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Long-term corrosion of copper alloys in the soil: new aspects of corrosion morphology in archaeological vessels from south-western Iran

Heritage Science volume 12, Article number: 73 (2024) Cite this article

Abstract

A group of copper-based objects excavated at Deh Dumen cemetery, in south-western Iran, was  studied and analysed to examine the long-term corrosion morphology and mechanism in the soil burial environment. For this purpose, twenty-two samples from twenty-one copper-based vessels were studied and analysed using X-ray diffraction, scanning electron microscopy—energy dispersive X-ray spectroscopy, micro-Raman spectroscopy and metallography techniques. The results of the analyses showed that the majority of vessels are made of tin bronze, along with two arsenical copper samples. The extent of corrosion observed ranges from very thin corrosion crusts to thick crusts and entirely corroded structures. These three identified corrosion morphologies display a multi-layered corrosion stratigraphy as well as the preserved limit of the original surface. The corrosion crusts include internal tin-rich and external copper-rich layers, and the main corrosion mechanism for the formation of multi-layered corrosion crusts is decuprification or selective dissolution of copper during the long-term burial time in a moderately Cl-contaminated soil. The three identified corrosion morphologies are similar to the previously published morphologies, but some clear deviations are apparent and are discussed here.

Introduction

Archaeological copper alloys, including tin bronze, brass, and arsenical copper, show variable morphological aspects of corrosion after long-term abandonment in burial environments such as soil and seawater [1,2,3,4,5,6,7,8]. These morphological aspects are strongly related to the corrosive factors in the burial environment, although the metal/alloy composition and metallurgical characteristics also impact the corrosion mechanism and rate [3, 9,10,11]. The corrosion of archaeological copper alloys—and tin bronze, in particular—has been studied extensively during recent decades, and some typological morphologies and typical mechanisms have been identified and established [2, 12,13,14,15]. More recent studies revealed some new and interesting characteristics [16,17,18,19,20,21]. Therefore, it is necessary to continue investigation on excavated archaeological copper alloys to better understanding long-term corrosion mechanisms, and these results could also be useful for conservators.

Robbiola et al. suggested a model for corrosion morphology in archaeological tin bronzes buried in aerobic burial environments in which two different corrosion morphologies form in single-phase α-tin bronzes according to the corrosivity of the soil environments [2]. This finding is based on the determination of the dissolution factor of alloy components and the presence or absence of the limit of the original surface within the corrosion layers. This classification explained the corrosion morphologies in archaeological bronzes, but some deviations were observed in other studies [3, 22]. Supplementary models of corrosion morphology and mechanism for archaeological tin bronzes have been presented elsewhere [17, 23, 24], focusing on wrought structure [17], but Robbiola’s work has been the basis on which many hypotheses have been developed to understand bronze corrosion in the soil.

The presented research provides scientific analysis of corrosion morphology of archaeological tin bronze vessels excavated from Deh Dumen archaeological site, south-western Iran, and provides detailed documentation of the conservation condition in the metallic collections. The current work also introduces an update of knowledge concerning these issues based on analysis of some bronze (and arsenical copper) objects in order to interpret their corrosion morphology and places the obtained data in the context of previously published results. The aim of this paper is to understand the corrosion morphology in excavated and untreated archaeological vessels in the 2010s from the Bronze Age (ca. 3000–1500 BCE) of south-western Iran [25]. The results are compared with published corrosion profiles identified in bronze artefacts.

The ancient cemetery of Deh Dumen is located in south-western Iran, about 70 km north-west of the city of Yasuj, the capital of the Kohgiluyeh and Boyer-Ahmad province (34° 46′ 84′′ N, 51° 02′ 99′′ E) (Fig. 1a), partially encircling the Zagros fold and thrust belt chain [26]. The archaeological site is in the Khersan river valley on the western side of the river (Fig. 1b). The Deh Dumen cemetery is an important Iranian archaeological site due to its cultural and trade relationship between the western and eastern regions of the Iranian Plateau and the Indus valley, as evidenced by the presence of objects that are similar to those found in other sites [27,28,29,30]. Archaeological excavations in the cemetery of Deh Dumen have been carried out in four field campaigns from 2013 to 2023. The samples are taken from objects excavated in 2013 and 2019 (Table 1).

