The advantages gained by the use of bronze rather than stone for edge tools derive primarily from the superior toughness of bronze and the possibility of shaping the metal tools to slimmer or more complicted forms than can be made in stone. Some work previously done with stone tools could be done more efficiently with bronze ones, as where a slim wedge is required for splitting an object. A bronze wedge could have sufficient toughness to be hammered upon and sufficient stiffness to propagate a crack in rock as well as wood or other organic matter. New tasks, which could not be accomplished with tools made of stone, could be undertaken when bronze tools became available. These would include work that requires the use of relatively delicate, slim edge tools, such as making deep, rectangular holes or re-enterant angles. The laboratory results show that a substantial amount of metal was being used for utilitarian purposes in and around Machu Picchu in immediately pre-Columbian times. Acknowledgements
I thank J. L. Hollowell for information about stone-working technology in preColumbian Peru, J. W. Rutledge for detailed descriptions of the metal artifacts from Machu Picchu in the Peabody Museum, Alan Pooley for assistance with the microprobe analysis, and Heather Lechtman for a critical reading of the first draft of this paper.
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References Bingham, H. (1930). Machu Picchu, Citadel ofthe Incus, chapter 7. New Haven: Yale University Press. Brandes, E. A. (Ed.) (1983). Smithells Metals Reference Book, pp. 22-26. London: Butterworths. Eaton, G. F. (1916). The collection of osteological materials from Machu Picchu. Memoirs of the Connecticut Academy of Arts and Sciences5,1-96. Foot, H. W. & Buell, W. H. (1912). The composition, structure and hardness of some Peruvian bronze axes.American Journal of Science34, 128-132. Gordon, R. B. (1984). Metallurgy of bronze tools from the Machu Picchu. Proceedingsof the 1984 Archaeometry Symposium. Washington: Smithsonian Institution. Hayden, Brian (Ed.) (1979). Lithic Use-Wear Analysis. New York: Academic Press. Lawn, B. R. & Marshall, D. B. (1979). Mechanisms of microcontact fracture in brittle solids. In (B. Hayden, Ed.) Lithic Use-Wear Analysis, p. 63. New York: Academic Press. Letchman, H. (1980). The Central Andes: Metallurgy without iron. In (T. A. Wertime & J. D. Muhly, Eds) The Coming of the Age of Iron, p. 322. New Haven: Yale University Press. Lechtman, H. (1984). Andean value systemsin the development of prehistoric metallurgy. Technology and Culture 25, l-36. Mathewson, C. H. (1915). A metallographic description of some ancient Peruvian bronzes from Machu Picchu. American Journal of Science40,525-6 16. Parker, G. (1982). Metallurgical notes on three Bronze Age implements found in the West of England. Journal of the Historical Metallurgy Society 16,4&49. Rutledge, J. W. (1984). The Metal Artifactsfrom the Yale Peruvian Expedition of 1912, Catalog and Commentary. M.Sc. Thesis, Yale University. Samuels, L. E. (1982). Metallographic Polishing By Mechanical Methods, p. 36. Metals Park (Ohio): American Society for Metals. Tylecote, R. F. (1979). The effect of soil conditions on the long term corrosion of buried tin bronze and copper. Journal of Archaeological Science64,345-3X
Appendix Definitions of Technical Terms Brittle fracture is breakage %ithout any accompanying permanent (plastic) deformation,
as in the propagation
of a crack through a glass plate.
Compliance is the capacity of a material to deform elastically under force; it is the
inverse of stiffness. Delta phase is the hard, brittle constituent found in bronzes with high tin contents. The
delta phase is present in some of the bronzes from Machu Picchu because of rapid, non-equilibrium cooling of castings (Mathewson, 1915); in this case it can be dissolved by a prolonged anneal at high temperture. Ductile fracture is breakage due to plastic flow, as when a high-purity copper bar is pulled apart. Most fractures in metals are neither perfectly brittle or perfectly ductile; the fractures observed in the bronzes from Machu Picchu show very limited ductility. Elastic deformation is completely recovered upon the removal of the deforming force and is to be contrasted with plastic deformation, which remains after the application of force. Eutectoid in the bronze artifacts studied here is the microstructural constituent containing the delta phase. Hardness as used in this paper is the resistance of a material to the penetration of an
indenter and is an indication of the plastic yield strength of the material. Necking is the reduction of cross-sectional area that accompanies ductile fracture. It is
due to localized plastic deformation and the amount of necking visible at a fracture is an indication of the degree to which the fracture is ductile. Only traces of necking are found at the fractures of the artifacts studied here.
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Stzjkss is the resistance that a material offers to elastic deformation. A stiff material has a high modulus of elasticity and a large force must be applied to deform it. Toughness is the capacity of a material to resist brittle fracture. Most lithic materials have low toughness; bronze of good quality has a high toughness. In metals, superior toughness is usually associated with ductility, the capacity to deform without breaking, and low hardness; when metals are hardened, toughness is usually sacrificed to gain superior hardness. Many of the artifacts from Machu Picchu are soft but lack toughness because of porosity and non-metalic inclusions.
