Archaeometallurgy in the Paraná Delta (Argentina): Composition, manufacture, and indigenous routes

TL;DR: The results of macroscopic, microscopic, and metallographic studies of archaeological metal objects from the Delta of the Parana River (Argentina, South America) are presented in this paper.

About: This article is published in Journal of Anthropological Archaeology.The article was published on 2017-09-01 and is currently open access. It has received 9 citations till now. The article focuses on the topics: Archaeometallurgy.

Summary

1. Introduction

• Metal objects have been one of the most valued goods for human societies all over the world since the emergence of metallurgy.
• In most parts of the South American lowlands, indigenous people did not make metal objects, but they obtained them through exchange with Andean populations (Steward, 1944–49).
• This is the main idea that this paper seeks to asses.
• The results of macroscopic, microscopic, and metallographic studies of 11 archaeological metal pieces from the Delta of the Lower Paraná River are presented.
• The objectives of these studies were to determine the chemical composition of the objects and the existence of any sort of alloy, as well as to obtain information about the metallurgical manufacturing techniques.

2. Materials and methods

• The studied metal objects are part of the Luis María Torres Collection (División Arqueología of the Museo de La Plata, Universidad Nacional de La Plata).
• He presents the results of his fieldwork on these three earth mounds where, besides the pieces analysed here, he also recovered abundant smoothed and incised pottery, bone tools, lithic artefacts, endocarps of palm fruits, and faunal remains of fish, deer, felines, and freshwater molluscs, along with numerous human skeletons (for a review of Torres Collection see Bonomo et al., 2009).
• 578) published wet-chemical analysis of four pieces1 from TIIPG, performed by Herrero Ducloux, the first doctor in chemistry in Argentina, who identified a marked predominance of copper (Cu), also known as Torres (1911.
• In the present study the following analyses were performed: (1) Macroscopic analysis:.
• In order to evaluate microstructurally the morphological characteristics and the manufacturing methods, all of the 11 pieces and the metallographic replicas made from three of them (one from each archaeological site) were observed with SEM at different magnifications.

3.1. Macroscopic analyses

• The metallic pieces are part of ornaments, possibly pendants and beads, and they all have holes to be suspended (Table 1).
• They have semi-lunar, trapezoidal, rectangular, and quadrangular shapes with rounded corners.
• In four of the pieces (TIIPG N 4, TIBG N 6, N 7 and N 11), it can be observed that the suspension hole was made through mechanical action with a pointed element perforating from one of the piece surfaces and leaving an irregular burr of metal material around the hole on the opposing surface.

3.2. Chemical composition and corrosion

• The metal used for manufacturing all the pieces is mainly pure Cu, as indicated by EDS analyses (Fig. 2a and b).
• The composition of the semi-lunar pendant is an exception in the entire object assemblage (Fig. 2c).
• The intentional incorporation of Sn improves the mechanical properties and decreases the copper’s reddish colour, making the pieces look more golden (González, 2002; Palacios, 2011).
• Precisely, in metal pieces from funerary contexts, such as the ones studied here, the corrosive processes are intensified due to the combination of environmental conditions and alterations generated by the organic substances during the decomposition of the bodies (Pifferetti, 2001:787).
• On their external surfaces, a patina has been formed by quite stable Cu carbonates that naturally protect the metal.

3.3. Metallographic and scanning electron microscope analyses

• The two suspension holes on one of the ends of this semi-lunar pendant (TIPG N 1) were studied: a complete hole (0.3 0.5 cm) and an incomplete hole (Fig. 1a).
• Even though the edge of the complete suspension hole (Fig. 3a) is very irregular due to corrosion, it does not present any evidence of mechanical work by pounding and it preserves the signs of the primary manufacture process, that is, the casting.
• The metallographies indicate that the initial manufacture stage involved alloy melting in the form of a small ingot or a bigger chunk of pure Cu. Precisely, the micro-shrinkage cavities observed in Fig. 6a are defects generated during ingot melting.
• This type of mechanical deformation that increases the sheet hardness can be discerned by the oxides orientations (Fig. 6b), which follow the hammering direction.
• In Fig. 6c and d the matrix with annealing twins was observed over a previous inter-dendritic phase, which is characteristic of molten material prior to this hot forming.