Fig. 1
figure 1

a Map of Iran and location of Deh Dumen archaeological site in the Kohgiluyeh and Boyer Ahmad province, south-western Iran; b the location of the Deh Dumen site at the western side of the Khersan river

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Table 1 Characteristics of twenty-two samples from twenty-one copper-based vessels from Deh Dumen, analysed in this research
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The vessels studied in this paper are excavated from the Deh Dumen Bronze Age cemetery in the Dena region, south-western Iran. Some experimental studies have been conducted on corrosion features of this site [31], but the current work is the first to investigate the corrosion morphologies comprehensively in these objects. Also, the Deh Dumen bronze collection provides this opportunity to study the corrosion morphology in a well-documented large scale bronze collection from the Bronze Age of Iran. Therefore, study on corrosion morphology in this bronze collection can help conservators to understand the collection’s condition, help them in conservation decision-making, and in developing a long-term preventive conservation approach for this bronze collection.

Materials and methods

Archaeological materials and sample preparation

To study the corrosion morphology of excavated bronzes from the Deh Dumen cemetery, twenty-two samples from twenty-one copper-based vessels excavated from 2013 to 2019 were selected (Fig. 2, Table 1). These investigated objects are different untreated, uncleaned, and unrestored vessels, stored in polyethylene bags and boxes after excavation and kept in an environment with relative humidity less than 50%, some of which have been analysed previously to characterize the microstructure, alloy composition and provenance studies [27, 28, 32]. The investigated objects include the vessels that were broken during burial and now are used for experimental analysis. These broken objects were selected to facilitate the preparation of samples for experimental investigations. One small piece from each broken object was selected to perform analyses, and two samples were selected from vessel No. DP-190 (DD-18 and DD-19). Analysis of surface corrosion layers took place on a cross-section of each sample. The samples were embedded in epoxy resin and then ground by abrasive paper (240 to 5000 grid size). Finally, the cross-sections were polished with 3 and 1 μm diamond pastes.

Fig. 2
figure 2

a Some studied copper-based vessels from Deh Dumen; b two tin bronze vessels at the situation that they were discovered in burial site (DD-21 and DD-22) [28]

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Instrumentation

To study the corrosion morphology, the cross-sections were observed using optical microscopy (OM). For metallographic observations, a Zeiss Axio Imager M2m microscope, with 50 × , 100 × , 200 × , 400 × , and 500 × magnifications, an Axiocam HRc digital camera, and AxioVision 4.X.X software were used. All cross-sections were observed in bright-field and dark-field illuminations.

The cross-sections were then studied and analysed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDS) technique. The microstructure and corrosion layers of the samples were observed at different magnifications in the backscattered electron (BSE) mode. The chemical composition of the corrosion layers was detected using EDS analysis. The cross-sections were carbon-coated to make a conductive surface for SEM image preparation. Scanning electron microscopy-energy dispersive X-ray spectrometry analyses (SEM–EDS) were performed with a FE-SEM Zeiss Σigma HD equipped with an Oxford Instrument X-MaxN 80 SDD detector. Backscattered electron (BSE) images, EDS analysis, and X-ray mapping were conducted with an accelerating voltage of 20 kV in high vacuum, with a spot size of ~ 1 nm. The size of the analysed area for measurement of alloy composition was ca. 150 × 150 microns and for the corrosion layers was based on the thickness of the layer.

Analysis of corrosion products was performed using X-ray Diffraction (XRD) technique. For this purpose, 0.5 g of surface corrosion products from all samples was removed mechanically by a scalpel and examined by XRD equipment, D8 ADVANCE model (Bruker, Germany), CuKα source with wavelength 1.54 Å and 2θ between 5° and 80°, at the Central Laboratory of the University of Isfahan, Iran. The X-ray diffractograms were characterized by using the Powder Diffraction File (PDF), a database produced and maintained by the International Centre for Diffraction Data (ICDD®).

Raman analysis was performed on the corrosion layers of some selected samples using a Bruker Senterra spectrometer equipped with an Olympus 50 × long working distance microscope objective and a charge-coupled device (CCD) detector. A Spectra Physics Cyan solid-state laser and a continuous wave diode laser emitting at 632 nm was used as the excitation source, and two holographic gratings (1800 and 1200 rulings/mm) provided a spectral resolution of 3–5 cm1. The output laser power was 20 mW, but the number of scans, and integration time were adjusted according to the Raman response of the different samples.

Results

Chemical analysis of core metal

The preliminary observation of the samples cross-section with optical microscopy revealed that objects could be classified into two different groups according to their corrosion layers: partially corroded objects with surface corrosion layers and significant amounts of metal under the corrosion layers (nineteen samples) and, completely corroded objects in which no significant uncorroded metal is retained (three samples).