Science 1985,12,3 1 l-327
Laboratory Evidence of the Use of Metal Tools at. Machu Picchu (Peru) and Environs Robert B. Gordon0 An interpretation of use-wear marks on metal artifacts is developed from the principles of metal cutting and brittle fracture and applied to surficial markings and microstructural damage on bronze tools from Machu Picchu and environs. Most of the tools have blunt edges, relatively low tin contents, and were not work hardened before use; they appear to have been designed for work that involved breaking chips from hard, brittle material. Use-wear marks on these tools are interpreted as due to sliding contacts and impacts with rock. One tool with a relatively sharp edge has a higher alloy content than those with blunt edges and has been work hardened; it appears to have been designed for cutting wood and use-wear markings suggestit was so used. A long bronze bar carries markings that suggest use by stonemasons. Many of the tools are broken and study of their microstructures shows that the bronze used has poor mechanical properties because of porosity and bands of sulphide inclusions. PERU, MACHU PICCHU, TIN-BRONZE, METALLURGY, USE-WEAR MARKS.
Keywords:
TOOLS,
Introduction The use of metals for decorative and symbolic purposes was an important stimulus for the development of metallurgical skills in the pre-Columbian culture of the Andes region but metals also served a variety of utilitarian functions there (Letchman, 1980, 1984). The 150 metal objects collected at Machu Picchu and environs by Hiram Bingham’s expedition of 1912 (Bingham, 1930) are one of the largest groups of artifacts of known provenance from pre-Columbian Peru and are a sample of the metal goods in use immediately before the arrival of Europeans. Some of the artifacts in the collection are described in papers by Foote & Buell(l912) and by Mathewson (1915), whose study was one of the first to use metallographic methods for the examination of archaeological materials. Thirteen of the artifacts seem to be intended for heavy work, these 13 items accounting for two-thirds of the weight of bronze in the collection. There is not yet agreement on the role of Machu Picchu in the Inca empire so it is not known if these artifacts are representative of the use of metals throughout the central Andes region, but, if they are workman’s tools, it is less likely that they would be unrepresentative than if they were ceremonial or military items. I report here the results of an examination of these artifacts undertaken to determine how they were used. The evidence obtained includes the size and shape of the objects, the composition and properties of the alloys of which they are made, and the wear and use marks visible on their surfaces. “Kline GeologyLaboratory, YaleUniversity,New Haven,Connecticut06511, U.S.A. 311 0305-4403/85/040311+17rs03.00/0
0 1985AcademicPressInc. (London) Limited
R. B.
312
GORDON
Table I Objects
examined
Identification Artifact
Register*
Other?
Weight
1 2
17898 17900
F&B F&B2
1
3 4
17902 17967
CHM
15
5 6 7 8 9 10
17969 17974 17899 17907 17964 17966
CHM CHM
8 10
CHM CHM
6 16
242 62 560 85 224 100
11 12 13
17975 17908 18478/9
CHM
20
65 345 1070
410 525 725 414
*Register number, tF& B=Foote& IRutledge (1984).
(g)
Where
found$
Espiritu Pampa on the Conservidayoc River. Near Rosahna in Urubamba River Valley, 90 km from Machu Picchu. No record. Machu Picchu, foot of north wall of Room at 24A at depth of 0.3 m. Near Snake Rock in the centre of Machu Picchu. Room 34A at Machu Picchu. No record. No record. Eastern terraces of Machu Picchu. Section 40A of Machu Picchu, a “kitchen midden”, 0.3 m deep. No record. No record. may not be from Peru. Reported from Cave 52 and from Section 44A of Machu Picchu. Peabody Museum of Natural Buell(l912); CHM=Mathewson
History, Yale University. (1915).
Figure 1. Outline drawing of the artifacts studied (heavy lines) showing the locations of metallographic samples (dotted lines) and the reconstructed shape where part of an artifact is missing (light lines). The dashed line at 12 shows the shape of a flat axe from Britain for comparison.