4. Discussion

• The metal objects found in the Paraná Delta are the final product of specialized technology.
• Since the beginning of the European conquest in the sixteenth century, several references to the Paraná River and neighbouring areas have been made about individuals from different indigenous populations having metal objects: Chaná-timbú, Guaraní, Quer- andíes, and different indigenous Chaco groups (Fig. 7).
• The Chanátimbú (including Timbú and Carcaraes people) were canoe groups that lived mainly in the alluvial plain and islands of the Middle and Lower Paraná River, systematically exploited the fluvial resources, and cultivated maize, squash and beans.
• In order to answer this, it is necessary to assess what the archaeological and ethnohistorical evidence shows about metals in neighbouring areas of the Paraná River.

5. Conclusions

• The obtained results allow us to reach the following conclusions: 1. The metal objects recovered in the Paraná Delta were mainly manufactured in pure Cu and, in one case, with a bronze alloy.
• The analysed objects could have been made by melting small ingots, prills or pieces of scrap metal, and then casting in open moulds.
• Regarding the production sequence in the Delta collections, the authors have only finished ornaments, although unfinished sheets may have reached the area.
• Hypothetically, metals may have come through three indigenous routes: two of them, the Carcarañá River across the Mountain Ranges of Cordoba and the Salado-Dulce Rivers through the plains of Santiago del Estero, used by Chaná-timbú groups; and a third one, the Paraná-Paraguay River route through Chaco, followed by Guaraní populations.
• Imported objects could have generated differences within the groups, giving greater status to local individuals that possessed them and symbolizing their special privileges.

Figures

Fig. 5. Scanning electron microscope micrographies. (a) Superficial patina on TIBG N 9, (b) microstructure of TIBG N 10, (c) micro-structural variations due to mechanical deformations in TIBG N 10, (d) crater-shaped morphology of the perforated zone in TIBG N 6, (e) conglomerated material around the trim in TIIPG N 4, and (f) microstructure with small twins and corroded material around the trim in TIIPG N 4.

Fig. 6. Optical metallographic macrophotos of TIBG N 10 microstructure. (a) Detail of the micro-shrinkage cavities in the perforated zone, (b) oriented precipitates on the perimeter edge, (c) matrix where the arrow marks annealing twin on the perimeter edge, and (d) morphology of the phases in the perimeter edge.

Table 2 Archaeological sites with metallic objects from Paraná and Uruguay rivers (Middle and Lower sections) and the Northern Pampas region

Table 1 Weight, dimensions, morphology and possible function of the metal pieces from the sites Túmulo I and Túmulo II del Paraná Guazú and Túmulo I del Brazo Gutiérrez.

Fig. 1. Location of the archaeological sites with metal objects of the Torres Collection: (a) Túmulo I del Paraná Guazú and (b) Túmulo II del Paraná Guazú, c. Túmulo I del Brazo Gutiérrez.

Fig. 7. The three potential indigenous routes for metals (a) Francisco César (1528) and Francisco de Mendoza (1545) expeditions, (b) Francisco de Mendoza (1545), (c) Aguirre (1566) and Sotelo de Narváez (1583), (d) Guaraní Itatines (1528), (e) Ayolas (1537–38), and (f) Ribera (1544). Tentative location of observations of indigenous people with metal objects in the sixteenth century: 1, 2 and 3 Sebastián Caboto expedition (1526–30): 1. Querandíes with ‘‘plumages” on their heads of low quality gold and good silver; 2. Guaraní wearing plates, ear-ornaments and metal axes (of gold and silver); 3. Guaraní wearing many ear-ornaments and plates of very good gold and silver; 4 and 5 Martim Afonso de Sousa expedition (1530–32): 4. Indigenous people wearing copper pieces in their nose holes; 5. A Chaná-beguá woman with metal objects on her ears; 6. Pedro de Mendoza (1535–37; journey of Juan de Ayolas in 1537): silver sheets and worn-out golden copper jewellery; 7. Alvar Núñez Cabeza de Vaca expedition (1541–44; journey of Francisco Ribera in 1544): non-Guaraní indigenous person wearing a big silver low lip plug in his lower lip, and silver ear-ornaments, and silver plates, hatchets and bracelets.

