Pre-Columbian Peruvian MetalsSample 24 |
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Copper Ax Inca Peru Highlands (Cuzco area?) ca. 1200-1500 AD? |
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Report by : Wendi Croft Feb 10, 2003 |
| Introduction |
Figure 1: Copper Ax
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| The object under observation is referred in this document as a simple copper ax. This type of tool was very common in both the New and Old Worlds and its manufacture was fundamentally standardized [1]. Typically a copper piece like the Ax would have been cast, and work hardened with some form of a hammer-like tool. This process was very common during the Late Intermediate Period (1200-1500AD) in Peru, where a sufficient amount of copper-arsenic alloys were produced. [1,2,3]. Possible applications for this ax could have ranged from a cutting tool used during food preparation, to soil upheaval or construction uses. Another example of this type of piece can be seen in Figure 2. | ||
| Sample Preparation | |
| A piece of the received artifact (Figure 1) was cut with a diamond jeweler's saw near a corner of the cutting edge (Figure 3). The piece was then mounted in Epoxy between plastic clips, ground to 800 grit, and polished to .25 m. Following that, it was etched with a common copper alloy etch of 1 part H20, 1 part H2O2, and 1 part NH4O4. It was observed under the light optical microscope (LOM) before and after etching. Digital images were taken at various stages using a Nikon digital camera in coordination with a LOM. | |
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Figure 2: Comparison sample found in the American
Museum of Natural History
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Figure 3: Macroscopic close up of the area
sampled for observation.
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| Results | |
| From the microstructure pictures taken, the
manufacturing processes of the ax were exposed. Due to the noticeable second phases that are seen in Figures 4 and 6 it was assumed the artifact went through a smelting process that did not completely remove all of the impurities. Smelting is a thermal processing technique used to make liquid metal from an beneficiated ore [4]. Through this process impurities can be taken out of the raw material before it is formed into its desired shaped. Therefore, it was assumed this particular sample derives from smelted ore, however not fully, leaving some particles behind that appear within the material. Furthermore, some of the inclusions were thought to possibly be a cuprous oxide or a sulfide, which are typical inclusions of these types of tools that have undergone smelting [1]. However no conclusive data was established to justify this hypothesis. |
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Figure 4: View of the un-etched sample tip
showing slag inclusions, casting voids, and corrosion.
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Figure 5: The etched sample showing elongated
slag inclusions, mechanical twinning, and possible corrosion.
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| Discussion | |
| After smelting the material, the piece was
presumably cast into form using a simple open mold [1]. An example of
a typical ceramic mold can be seen in Figure 4. Evidence of the casting
process can be seen through the coring lines that can be faintly noticed
in Figures 5 and 6. The coring lines look like darker areas of etched
material regardless of the grain structure. Coring is the compositional
gradient within a typical slow cooling casting structure know as dendrites.
It is noted as being very common in Copper- arsenic alloys [5]. This type
of forming was easy and efficient for this quality of tool for this time
period. Subsequent to forming the sample, the piece was most likely finished through a cold working and annealing cycle. This can sometimes be termed hot working [5]. This type of process typically occurred by, first, deforming the piece with a hammer-like tool, creating a sharp edge. Following this the sample would have been annealed, by brining it to a high temperature. These two steps would be repeated until the desired edge and hardness was achieved for the tool. Effects and evidence of this rotating process can be seen in the slag inclusions and annealing twins show in Figures 5,6, and 7 as well as the X-Ray image in Figure 8. The inclusions in these images are elongated which is proof that the sample was deformed after casting. Moreover, in the X-Ray image the edged of the piece are distinctively thinner. This could be attributed to cold working. Additionally, the grain structure is recrystallized and has annealing twins presents, both of which lend to annealing. A recrystallized structure has predominately equal sized grains. Annealing twins appear to be straight lines intercepting the grains of the sample. All three of these observations point to the sample having been hot worked [5]. The overall integrity of the sample was assumed to be good. In comparing it to other pieces of similar use and location it matched very closely. Moreover, the piece was visibly corroded which also lends to authenticity. In addition the sample was obviously worked with crude materials (a very large hammer head etc.) as could be seen macroscopically in it large dents. This all lead to the assumption that the piece is genuine. |
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| Figure 6: The etched sample showing elongated slag inclusions,coring lines (midshaded lines to the left) and anneaing twins. | Figure 7: Dark Feel view of the etched sample exaggerating the slag inclusions. |
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Figure 8: X-Ray Image analysis showing the material was consistent throughout the piece, yet thinner on the edges |
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Referneces: 1. David A. Scott, "Metallography and Microstructure of Ancient
and Historic Materials", The J Pual Getty Trust, Singapore, 1991
pg 92. |
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