Methods Used and Artifacts Examined
The artifacts examined are listed in Table 1 and all but one (a long bar) are illustrated in Figure 1. They include all of the metal artifacts that appear to be workman’s tools among the items brought back from Peru by Hiram Bingham’s expedition and now in the collections of the Peabody Museum of Natural History, except for one item that was not available for study. The collection of the artifacts in Peru was done by a small scientific party in a short period of time under conditions that precluded recording and numbering each object in the field. The manuscript field notes of the expedition members
USE OF METAL
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have been searched by Rutledge (1984) to match descriptions with items in the collcction; additional information was obtained from Eaton’s (1916) account of the osteological finds. Room and location numbers at Machu Picchu used here are those shown in the manuscript map now on the Peabody Museum. Although locally pitted by corrosion, the artifacts were in good condition when found. Previous investigators cleaned (and in some cases, machined) parts of the surfaces of some of the artifacts with procedures that were not recorded. This surface alteration is localized and a thin layer of soil was found to be still in place on most of the metal surfaces. Only surficial markings that emerged from under a cover of soil after gentle washing are reported here. The metallographic sections originally made by Mathewson were repolished and etched; new samples were prepared for the artifacts which had not been previously sampled. Microhardness was measured on the metallographic specimens with a diamond pyramid indenter and a load of 0.3 kg. The profiles of the edges were measured on the metallographic specimens or, where no section was available, with the aid of a telescope and micrometer microscope stage. New determinations of chemical composition were made with an electron microprobe attached to a scanning electron microscope. A series of copper-tin alloys was prepared to serve as composition and hardness standards. Technical terms used to describe the mechanical and metallurgical characteristics of the tolls are defined in the appendix. Sources of Evidence on the Use of Metal Tools Methods are available for inferring the previous use made of stone tools (Hayden, 1979) but no corresponding set of techniques applicable to metal tools has yet been advanced. The sources of evidence used here are described below. Toolform The possible shapes that a hand-held tool intended for a particular task, or range of tasks, can assume without becoming impossibly clumsy is limited. Unless artificers have no control over the design of the tools supplied to them, it is expected that a tool form best adapted to the requirements of a particular task, set of materials, and place will be developed. When what appears to be a useful form of tool is recognized, it is not proof that the tool was actually used for the purpose envisaged, but if a tool appears to be ill-suited to a particular use, it is likely that it was not used for this task. Distortion and breakage Use of a metal tool in any kind of hard mechanical work will usually result in distortion of some part of the tool. Distortion may be detected from external evidence, as when what was intended to be a cutting edge is blunted, bent or broken. Deformation markings in the microstructure that are unrelated to any process used in the manufacture of an artifact are internal evidence of distortion. If an object is found to be broken where it would be weakest in performing a certain function, it suggests that it was used for that function. Wear, attrition, or damage caused by the use of a tool may be corrected by reforming or resharpening; evidence of repairs may be found in the microstructure or in markings on the surface. Use-wear marks Unless a tool is used very gently or is remarkably hard, evidence of its contact with the material being worked will be left on its surfaces. Metal tools employed in mechanical work will usually be marked with indentations and striations whose form depends on the class of work that has been done. In the case of edge tools, the possible classes of work
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R. B. GORDON
Clearance
:
i
Figure 2. Illustration of three modes of working with edge tools, cutting a chip from a plastic material (right), breaking a chip out of a brittle material through the formation of cracks (centre), and splitting (left). The definitions of the clearance and rake angles of a cutting tool are shown; the chip is being formed by intense local plastic deformation in the shear zone.
are cutting, breaking, or splitting. These three modes of working are illustrated in Figure 2. Cutting is the removal of material from the workpiece as small bits ~ chips ~ formed by a combination of plastic deformation and ductile fracture of the work under the action of the tool. When a tool is used for breaking the chips are formed by brittle fractures rather than plastic deformation. Splitting is the division of the work into approximately equal pieces by the propagation of a crack through it. Each of these modes of working with an edge tool requires a different combination of tool form and material properties. The basic design characteristics of a cutting tool are the clearance and rake angles, which are defined in Figure 2. Cutting can be achieved with a rake angle that is small or even negative but the corner between the tool faces must be sharp. Making the rake angle more negative will, when a critical angle is reached, cause a transition from cutting chips to plowing, the formation of a furrow and ridges without the removal of chips (Samuels, 1982). Successful cutting of compliant materials such as wood or flesh requires a large, positive rake angle so as to avoid excessive elastic deflection of the work near the point of the tool. Stiffer work, such as bone or metal, is best cut with a tool having a smaller (or even slightly negative) rake angle. A tool used for cutting must have a sharp edge so as to localize the plastic shear zone and the edge of the tool must be harder than the work so that the sharpness can be maintained. Chips may be cut from bronze when the rake angle is as small as - 35”. Hence, grooves caused by the removal of chips may be formed on a bronze tool by rubbing it over a rough rock surface. A set of use-wear marks believed to have been so formed is shown in Figure 3. The absence of ridges adjacent to the groove and the long, continuous striations within the grooves show that the chips were being removed from the side of the tool by sliding contact with material hard enough to retain a sharp edge while moving over the bronze. Chips can be formed in a brittle material by creating tensile cracks below the area of contact between the tool and the work (Lawn & Marshall, 1979). In this case the function of the tool is to develop a high stress concentration at the point of contact. The tool need not have a sharp point but it will function better if made of material that is stiffer and tougher than the work; it need not be harder though if it is excessively soft it will become too blunt to be effective after slight use. The tool may be hammered (as with a percussion rock drill) or may be dragged over the surface of the work (as with a rotary rock drill). The essential function of a splitting tool is to wedge apart the two halves of the workpiece. If a slot has been previously cut where the split is to be developed, the tool need
USE OF METAL
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Figure 3. The side of the blade of artifact 5 showing deep striations, ing the fine grooves characteristic of the removal of chip by sliding rough rock surface. Length of scale bar = 1 mm.