Fig. 3. Optical metallographic macrophotos of TIPG N 1 microstructure. (a) Edge of the complete suspension hole (marked by an arrow), (b) dendritic microstructure, (c) subgrains with precipitates (marked by an arrow), and (d) primary solidification microstructure of the edge of the incomplete suspension hole.

Fig. 2. Energy dispersive X-ray micro-analysis. (a) Particle of TIBG N 10, (b) Matrix of TIIPG N 4, (c) Piece TIPG N 1, and (d) Superficial patina of TIBG N 6.

Fig. 4. Scanning electron microscope micrographies of TIPG N 1. (a) Suspension hole morphology and (b) precipitates of the interdendritic spaces.

Frequently asked questions

Q1. What are the contributions in "Archaeometallurgy in the paraná delta (argentina): composition, manufacture, and indigenous routes" ?

The aim of these studies was to determine the chemical composition and the manufacturing techniques of these allochthonous objects frequently placed in human burials. 

Q2. What could be the reason for the differential treatment performed on these individuals when they died?

The differential treatment performed on these individuals when they died could be a sign of asymmetrical social relationships in which only some members of the society had preferential access to certain exchange goods. 

Q3. What are the common types of ornaments found in the Andes?

They are mainly ornaments often found in funerary contexts, which have been interpreted as prestige goods (Taboada, 2014), many of them as symbols of power and social status, which were possibly special gifts from the Incas to local Santiago del Estero populations as part of alliances or in exchange for some services. 

Q4. What was the main reason for the Guaran to travel to the west?

Even the search for metals without intermediaries and the direct theft from mining settlements were powerful incentives for the repeated population movements towards the west made by the Paraguay river Guaraní (named Itatines; Combès, 2015). 

Q5. What were the objectives of these studies?

The objectives of these studies were to determine the chemical composition of the objects and the existence of any sort of alloy, as well as to obtain information about the metallurgical manufacturing techniques. 

Q6. What did Francisco Ribera learn about the trade circuit?

He was informed there about the interethnic trade circuit that metal objects followed: they got the gold and silver from ‘‘the Payzunos, who are three days away from their land, they give them in trade for bows and arrows and slaves that they take from other nations, and the Payzunos obtain them from the Chanés and Chimenos and Carcaraes and Candires, who are other Indian people, who have lots of them. 

Q7. What are the contaminant elements that have been formed on the surface of the metal?

Beneath these layers made by salts, continuous reddish surfaces have been generated, which constitute passivation patinas formed by the oxidation of Cu. 

Q8. How long have metal objects been transported?

They have been transported for millennia for very long distances, from the sources and metal production workshops to the places of use, as observed in the southern South American Andes (e.g., González, 1992; González et al., 2011). 

Q9. What is the main route of metal ornaments and other products to the Paraná Delta?

According to early chronicles, one of the main indigenous routes through which metal ornaments and other products reached the Chaná-timbú people in the Paraná River was following the course of one of its tributaries, the Carcarañá River. 

Q10. What did the cultural developments in the NWA have on the Paraná Delta?

The cultural developments in the NWA did not have a direct impact on the Paraná Delta, although they possibly did affect a greater circulation of goods to remote areas. 

Q11. What are the only archaeological sites in Argentina that are not included in this paper?

They are only limited to indigenous settlements from the initial period of the Hispanic contact (Las Conchas site, Serrano, 1934 and possibly Arroyo Fredes site, Loponte et al., 2011) and settlements of European origin, which are not included in this paper, such as Santa Fe la Vieja city founded in 1573 (e.g. Fester and Retamar, 1955) and the Franciscan missions of Santiago del Baradero founded in 1615 (Debenedetti, 1910; Tapia et al., 2009). 

Q12. What is the morphology of the matrix with annealing twins?

In Fig. 6c and d the matrix with annealing twins was observed over a previous inter-dendritic phase, which is characteristic of molten material prior to this hot forming. 

Q13. What is the manufacture hypothesis of the rectangular pendant?

the manufacture hypothesis is that this piece was made by casting in an open mould or bivalve whose interior had the shape of the pendant and contained some sort of implement with an oval section that could craft the holes. 

Q14. What is the significance of the first European observations of the local indigenous people?

the first European observations of the local indigenous people can be very useful for understanding how the metals came to the Paraná River.