SEM–EDS analysis of the alloy composition of the first group of objects is presented in Table 2. Results are the average of three EDS analyses for each sample. Results showed that eighteen objects are made of binary tin bronze with tin content between ca. 7 and 12.5 wt% of tin. One sample displayed a different composition: the metal fragment inside the vessel’s base (DD-18) is made of arsenical copper with 2.6 wt% of arsenic. The composition of the second group of objects was not calculated by SEM–EDS, but the semi-quantitative analytical data on the internal part of the cross-section showed that two objects are also made of tin bronze (DD-07 and DD-20) while DD-14 is likely made of arsenical copper.

Table 2 Results of quantified SEM–EDS analysis of the alloy composition in nineteen partially corroded copper-based samples with remains of metal structure (wt%)
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Twenty vessels are made of tin bronze while one vessel and one vessel’s base are made of arsenical copper (DD-14 and DD-18). These arsenical copper objects may have been produced due to smelting As-bearing copper ores, as was explained before in detail [28, 32]. It should be mentioned that the microstructure of all objects presented a single-phase solid solution of copper with no evidence of segregated high-tin phases or compounds.

Stratigraphy of corrosion layers

Eighteen partially corroded tin bronze objects show a complex corrosion crust including multiple distinct layers. The multi-layered corrosion structure in these tin bronze vessels shows two different morphologies. In the first morphology, a two-layered stratigraphy is observed in some bronze vessels (M1) in which the two main corrosion layers developed under the limit of the original surface (OS) (Fig. 3): layer A formed under the original surface as an almost uniform corrosion layer. It is very thin (20–50 µm) and dark-grey in colour. The original grain microstructure of bronze is retained as a pseudomorphic structure in this layer, and layer B formed between layer A and the alloy substrate, with very low thickness (< 10 µm). It is more visible as an inter- or transgranular attack, red in colour. It should be noted that two additional very thin layers are observable in some samples as external corrosion crusts (Fig. 3): layer C is an irregular red corrosion layer formed over the original surface with a very low thickness. This layer does not cover the samples entirely and is not present in all samples of this morphology, and layer D is an irregular green corrosion layer formed over the original surface with a very low thickness. This layer does not cover the samples entirely and also is not present in all samples of this morphology.

Fig. 3
figure 3

Optical and SEM-BSE micrographs of three samples of the M1 morphology with very thin internal smooth patina (layers A and B) and some thin external corrosion layers (C and D). The limit of the original surface (OS) is obviously visible

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In the second characterized morphology, the other bronze vessels display a more complex multi-layered corrosion (M2) in which four corrosion layers are well-developed under and over the limit of the original surface (Fig. 4): layer A formed under the original surface as an almost uniform, thick corrosion layer. It is variable in thickness (20–200 µm), varied in colour from dark-grey to red and orange in different samples. The original grain microstructure of bronze is retained as a pseudomorphic structure in this layer. layer B is formed between layer A and the alloy substrate. Layer B is variable in thickness, red in colour, and more visible as an inter- or transgranular attack revealing the twin bands and slip lines and the grain structure of the bronze matrix, sometimes covering significant parts of the metal substrate, layer C formed over the original surface, red in colour and low in thickness (20–200 µm), and layer D formed over Layer C as the outermost corrosion layer. It is green in colour and sometimes is mixed with soil particles. Its thickness is variable in different samples and also within areas of the same sample (10–500 µm). This layer is also covered with a thin layer of soil contamination.

Fig. 4
figure 4

Optical and SEM-BSE micrographs of four samples of the M2 morphology with thick internal corrosion layers (A and B) and thick external corrosion layers (C and D). The limit of the original surface (OS) is obviously visible

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Layer B observed in both corrosion morphologies introduced above, mostly includes inter- or transgranular attacks and, in fact, resembles localized corrosion. This thin red (or orange) layer represents the early stages of corrosion/oxidation/dissolution of copper caused by a corrosive solution penetrating into the metal through the crack. If this becomes extensive, this localized corrosion may result in cracking and embrittlement of archaeological tin bronzes, especially if they retain large amounts of deformation in the microstructure due to thermo-mechanical operations [33, 34], as has been identified before in Deh Dumen vessels [28, 31, 32].

