A pre-Columbian galvanic technique able to explain the gilding of copper in northern Peru

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Highlights

  • Gilding of copper artifacts made by Moche cultures of Peru is not well understood.

  • Electrochemistry has been suggested but it yields about 1 μm thick films.

  • 1 μm thick films do not match the spectrum of thicknesses found on the artifacts.

  • A galvanic method based on iron pyrite as anode produces thicker films.

  • Film characteristics obtained by the galvanic method match well with artifacts.

Abstract

Around 150 BCE and 700 CE, pre-Columbian goldsmiths in the Peruvian northern coastline developed a method for gilding copper. The characteristics of the resulting pieces are substantially different from those obtained by hammering, embossing, and casting. We discuss two electrochemical gilding methods that were possibly developed by pre-Columbian goldsmiths. The first method consists of electrochemical replacement where copper is immersed in a solution containing dissolved gold. The second method utilizes the same solution but includes pyrite to form a galvanic cell with copper as the cathode and pyrite as the anode. Characterizations via X-ray fluorescence (XRF) and Scanning electron microscopy with energy dispersion spectroscopy (SEM-EDX) allowed a comparison of the results of the two methods with archaeological samples. The electrochemical replacement method does not reach thicknesses observed in archeological samples and delivers irregular gilding due to the formation of anodic spots on copper. Meanwhile, including pyrite as an anode in the electrochemical cell leads to a more homogeneous deposition of gold with layer thicknesses similar to those found in archaeological samples. This work contributes to understanding pre-Columbian techniques lost before the Inca Empire.

Introduction

Through the centuries that preceded the Spanish conquest, ancient Peruvian metalworkers developed gilding techniques to transform copper surfaces into different hues of gold and silver (Bray, 1993, Niece and Meeks, 2000). Those gilded metal objects were part of the costumes of the elite desiring to express their power through the use of ritual paraphernalia (Velarde and Castro de la mata, 2010). Most gilded copper objects recovered from burial sites are now part of museum collections where severe corrosion put their integrity at risk. Therefore, understanding the gilding techniques is essential to enrich our knowledge of the Andean metallurgical development and provides scientific grounds for proposing conservation strategies to preserve our metal cultural heritage.

Among the limited research done in this field, H. Lechtman first suggested and recreated the use of electrochemical replacement as a gilding technique used by the Moche, Vicus, and Recuay cultural groups from the north coast of Peru during the Early Intermediate Period (150 BCE–700 CE) (Lechtman, 1979, Lechtman et al., 1982, Lechtman, 1984, Centeno and Schorsch, 2000). The study was based on the fact that the characteristics of the gilding layer did not correspond to those obtained by other techniques, such as hammering, embossing, and casting. As a result of Lechtman’s experimental work, dipping copper substrates in a gold solution composed of salts that simulate aqua regia ions, produced up to a 1 μm layer of gold coatings. However, analyses of Moche, Vicus, and Recuay ornaments have shown that the gilding layer can sometimes be more than 4 μm thick (Velarde and Castro de la mata, 2010).

As an alternative to the self-exhausting electroless method proposed by Lechtman (Lechtman, 1979, Lechtman et al., 1982, Lechtman, 1984), we are introducing pyrite (FeS2) as a sacrificial electrode that prolongs the plating process in order to get a thicker gilded layer. Pyrite was well known to the northern cultural groups who took advantage of its reflective properties and used them as mirrors and decorative objects. Further, there is also evidence that pyrite was used as dust in funerary rituals (Petersen and Brooks, 2010). Thus, based on the materials known at the time, we propose an alternative method to obtain gilded copper objects, by changing the reaction principle from purely electrochemical replacement to a galvanic. We provide evidence that by considering pyrite as a sacrificial anode, it is possible to produce gilded copper that matches the characteristics of archaeological samples taken from several sites in northern Peru.

Section snippets

Materials and methods

Copper coupon preparation. Copper coupons (5 cm length × 2.5 cm width × 0.1 cm thickness) were polished with sandpaper with grit sizes of 500, 800, 1200, and 2000 µm (SiC-Paper, Struers) to remove the oxide layer formed during annealing. Before deposition, each copper coupon was polished with 1.0 µm alumina slurry and cleaned in an ultrasonic bath (WUC-A01H, Witeg) of ethanol (99.9 %, Merck), followed by acetone (99.8 %, Merck) for 5 min in each bath.

Electrolytic bath preparation. The

Description of archaeological samples

Archaeological samples found in northern Peru show a gold thickness of up to 6 μm (see Table 1). Lechtman reported that electrochemical replacement (or displacement) gilding could lead to a layer thickness of 3 μm (Lechtman et al., 1982). However, theoretically speaking, replacement deposition should provide a maximum thickness of around 1 μm only (Ali and Christie, 1985), generating smooth thickness profiles not susceptible to the dog-boning effect (Scheunert et al., 2015). As commented by

Conclusions

We have conducted experiments to understand the deposition of gold layers that are seen in the gilded copper artifacts found in northern Peru. This understanding forms the basis of possible intervention strategies that are needed to be implemented to prevent further corrosion of these heritage objects under current atmospheric conditions. Previous work in the literature describes a possible process for depositing the gold layer on copper objects by electrochemical deposition. This is certainly

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been funded by the CONCYTEC (Peru) project Improvement and Expansion of the Services of the National System of Science Technology and Technological Innovation 8682-PE, through its executive entity ProCiencia (contract number 035-2019). We thank the Department of Chemical Engineering at UTEC for facilitating the use of its laboratories and electrochemical cells. Karinna Visurraga is acknowledged for their administrative support.

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