many containcontact with a
not have a sharp point. Since splitting is most easily managed with an approximately equal division of the work, a splitting tool is usually made symmetrical about the splitting plane. The included angle chosen depends on the magnitude of the splitting force that is available; if it is small, a small angle will be wanted so as to gain a greater mechanical advantage. The tool need not be harder than the work but must have sufficient strength and stiffness to avoid distortion in use. A stone tool may be used to split wood or bone but use of a tool made of a tough alloy will greatly facilitate splitting rock, particularly when close control over the crack path is wanted. Tool material
Edge tools that will cut wood, bone, and flesh can be made of stone because of its high hardness and the sharp edges that can be prepared on it. Bronze offers little or no advantage in this kind of work unless the tool is to be hammered upon, in which case its superior toughness may be judged by the user to compensate for its inferior hardness. Because it can be cast or forged to convenient forms and because of its stiffness and, if of good quality, toughness, bronze offers a very great advantage over stone in tools intended for breaking and splitting rock, particularly where fine work is to be done. While bronze was a scarce and expensive material we expect that the preferred use for it in tools would be in those applications where it offered the greatest advantage over working with non-metallic tools. Where the capacity to make a range of alloys existed and the alloys chosen for individual tools enhances their capacity to perform their apparent functions, it suggests
316
R. B. GORDON Table 2. Composition
of Artifacts cu
Sn
Fe
&
s
1* 2* ::
876 93.7 95.2 94.3
12.0 5.6 50 4.0
0.08 0.0 0.0 0.3
0.0 0.65 0.4 0.0
0.35 0.08 0.4
51 7t
98.7 94.6 98.7
5.0 5.1 0.6
0.9 0.0 0.0
0.0 0.0 0.0
0.4 0.3 0.7
iI 101 llt 12t 13$
96.3 93.1 93.7 91.5 100.0 94.6
3.7 6.0 5.5 8.3 0.0 5.4
0.0 0.5 0.6 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.4 0.2 0.2 0.0 tr.
Artifact
6:
*Wet chemical analysis reported by Foote & Buell(1912). tElectron microprobe analysis. $Wet chemical analysis reported by Mathewson (1915). All analyses are in weight percent. Arsenic was not detected in any of the microprobe analyses nor reported for any of the wet chemical analyses.
that the tools were designed for those functions. For example, if a tool with an edge suitable for cutting wood were made of hard, high-tin bronze and another with a blunt point suitable for striking blows were made with tougher, low-tin alloy, the match of material properties to apparent tasks suggests that the tools were made to perform those tasks. Use Made of the Peruvian Tools Material and manufacture Alloys. All of the artifacts studied are made of arsenic-free tin bronze and most contain about 5% tin, the composition of bronze commonly found at Machu Picchu (Rutledge, 1984). The analyses, Table 2, show that the principal impurities are iron and sulphur. Annealed, 5% tin bronze of good quality would have a strength of 350 MPa and 55% elongation (Brandes, 1983) but a tensile test of artifact 13 gave an ultimate strength of 192 MPa and an elongation of 6% in a gauge length of 50 mm (Mathewson, 1915). The poor strength and ductility of the bronze in this and most of the other artifacts studied is due to porosity and/or bands of sulphide inclusions (Gordon, 1984). Artifacts 1 and 11 have unusually high tin contents. In artifact 1 the eutectoid structure which would ordinarily be present in a bronze casting of this composition is not found, showing that it must have been annealed at a temperature between about 600 and 800°C. This anneal, by dissolving the brittle delta phase, would have made subsequent forging of the blade easier. The high tin content may be responsible for the relatively good surface finish that has been preserved, since the composition is near the optimum for resistance to corrosion in soil (Tylecote, 1979). Artifact 11 is the only one in which there is large-scale tin segregation and its microhardness ranges from 163 on one side of the blade to 84 in a copper-coloured region on the other side; the eutectoid structure is visible in the harder regions. The tin content of artifact 7 is unusually low and its microstructure consists of recrystallized grains of copper-rich solid solution with a few sulphide inclusions.
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Manufacture. Evidence of plastic deformation in the microstructures of the artifacts is confined to localized areas; this and the absence of forging marks on most of their surfaces shows that the artifacts were cast nearly to their final forms. Most have bilateral symmetry that could not be attained in open moulds but have no parting lines such as are often found on bronze objects cast in bivalve moulds. Annealing twins are common in the microstructures and may have originated in thermal strains developed during cooling of the castings. Only artifacts 1, 6, 9 and 10 show evidence of having been further shaped by forging after they were cast. The most satisfactory evidence of this is the elongation of the sulphide inclusions, which persists even when the deformed metal has been recrystallized (Gordon, 1984). Mathewson (1915) used a substantial part of artifact 9 for a hotworking experiment in which he duplicated the microstructure found near the edge, thereby showing that the pointed end was formed by hot forging. In artifact 10 the sulphides are elongated for a distance of about 4 mm back from the working edge while in a region extending about 15 mm back from the edge, and in another reaching 30 mm down from the fracture, deformed and undeformed grains are adjacent to one another in a structure similar to, but finer than, that formed in artifact 9 by hot working. These two parts of the blade were probably forged while the object was cooling from its last anneal. The elongation of sulphide inclusions shows that the metal near the edge of artifact 1, which is one of the sharpest found in the collection, has been forged and, since deformation marks and the distorted grains are present in the microstructure, some of the forging must have been done after the last anneal. The final cold forging is responsible for the hardness gradient, shown in Figure 4, which extends about 20 mm back from the edge. Since elongated sulphide particles intersect the surface of the point for a distance of 5 mm back from the edge, at least some of the final shaping of the blade must have been done by the removal of metal. In artifact 6 the sulphide inclusions within 5 mm of the edge are elongated. Deformation markings are present in a zone extending 5 mm inwards from both surfaces, except at the edge, where there are no markings. The longitudinal hardness profile (Figure 4) confirms that the edge has not been work hardened but there is hardening at the sides of the blade associated with the bands of deformation marks. The sides were probably hammered lightly after the last anneal but it is possible that the