As mentioned above, three vessels showed a heavily corroded microstructure in which the metal substrate is not visible in the cross-section of the sample. These vessels also include multiple corrosion layers, although the limit of the original surface is retained between the corrosion crust. The distinct layers observed in these objects include (Fig. 5): layer A formed as an almost uniform layer under the original surface. It is red or dark-red in colour and its thickness is between 100 and 200 µm in samples DD-07 and DD-20. In sample DD-14, this layer covered all parts of the internal layer below the original surface, layer B is green in sample DD-07 and DD-20 and covered almost all internal parts of the object below layer A, although sometimes some islands of the dark red corrosion product are visible in this thick internal matrix. This is not observed in sample DD-14 as a uniform layer, but rather some green islands are scattered in the internal red matrix of this sample, layer C formed over the original surface as an almost uniform dark red corrosion layer with 50–100 µm thickness, and layer D is a green layer covering the layer C. It is the outermost corrosion layer, though it is sometimes covered with a soil contamination layer. Its thickness is variable from 50 to 200 µm.

Fig. 5
figure 5

Optical and SEM-BSE micrographs of three samples of the completely corroded M3 morphology, DD-07, DD-14 and DD-20, with internal corrosion layers (A and B) and thick external corrosion layers (C and D). The micrographs of the arsenical copper sample (DD-18) are very similar to M2 morphology. The limit of the original surface (OS) is obviously visible

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The only arsenical copper sample with metal residue (DD-18) also shows a partially multi-layered corrosion microstructure similar to most of the tin bronze vessels, although in contrast, external red and green layers (C and D) are mixed in many areas and layer B is not visible similar to tin bronze vessels despite some very small corrosion attacks that are visible below Layer A. However, it should be mentioned that the corrosion crust of this Cu-As object could be classified as a four-layered corrosion morphology as follows (Fig. 5): layer A formed under the original surface in dark-red to dark-brown colour. Its thickness is variable from 50 to 300 µm in different areas of the object, while some very tiny intergranular attacks are formed under this layer, layer B formed as some local red–orange attacks under layer A, layer C formed as a non-uniform corrosion layer over the original surface and with very low thickness (10–20 µm) with some thicker projections, layer D covered the original surface and is green in colour with numerous islands of dark-red corrosion phase. Its thickness is significant and sometimes reaches 400–500 µm.

The two internal layers (A and B) are common in both M1 and M2 morphologies of tin bronze vessels, although their thickness is different. Figure 6 shows SEM-BSE micrographs of samples from different morphologies in higher magnifications in which the internal corrosion layers (A and B) show pseudomorphic replacement of the alpha matrix with the corrosion products. In fact, the ghost microstructure of the bronze matrix including worked and annealed grains is well-preserved in the internal corrosion layers due to long-term corrosion processes in the burial environment, as also observed previously in corroded bronze objects [3, 12, 22, 24, 35,36,37,38]. This ghost microstructure can be deduced only in the internal corrosion layers under the limit of the original surface and is very helpful to characterize the original surface’s limit [39], while the extensive inter- and transgranular attacks (Layer B) could be visible in the alloy/corrosion interface of the M1 and M2 morphologies, as the results of corrosion penetration.

Fig. 6
figure 6

SEM-BSE micrographs of some details of the microstructure of the corrosion layers including the pseudomorphic replacement (ghost structure) formed in the internal corrosion layers (A and B)

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Results of semi-quantitative EDS analysis as well as elemental EDS mapping of partially corroded morphologies (Fig. 7, DD-03) showed that in M1 and M2 morphologies, layer A is tin-rich and has a lower Cu/Sn ratio in comparison with bronze matrix, and some evidence of the metallurgical characteristics of the original metal, such as pseudomorphic retention of the microstructure and elongated inclusions [17], are visible in the layer structure. It is worth noting that these inclusions are previously analysed and are composed of copper sulphide [28, 32]. Layer B, or the inner part of the internal crust, has a red/brown colour and an elemental composition containing Cu, Sn and O and a much higher Cu/Sn ratio than Layer A. The external layers are more commonly observed in the tin bronze vessels with M2 morphology, and their EDS analysis also showed that they are composed of copper with variable amounts of oxygen, as observed in sample DD-03. Furthermore, the elemental EDS mapping of an entirely corroded sample from M3 morphology (DD-20) showed high concentrations of copper and tin in the internal layers, while some copper- and chlorine-rich corrosion products with almost no tin are formed within the internal layers. The external corrosion layers (C and D) are similar to other morphologies from a chemical viewpoint.

