Abstract
A large number of metal artefacts with exceptional artistic value of the Moche culture have been found in the tombs of the Lords of Sipan (Lambayeque, Peru) and of the Lady of Cao (El Brujo, Peru) characterised by different burial conditions. Some of the objects, dated around 300-400 AD, are constituted by substrates of Cu- or Ag-based alloys coated by uniformly distributed thin films of precious metal (1 - 4 microns) that create also polymetallic bicoloured surfaces with “gold” and “silver” areas. In order to investigate the corrosion product structure and composition as well as to identify the techniques used to give the gold or silver appearance, an integrated analytical approach has been adopted. The selected complementary methodologies were scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and optical microscopy (OM). The findings reveal that the substrates are mainly composed by Cu-Ag-Au alloys that at the site of Sipan have been almost completely corroded during the burial. Furthermore, the results show that the main aggressive agent is Cl- coming from the soil and that the degradation phenomena were likely enhanced by the galvanic coupling between the precious metal layer and the less noble substrate. The degradation products have formed mainly layered structures containing chloroargyrite (AgCl), cuprite (Cu2O), nantokite (CuCl), and atacamite (CuCl23Cu(OH)2) polymorphs. These latter species warn that dangerous copper cyclic corrosion is occurring, a harmful phenomenon, commonly defined as “bronze disease”, which must be be firmly mitigated. Finally, the findings reveal that the Moche metal workers used the depletion gilding to selectively modify the surface chemical composition of the artefacts to produce the Ag or Au thin films. According to this subtractive method, the surface of the Cu-Au-Ag alloys was enriched with a layer of precious metal by means of cycles of thermal treatments and removal of Cu or both Cu and Ag from the outermost region by using pickling solutions.
Highlights
In 1987, great treasures composed of spectacular metal artifacts were discovered in the unlooted tombs of the Lords of Sipán (Huaca Rajada, Lambayeque, Peru), and in 2006, in the tomb of the Lady of Cao (El Brujo, Trujillo, Peru)
This structure suggests to the conservators to pay a great deal of attention during the cleaning procedure to avoid the loss of the thin gold layer that could be peeled away (Scott, 1990, 1991, 2002; Ingo et al, 2013b)
This structure is similar to that observed in the nariguera found at Sipán in the tomb 16 and shown by the scanning electron microscopy (SEM) and optical microscopy (OM) images reported in Figure 3; from the comparisons between these structures, we can evaluate the effect of the long-term corrosion phenomena that occurred in the soil to the buried Ag-Cu-Au artifacts
Summary
In 1987, great treasures composed of spectacular metal artifacts were discovered in the unlooted tombs of the Lords of Sipán (Huaca Rajada, Lambayeque, Peru), and in 2006, in the tomb of the Lady of Cao (El Brujo, Trujillo, Peru) These sites are characterized by different burial conditions that have influenced the long-term corrosion behavior. In order to investigate the microchemical surface and bulk structure and the degradation products formed during the longterm burial, we have adopted an integrated analytical approach by using complementary methodologies such as scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and optical microscopy (OM). We point out that the selected artifacts were not objects of common use but ornaments for an elite exhibiting power and rank They include high-status ceremonial objects and spectacular nose ornaments, i.e., the narigueras, large jewels to be hooked to the nostrils. Other experimental details are reported elsewhere (Ingo et al, 2000, 2002)
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