sides may have been deformed by the process that produced the striations discussed below. Interpretation. Two of the objects have an unusually high tin content, two are nearly pure copper, and the rest contain approximately 5% tin. If a bronze tool.with a sharp edge is to retain its sharpness, it must be made hard by a high tin content or work hardening or both. All three characteristics are found together in artifact 1, which suggests that it was designed for use in cutting wood or other organic matter. No positive evidence of intentional alloying to particular compositions can be adduced for the other objects but it is interesting to note that the 5% tin alloy would, in the absence of porosity and sulphides, have a combintion of toughness and hardness that is appropriate for tools to be used for heavy breaking and splitting work and that work hardening the edges of such tools would be undesirable. Artifact 7, which has only a trace of tin, also has a very blunt edge. Form Shape. All of the objects shown in Figure 1 can be described as consisting of a blade with
a more-or-less sharp edge; some have a cross-bar at the opposite end from the edge. Artifacts l-7 have an approximately common form and size such that any of them will fit easily into one’s hand in an attitude convenient for making chips on a work surface by hammer blows on the top of the cross-bar. The bar then protects the user’s hand against
318
R. B. GORDON
d km)
Figure 4. Hardness gradients measured and 9. The distance from the edge is d. fully annealed copper-tin alloys having data for artifact 2 are taken on the most d; they represent an upper bound to the
along the central axes of artifacts I, 2, 6 The lines marked “a” are the hardnes’s of the same tin content as the artifacts. The heavily deformed grains at the indicated hardness attained along the section.
badly directed blows. Artifacts 8811 are also edge tools and the absence of cross-bars and would be make them suitable for insertion into the workpiece. A long bar, artifact 13, is of a size and shape to be conveniently manipulated as a hand-held tool. None of the artifacts is decorated. The shape of artifact 12 is different from all the others in the collection but is very similar to the form of, for example, flat axes of the British Early Bronze Age; a comparison with a flat axe found near Bristol (Parker, 1982) is made in Figure 1. Because of this, and our inability to find any identification of artifact 12 in the expedition records, it is not discussed further here. Edge projile. The profiles of the working edges of the artifacts, shown in Figure 5, range from quite sharp to blunt. The two badly damaged edges, 2 and 9, appear, on the basis of microstructural evidence, to have been originally blunt as well. The blade profiles of artifacts 4, 5 and 6 are of intermediate sharpness (the slight asymmetry of artifact 4 is part of the original casting). The similarity of these profiles suggest a more-or-less standardized tool design; this is the basis of the reconstruction of the forms of artifacts 5 and 6 shown in Figure 1. Only two artifacts, 1 and 10, are sharp enough to be useful in cutting wood or other organic matter. They are compared with what is now considered to be the optimum point profile of an axe for chopping wood in Figure 6; both are slightly blunter than the optimum and they would only be effective in cutting under blows from a hammer - they are not sharp enough to be useful in carving wood, for example. The artifacts with the blunt points would be useless for cutting but are suitable for breaking or, with the aid of a starting notch, splitting. The other working edges are of intermediate shape; they are too blunt for cutting and could be used for breaking or splitting.
USE OF METAL
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\_---’
Figure
5. Measured
cross-sections
Figure 6. Comparison of the profiles with the optimum shape of a modern
5mm
4
of the edges of the tools.
of the tips of artifacts 1 and 10 (heavy axe for chopping wood (light lines).
lines)
Breakage
All but two of the artifacts have been fractured; four (5, 6, 10 and 11) have been broken bodily with fractures that show very little ductility and four (2, 3, 4 and 13) contain smaller breaks. Incomplete fractures or small cracks are present in artifacts 1, 2, 4, 8, 9, 11 and 13. The fractures are all brittle or nearly brittle, although in the case of artifact 10 there was enough ductility in the metal to permit the object to be twisted through about 12” before it fractured. Sometimes a trace of necking (localized plastic deformation preceding fracture) can be seen, as in artifacts 10 and 11, where the small necks on one side show the presence of tension, from which we infer that breakage was by application of lateral force. The fracture surfaces are rough and the fracture paths usually follow
320
R. B. GORDON
bands of sulphide particles. The high degree of porosity in artifact 13 is the cause of the brittleness of this artifact. While the items fractured bodily might have been broken by their owners to prevent their being used by others, it is more likely that they were damaged in use; the other breakages must have been failures in service. Because of the limited ductility of the bronze, nearly brittle fracture is expected to be the most common type of failure encountered in the use of these artifacts. Microstructural
evidence of use
It would be nearly impossible to use a bronze tool in any sort of heavy work without producing deformation markings in the microstructure; where the damage is more extensive, the resultant work hardening can be detected with microhardness measurements. These features will be preserved unless the tool has been subsequently annealed at high temperature. Most of the artifacts examined contain evidence of deformation that cannot be accounted for by manufacturing procedures. Working edges. Artifacts 2,3,5,9 and 1 I show evidence of deformation near their working edges that is unrelated to the manufacture and in some of them the working edges are severely distorted. In artifact 2 sulphide inclusions near the edge of the blade have not been elongated and the coring in the as-cast microstructure is retained, showing that the tip of the blade has not been forged or subjected to prolonged annealing and that any deformation markings found in the microstructure must be due to damage incurred during use. Heavy plastic deformation is visible in the microstructure to about 3 mm back from the edge; at the edge the grains are elongated perpendicular to the centre plane of the blade and two cracks have developed as the metal at the tip has spread laterally. These features are visible in the section of the edge of the blade shown in Figure 7. There is strong work hardening within a few millimeters of the tip (Figure 4) but back from the edge the hardness (HV 115) is only slightly greater than that of annealed bronze with a comparable amount of tin but no silver (HV 97). The edge has been upset and in some areas turned over and partially detached; one corner of the blade has been broken Off.