Fig. 7
figure 7

EDS elemental map of the corrosion layers in two corroded tin bronze samples showing the different Cu/Sn proportion in the internal corrosion layer and the metal parts in sample DD-03 and presence of chloride corrosion product in internal part of sample DD-20

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Mineralogy of corrosion products

X-ray diffraction

Analysis of corrosion products of the vessels revealed a variety of copper corrosion products and soil minerals (Table 3 and Fig. 8). Cuprite (Cu2O) was detected as the major corrosion product in all samples while malachite (Cu2CO3(OH)2) was identified as the major and minor product in all samples (except for sample DD-01). Tenorite (CuO) and copper trihydroxychlorides (Cu2(OH)3Cl) were also found as minor phases in the majority of analysed samples. Cuprite and malachite are typical corrosion products in buried copper alloys, and they are formed together in corrosion layers of archaeological objects in moderately corrosive soils, in the presence of high moisture and CO2 content. In fact, cuprite is the first product of copper oxidation in soil, but in slightly alkaline soils, bronze will develop a green layer of malachite [40,41,42]. The presence of tenorite is an interesting aspect of corrosion in the Deh Dumen copper-based objects, as this copper oxide mineral forms in very specific conditions, including high temperature, high pH environment or high concentration of oxygen [40]. Nevertheless, it shows that the pH may has been alkaline in the burial environment (as is measured before about 8 [31]), or the concentration of oxygen may has been fluctuated during the long-term burial of the metal objects, as also has been observed in another copper-based collection from Iran and other regions [3, 43, 44]. An isomer of malachite, azurite (Cu3(CO3)2(OH)2), is a more rare corrosion product on copper and bronze objects in soil, and its formation strongly depends on the presence of environments rich in moisture and bicarbonate anion (HCO3−) [45, 46].

Table 3 Results of XRD analysis of corrosion layers in twenty-two samples from twenty-one copper-based vessels from Deh Dumen; XXX: major phase, XX: minor phase, X: trace phase
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Fig. 8
figure 8

X-ray diffractogram of corrosion products of some samples from Deh Dumen; Cup cuprite, Tnr tenorite, Mal malachite, Ata atacamite, Pata paratacamite, Cu copper, Bct brochantite, Qtz quartz, Cal calcite

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Copper trihydroxychlorides, or basic copper chlorides, are another group of corrosion products commonplace in archaeological copper alloys. The presence of chloride ions in burial environments may lead to the formation of nantokite (CuCl). During burial or after excavation, nantokite will form atacamite or paratacamite in the presence of moisture and oxygen. This so-called phenomenon "bronze disease" or “active corrosion” is a cyclic reaction that can take place in the presence of oxygen or chlorine ions, and the reaction will continue until all copper is transformed into copper corrosion products [40, 47,48,49]. This event is actually the reaction of copper with chlorine in the presence of moisture and oxygen that causes the formation of copper trihydroxychlorides (e.g., atacamite, paratacamite) and  will continue in the presence of moisture [40, 50].

Brochantite (Cu4SO4(OH)6) was detected as a minor corrosion product in one sample (DD-20). Basic copper sulphates (including brochantite) are rare corrosion products in buried copper-based objects and have been observed as the main corrosion products in metal monuments exposed to urban environments [40, 51, 52]. Brochantite has also been observed as a corrosion product in some archaeological bronzes and has been attributed to the oxidation of copper sulphides which result from bacterial activities in the burial environment [53], or from high concentrations of soluble sulphate anion in the burial environment [3].

Copper is also detected as a minor phase in a significant number of samples. It may be introduced from the metallic substrate under the corrosion layers that has been mixed with the samples during mechanically removing the corrosion crusts for XRD analysis.

Micro-Raman spectroscopy

Raman spectra of four different layers in mounted cross-section (before carbon coating) from five objects of M1 and M2 morphologies are presented in Fig. 9. Table 4 also presents the peaks and identified phases in the different layers. The Raman spectra of layer A in three samples (DD-01, DD-08 and DD-15) show notably weak Raman peaks. In fact, layer A demonstrates no specific peak in sample DD-01, while some partially intensive peaks are visible in 84 and ~ 270 cm−1 in two other samples (DD-08 and DD-15). Additionally, one weak Raman peak at 620 cm−1 in sample DD-08 corresponds to cassiterite (SnO2) [54, 55]. This tin oxide compound forms more as an amorphous or non-crystalline corrosion product in the internal corrosion layer of tin bronzes and it is not easy to detect through techniques such as XRD, but µ-Raman is a helpful technique to find traces of cassiterite [10]. Layer B analysed in three samples (DD-03, DD-08, DD-13) shows very similar Raman spectra with intense peaks at 148, 220 and 624 cm−1 that correspond to cuprite, although a peak is also observed in 308 cm−1 in sample DD-08 that can be attributed to tenorite [56,57,58], as was also detected with XRD technique as a minor/trace corrosion product. The Raman spectra of layer C in the same samples also displayed similar peaks at 136–137, 220–221 and 627 cm−1 corresponding to cuprite.