The working edge of artifact 9 has been deeply indented, upset, and turned over. The metal near the working edge has split along lines of sulphide particles that were elongated during the hot forging of the pointed end. This has widened the edge, as shown by the dotted line in Figure 5, but the original working edge was almost certainly blunt rather than sharp. The hardness gradient (Figure 4) along the axis of the blade extends well back from the edge that has been damaged by use and so must have resulted from a final cold forging during preparation of the artifact. Since the working edge shows such heavy damage, it is surprising that the surviving part of the top shows no evidence of deformation in its microstructure. The microstructure of artifact 5 taken near the fracture shows residual coring, no annealing twins, and abundant sulphide inclusions. This appears to be the as-cast structure unaltered by subsequent forging or annealing. The metal 5 mm back from the tip along the centre line of the blade is recrystallized but the sulphide inclusions are not elongated and there are few deformation marks. Thus, the blade has neither been forged nor work hardened. But, deformation markings are found in the grains along the sides of the blade and these become more intense as the working edge is approached. The metal in a zone about 0.25 mm thick at the tip is so heavily deformed that the grains are elongated perpendicularly to the blade axis. A structure such as this would result if the edge had been flattened against a hard object and then resharpened. The presence of undistorted sulphide inclusions in artifact 11 shows that it has not been forged but deformation marks are found near the fracture surface, along the
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Figure 7. Micrograph of a polished and etched section of the edge of artifact showing the flattening and splitting of the edge and the presence of silver-rich areas (dark) with traces of a second phase. Length of scale bar =0.5 mm.
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adjoining sides, and throughout the metal near the edge. These are interpreted as deformation caused by use of the tool. The edge, which now appears to be undamaged, has been reformed by grinding, as is shown by the striations discussed below. Other surfaces. The microstructures of five of the artifacts (1, 2, 3, 7 and 8) show that they have been deformed on their tops or sides in a way that is unrelated to any manufacturing procedure. The microstructure at the long end of the cross-bar of artifact 1 consists of recrystallized grains with undeformed sulphide particles and much porosity; deformation marks are found adjacent to the surface at the end of the cross-bar. Both the end and the centre part of the top of the cross-bar have been indented by blows from a blunt object harder than bronze (as described in the following section) and these blows caused the deformation markings in the metal near the surface. Deformation markings are also found in the metal near the end of the cross-bars of artifacts 1 and 3 and the indentations on the ends and tops of the bars suggest that they have been used as hammers and have been hammered upon. In artifact 7 there are a few indentations on the top of the cross-bar near its centre and deformation markings are found in a band about 2 mm thick at the end of the cross-bar that was sectioned. This artifact appears to be in nearly the as-cast condition and to have had very little hard use. Although no indentations are visible on the top surface of artifact 8, the metal below the top to a depth of 1.7 mm is heavily deformed. Below this zone of deformation the microstructure consists of undeformed, recrystallized grains, spherical pores, and sulphide inclusions. Summary. Microstructural evidence shows that two of the artifacts, 2 and 9, have been used in such a way as to cause heavy damage to the working edges; it is likely that the
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Figure 8. Indentations bar = 1 mtn.
and striations
in the flat edge of artifacts
3. Length
of scale
same would be found for artifact 3 (as discussed below). Two others, 5 and 11, also appear to have suffered internal deformation from use. Five of the artifacts show microstructural evidence of having been hammered upon or hammered with; the surficial evidence of this use is discussed in the following section. Use- wear marks Three types of use-wear marks have been found on the surfaces of the artifacts. Two types, the indentations found on the tops and on the edges, are associated with internal microstructural damage but the evidence of this association for the third type, the striations found primarily on the side of some of the artifacts, is weak. The tops and ends of the cross-bars of artifacts 1, 2, and 3 are heavily indented, in some places to the extent of turning the edge over. Artifact 7 is lightly indented on its top surface. The presence of the deformation markings in the metal below the surface shows that these indentations were made after the metal was last annealed. These markings have no discernible relation to the fabrication of the artifacts and are interpreted as the marks left by hammer blows. Those on the tops were probably made with hammer stones, such as the one illustrated by Bingham (1930), while the tool was being driven into a workpiece by blows. Those on the ends of the arms could be due to the use of the cross-bar as a hammer. The presence of these markings shows that these objects were not hafted and so should not be called axes. The second type of wear or use marks present are the striations and indentations on, or adjacent to, the working edges. The working edge of artifact 3 has been flattened, indented, and upset, as shown in Figure 8, and some of the indentations on the edge contain striations that suggest sliding motion against a hard material. No microstructure
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Figure 9. Side view of the point of artifact at the edge. Length of scale bar = 1 mm.