Fig. 9
figure 9

µ-Raman spectra of four different corrosion layers (A, B, C and D) observed in some selected samples from M1 and M2 morphologies

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Table 4 Results of µ-Raman analysis on different corrosion layers in some copper-based vessels from Deh Dumen. The bold values are the main Raman peaks corresponding to the identified corrosion products 
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Raman peaks related to the green layers revealed malachite as the main corrosion product. Intense Raman peaks are observed at 57, 74, 114, 120, 147–148, 169–173, 270–272, 433–436, 1095–1098 and 1491 cm−1 respectively [59,60,61]. It is worth noting that a moderate peak was observed at ~ 511 cm−1 in the green layer of three samples that may correspond to copper trihydroxychlorides (atacamite or paratacamite) [62, 63].

Figure 10 shows Raman spectra of different corrosion layers present in a completely corroded sample (DD-14) and the arsenical copper sample (DD-18). The red layers of sample DD-14 (layers A and C) show similar Raman spectra with intense peaks at 147–148, 220 and 623 cm−1 related to cuprite, while the moderate peak observed at 296 cm−1 is related to tenorite [56,57,58]. Two green layers show the Raman spectra of malachite, including strong peaks at ~ 120, ~ 149, ~ 180, ~ 270, ~ 440 and 1491 cm−1 [59,60,61], although a moderate peak is also observed at 515 cm−1 in layer D that corresponds to atacamite or paratacamite [62, 63].

Fig. 10
figure 10

µ-Raman spectra of different corrosion layers observed in sample DD-14 from M3 morphology and DD-18, the arsenical copper sample

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Discussion

Corrosion morphologies

Accordingly, three corrosion morphologies observed in the Deh Dumen copper alloy objects could be classified as follows:

  • M1 morphology is observed in some tin bronze vessels and includes a very thin two-layered internal corrosion crust with some evidence of a very thin external layer. The original surface is the outermost part of the corrosion crust.

  • M2 morphology is observed in most of the tin bronze vessels and an vessel's base arsenical copper and includes a four-layered corrosion crust with multiple internal and external corrosion layers, and the limit of the original surface is visibly retained between the layers.

  • M3 morphology in which the metal substrate is converted entirely to corrosion products, although the limit of the original surface is also visible between the multi-layered corrosion structure.

The results of observations and analyses showed that all samples were corroded forming a tin-rich corrosion layer (layer A) while there is a variable thickness of red corrosion attack in the interface of layer A and alloy (layer B). This tin-rich internal corrosion layer, known also as noble patina, is typically observed in moderately corroded tin bronze objects and is attributed to copper leaching or decuprification along with internal oxidation of tin during burial time [2, 55, 64, 65]. This process causes to relative enrichment of tin in the corrosion layer in comparison with the alloy [10, 66]. The internal tin-rich surface layer is composed of copper and tin oxides (non-crystalline tin oxide, cuprite and tenorite) and demonstrates a variety of colours influenced by the penetration of soil elements in the layer during corrosion [2, 12, 67]. The tin oxide IV or cassiterite, which forms in the noble patina, is difficult to detect using mineralogical analytical techniques [2, 3, 10]. This is the reason why copper oxides (cuprite and tenorite) were identified as major compounds in the noble patina or internal layer (A) and even the innermost internal layer (B).

Decuprification—dissolution factor

To confirm the identity of this event in the corrosion mechanisms of archaeological bronzes, dissolved copper in a corrosive environment has been calculated in relation to the amount of atomic tin in the alloy using the method for the determination of dissolution factor of alloy components developed by Robbiola et al. [2, 68] and Chiavari et al. [66]. The copper dissolution factor (fCu) has been calculated for both layers by the following equation:

$${\text{f}}_{{{\text{Cu}}}} = 1 - \frac{{\left(\frac{{X_{Cu,p} }}{{X_{Sn,p} }}\right)}}{{\left(\frac{{X_{Cu,a} }}{{X_{Sn,a} }}\right)}}$$

where XSn and XCu are respectively amounts of Sn and Cu, and in which p indicates the corrosion layers and a indicates the alloy, assuming \(X_{Sn,a} + X_{Cu,a} = 1\). The copper and tin amounts (XSn and XCu) in both layers and alloy have been obtained from results of quantitative SEM–EDS analysis based on atomic percent or atomic fraction (A%), by the average of three analyses. If the Cu/Sn ratio in the studied layer is equal to the ratio in the alloy, fCu = 0, and when all of the Cu has been dissolved and removed, fCu = 1. Thus, a higher value of fCu indicates more copper dissolution during the burial time [68].

For calculating copper dissolution factor, EDS analyses of layer A and B were performed on 18 tin bronze vessels with remnants of uncorroded alloy (three analyses for each layer). Then fCu was calculated based on atomic percent using the equation presented above. Then the mean fCu of three analyses from each sample were calculated. Based on the measured means, fCu in layer A in the investigated samples ranges from 0.56 to 0.93 where XSn,a ≤ 0.07. The calculated value for fCu in all samples is 0.82 with a standard deviation of 0.12 (0.82 ± 0.12), significantly lower than the value calculated by Robbiola et al. [2] and Oudbashi et al. [22]. Calculated fCu in tin bronze objects buried in Cl-contaminated corrosive environments is not constant and shows lower and variable content, indicating that the dissolution factor of copper is variable and lower in Deh Dumen vessels [69]. The value of fCu was also calculated for layer B from 0.41 to 0.82 where XSn,a ≦ 0.07. The mean calculated value for fCu in all samples is 0.59 with a standard deviation of 0.12 (0.59 ± 0.12).

Figure 11a shows the scatter plot of fCu versus XSn,a calculated in the 18 analysed tin bronze vessels. Figure 11a demonstrates that the factor of copper dissolution is not constant amount for both layers A and B but is variable in the studied samples. Of course, fCu is consistently higher in layer A than layer B. In fact, there is no apparent relationship between ƒCu and XSn,a in both layers, although the data seem to have a linear behaviour, with a constant slope.

Fig. 11
figure 11

a Bivariate scatter plot of ƒCu versus XSn,a, in alloy-layer A and alloy-layer B [2] based on atomic fraction of Cu and Sn measured with SEM–EDS technique; b Cu-Sn–O ternary diagram based on atomic fraction showing the relationships between Cu, Sn, and O of the external and internal corrosion layers (A, B, C and D) in the corrosion crusts for samples in M1 and M2 morphologies, measured with SEM–EDS technique [17]

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Figure 11b presents the Cu–Sn–O Atomic% ternary diagram based on the concentration of the elements measured with EDS analysis [17]. The plot shows that layer C, the external red layer formed directly over the original surface, is composed of cuprite (Cu2O) while layer D, the outermost green layer, is mainly composed of copper carbonates (malachite or azurite). It is worth noting that chlorine is detected with EDS as a minor constituent in the green corrosion layer (D), indicating the presence of copper trihydroxychlorides as minor corrosion products combined with malachite. On the other hand, the two internal corrosion layers (A and B) demonstrate a composition between copper oxides (cuprite and tenorite) and tin oxides, revealing that these are formed of a mixture of oxides of two main alloy constituents. Layer A should be mainly composed of tin oxide (probable cassiterite or SnO2) with some amounts of copper oxide, while layer B tends more to copper oxides.

The elemental EDS map of the cross-section of a heavily corroded vessel (Fig. 6, DD-20) shows that the internal corrosion layers contain chlorine as the main constituent. In fact, the internal green layer (layer B in M3 morphology) is composed of copper and chlorine, probably combined with copper carbonates.

Based on results of corrosion studies on the bronze samples, it is clear that the corrosion mechanism occurred in Deh Dumen objects is the selective dissolution of copper, followed by the internal tin oxidation and the reaction of dissolved copper with soil anions with deposition of basic copper carbonates (as well as chlorides) on the surface. This process causes the formation of a thin crust of corrosion products over the original surface. The main corrosion products include copper oxides (cuprite and tenorite), and to a lesser extent, malachite and copper trihydroxychlorides, the presence of which suggests that the burial environment is contaminated with soluble chloride ions. Previous analytical work on soil of the site corroborated the presence of low concentrations (14–18 µg/kg) of soluble chloride in the burial environment [31]. Presence of chloride in the burial environment usually leads to heavily corroded bronzes that in some cases may cause the complete transformation of metallic bronze to corrosion products [9, 40, 47]. In the Deh Dumen bronzes, evidence of bronze disease is observed due to low concentrations of basic copper chlorides determined in the chemical composition of some corrosion layers.