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2 showing
the striated
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burr turned
over
sample of this edge was available but the external evidence makes it quite likely that deformation markings would be found near the working edge in an etched section. In artifact 2 the edge has been turned over and the upset edge is deeply striated, as shown in Figure 9. The turned-over end of artifact 9 is also deeply grooved with longitudinal striations. The markings on all of these artifacts are interpreted as due to contact with material that is harder than bronze. Since the edges have been deformed, one possible interpretation is that they are due to work done that involved impact with rock, such as breaking out chips in dressing stone. Since there are no use marks on the sides of artifacts 2 and 3, the work was probably done on large, free stone surfaces where insertion of the tool into slots or cracks was not required. Striations on the sides of the artifacts are the third type of use-wear mark found. These are illustrated by the well-preserved grooves along the length of both sides of the upper half of artifact 10 (Figure 10). The edge is undamaged but the marks on the sides show that the blade has been inserted to a depth of at least 70 mm in a crack or slot. Similar grooves are found on the side of artifact 9. The fine striations within the grooves show that they were formed by cutting a chip from the bronze by the motion of hard material over the metal surface. Sliding contact with the rough surface of split granite, for example, would produce such marks because of the relatively large negative rake angle at which chips can be cut from bronze. These striations could arise in a number of ways; two possibilities are sharpening or dressing the bronze tool on stone or the insertion of the tool into a slot in the stone, as would be done in splitting a stone block. The presence of deep grooves on the sides of artifacts 10 and 11 suggests that they were deeply inserted into a slot. With the tool so placed, a badly directed hammer blow could easily break the bronze, which has poor ductility because of its high sulphide content and porosity, so
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Figure 10. Deep striations on the side of artifact IO formed chips. These were probably cut in the bronze by sliding contact face of hard rock. Length of scale bar = 1 mm.
by the removal of with a rough sur-
causing the observed fractures. The striations found at the edges of artifacts 5,6 and 11 are interpreted as marks left from sharpening or dressing rather than use. For example, there is no evidence of use of artifact 5 on the outside of the tip of the blade but the adjoining sides are covered with striations that are generally parallel to the edge and frequently cross one another, as shown in Figure 3. In artifact 11 they are associated with the bevel, which was probably put on the edge by sharpening against a stone, and in artifacts 5 and 6, with the removal of metal in forming the profile of the point. Artifact 13, a long, tapered bar, has not been sharpened, subjected to impacts, or used for splitting, but it has short grooves cut in the bronze by sliding contact with stone. Such marks would arise from levering stone blocks about in the construction work. Discussion
I interpret the accumulated evidence above as showing that most, if not all, of the artifacts described were designed as workman’s tools, that most have been used for their intended purpose, and that several of them have had hard use. Only one artifact, 7, would be of little use in utilitarian tasks; it is too ill-formed, soft, and blunt to accomplish much with. One of the tools, artifact 1, seems to have been designed and used for cutting wood or other organic material with heavy hammer blows while hand held. It has a unique combination of characteristics - a sharp point hardened by alloying with 12% tin and cold work, a form massive enough to sustain hammer blows, and a size and shape convenient to hold in one’s hand. It could be used either for splitting or for removing chips from the workpiece.
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At least three of the tools, artifacts 2, 3 and 9, were made with blunt edges and of an alloy having a low tin content which would be tough in good quality castings. They appear to have been intended for work such as breaking chips out of stone (as illustrated in the centre diagram of Figure 2) and they carry evidence of having been used in work that involved impact with stone. Artifacts 10 and 11 appear to have been in sliding contact with rough, hard surfaces, such as would occur in splitting stone while artifact 8 seems to have been made for splitting but not much used. The edges of artifacts 4,5 and 6 are not sufficiently sharp to be useful in cutting wood but would deform more easily than the blunt edges of artifacts 2, 3 and 9 in uses which involve impact with stone. The heavy deformation of the metal at the edge of artifact 5 shows that it has had hard use although the damage which we infer from the deformation markings to have been present has been repaired. These tools may have been intended for a finer class of work on stone than could be done conveniently with the blunt edges found in artifacts 2, 3 and 9. It seems likely that artifacts 5 and 6 received substantial blade damage in use, were reformed by stoning (leaving the sharpening marks now visible on their sides) and were then found to be cracked and unfit for further work. Tool 4 was not used, perhaps because a corner of the blade broke off when it was first placed in service. Because of the deficient ductility of the bronze used in the tools found at Machu Picchu, breakage in service must have been frequent. It is likely that the broken tools were remelted and used to cast new tools, which would, of course, reduce the number of broken tools to be found at the site. The actual work done with the tools that have been in contact with stone cannot be deduced from the evidence that they carry. It could have been the preparation of ashlars or other large stone blocks having recesses or sharp internal corners, work in which long, thin tools that could only be made of metal are needed to reach the working surfaces. These tools would also be useful in cutting decorations or figures on stone blocks but they are too large to be of much service in making small stone items, such as might be used for personal adornment. Field studies are needed now to identify the nature of the tasks that were being performed with metal tools at Machu Picchu and environs. Most of the tools examined thus appear to have been designed, manufactured, and used for work on stone and one seems to have been specifically designed for woodworking. The advantages gained by the use of bronze rather than stone for edge tools derive primarily from the superior toughness of bronze and the possibility of shaping the metal tools to slimmer or more complicted forms than can be made in stone. Some work previously done with stone tools could be done more efficiently with bronze ones, as where a slim wedge is required for splitting an object. A bronze wedge could have sufficient toughness to be hammered upon and sufficient stiffness to propagate a crack in rock as well as wood or other organic matter. New tasks, which could not be accomplished with tools made of stone, could be undertaken when bronze tools became available. These would include work that requires the use of relatively delicate, slim edge tools, such as making deep, rectangular holes or re-enterant angles. The laboratory results show that a substantial amount of metal was being used for utilitarian purposes in and around Machu Picchu in immediately pre-Columbian times. Acknowledgements
I thank J. L. Hollowell for information about stone-working technology in preColumbian Peru, J. W. Rutledge for detailed descriptions of the metal artifacts from Machu Picchu in the Peabody Museum, Alan Pooley for assistance with the microprobe analysis, and Heather Lechtman for a critical reading of the first draft of this paper.
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References Bingham, H. (1930). Machu Picchu, Citadel ofthe Incus, chapter 7. New Haven: Yale University Press. Brandes, E. A. (Ed.) (1983). Smithells Metals Reference Book, pp. 22-26. London: Butterworths. Eaton, G. F. (1916). The collection of osteological materials from Machu Picchu. Memoirs of the Connecticut Academy of Arts and Sciences5,1-96. Foot, H. W. & Buell, W. H. (1912). The composition, structure and hardness of some Peruvian bronze axes.American Journal of Science34, 128-132. Gordon, R. B. (1984). Metallurgy of bronze tools from the Machu Picchu. Proceedingsof the 1984 Archaeometry Symposium. Washington: Smithsonian Institution. Hayden, Brian (Ed.) (1979). Lithic Use-Wear Analysis. New York: Academic Press. Lawn, B. R. & Marshall, D. B. (1979). Mechanisms of microcontact fracture in brittle solids. In (B. Hayden, Ed.) Lithic Use-Wear Analysis, p. 63. New York: Academic Press. Letchman, H. (1980). The Central Andes: Metallurgy without iron. In (T. A. Wertime & J. D. Muhly, Eds) The Coming of the Age of Iron, p. 322. New Haven: Yale University Press. Lechtman, H. (1984). Andean value systemsin the development of prehistoric metallurgy. Technology and Culture 25, l-36. Mathewson, C. H. (1915). A metallographic description of some ancient Peruvian bronzes from Machu Picchu. American Journal of Science40,525-6 16. Parker, G. (1982). Metallurgical notes on three Bronze Age implements found in the West of England. Journal of the Historical Metallurgy Society 16,4&49. Rutledge, J. W. (1984). The Metal Artifactsfrom the Yale Peruvian Expedition of 1912, Catalog and Commentary. M.Sc. Thesis, Yale University. Samuels, L. E. (1982). Metallographic Polishing By Mechanical Methods, p. 36. Metals Park (Ohio): American Society for Metals. Tylecote, R. F. (1979). The effect of soil conditions on the long term corrosion of buried tin bronze and copper. Journal of Archaeological Science64,345-3X
Appendix Definitions of Technical Terms Brittle fracture is breakage %ithout any accompanying permanent (plastic) deformation,
as in the propagation
of a crack through a glass plate.
Compliance is the capacity of a material to deform elastically under force; it is the
inverse of stiffness. Delta phase is the hard, brittle constituent found in bronzes with high tin contents. The
delta phase is present in some of the bronzes from Machu Picchu because of rapid, non-equilibrium cooling of castings (Mathewson, 1915); in this case it can be dissolved by a prolonged anneal at high temperture. Ductile fracture is breakage due to plastic flow, as when a high-purity copper bar is pulled apart. Most fractures in metals are neither perfectly brittle or perfectly ductile; the fractures observed in the bronzes from Machu Picchu show very limited ductility. Elastic deformation is completely recovered upon the removal of the deforming force and is to be contrasted with plastic deformation, which remains after the application of force. Eutectoid in the bronze artifacts studied here is the microstructural constituent containing the delta phase. Hardness as used in this paper is the resistance of a material to the penetration of an
indenter and is an indication of the plastic yield strength of the material. Necking is the reduction of cross-sectional area that accompanies ductile fracture. It is
due to localized plastic deformation and the amount of necking visible at a fracture is an indication of the degree to which the fracture is ductile. Only traces of necking are found at the fractures of the artifacts studied here.
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Stzjkss is the resistance that a material offers to elastic deformation. A stiff material has a high modulus of elasticity and a large force must be applied to deform it. Toughness is the capacity of a material to resist brittle fracture. Most lithic materials have low toughness; bronze of good quality has a high toughness. In metals, superior toughness is usually associated with ductility, the capacity to deform without breaking, and low hardness; when metals are hardened, toughness is usually sacrificed to gain superior hardness. Many of the artifacts from Machu Picchu are soft but lack toughness because of porosity and non-metalic inclusions.