Figure 12, generated using the online DSS Microstructure and Corrosion of Metals Database (MiCorr) system [70], shows a schematic representation of the three different morphologies observed in the archaeological bronze vessels from Deh Dumen.

Fig. 12
figure 12

Schematic representation of the corrosion structure and different morphologies of Deh Dumen bronze artefacts generated by the online DSS Microstructure and Corrosion of Metals Database (MiCorr) system [70]

Full size image

It is worth noting that there are four specific layers in all morphologies, including two internal corrosion layers (A and B) and two external corrosion layers (C and D). The limit of the original surface is preserved in all morphologies. The internal layers present a pseudomorphic replacement of the bronze matrix with copper and tin oxides, although some copper carbonates (probably mixed with copper trihydroxychlorides) are formed in the internal layer of M3 objects. The external layers (C and D) are also formed of cuprite and malachite respectively.

M1 morphology observed in some tin bronze vessels is consistent with typical corrosion previously observed in archaeological bronzes. Formation of the two-layered (or three-layered) corrosion on the surface of buried bronzes with preserving the limit of original surface and pseudomorphic replacement could be referred to type I corrosion stated in literature previously. Furthermore, M2 morphology showed no similarities with type I or type II morphologies that are described previously:

  1. 1.

    This is a four-layered corrosion morphology.

  2. 2.

    A thick layer of cuprite formed in the external part, over the original surface.

  3. 3.

    The innermost corrosion layer (B) is very thick.

In fact, this morphology also has some similarities with type II corrosion described by Robbiola et al. [2], in which the limit of the original surface is an area in a thick cuprite layer, but the original surface is not visible. In M2 morphology, the original surface is located under a thick layer of cuprite and is clearly visible. The cross-section of samples also shows that objects of this group are corroded heavily, and a significant fraction of the internal material is converted to corrosion products. The morphology of the heavily corroded groups (samples DD-07, DD-14, DD-20) is similar to the known type II corrosion of bronzes, although the limit of the original surface is also observable in the cross-section. The significant concentration of soluble chloride ions in the burial environment may lead to formation of copper trihydroxychlorides as the main corrosion products, even in the presence of high concentrations of bicarbonate ions in alkaline soils [9, 71]. The formation of malachite as the main corrosion product in Deh Dumen bronzes with some traces of copper trihydroxychlorides could be due to a high concentration of bicarbonate ions and significant concentration of chloride ions in the low alkaline soil [31].

Some objects with M1 morphology from Deh Dumen are similar to previously established type I morphology [2] but others, as well as all M2 objects demonstrated some deviations from it: the presence of an external copper oxide layer formed over the original surface of both, and very thick internal corrosion layers are very clear differences, as it is possible to consider M2 as a new model for long-term corrosion of tin bronze in soil. Also, the M3 morphology (entirely corroded microstructures) is very similar to previously established type II morphology, but some deviations are also apparent: the original surface is retained well between two layers of red copper oxides and the internal red oxide layer (A) shows the pseudomorphic replacement. Some small deviations from the two previously established corrosion morphologies were observed before in the Iranian tin bronze objects [3, 22] but the Deh Dumen tin bronze vessels exemplify an alternative long-term corrosion of tin bronze in soil as an interstitial morphology, as some characteristics of both type I and II morphologies are combined in one object. Nevertheless, the corrosion morphologies established and developed by Robbiola et al. [2] are very useful yet to interpret the corrosion mechanism and layers in the archaeological bronzes, although development of analytical studies on corrosion of archaeological tin bronze can help us to clarify some unknown aspects of this topic or develop new models for corrosion morphologies.

Conclusions

Investigation on corrosion morphology in twenty tin bronze vessels, as well as one  arsenical copper vessel and one arsenical copper vessel's base, from Deh Dumen Bronze Age site, south-western Iran was performed by microscopic and microanalytical techniques. Results identified three corrosion morphologies in the corroded bronze vessels including surface thin multi-layered noble patina (M1), thick multi-layered corrosion crust (M2) and entirely corroded and multi-layered corrosion (M3).

The limit of the original surface is preserved and fully observable in the cross-section of all samples, even in entirely corroded ones. According to the limit of the original surface preserved in all analysed samples, these three corrosion morphologies show a two-layered internal and a two-layered external corrosion crust. The M1 morphology could be compared with the well-known type I corrosion morphology of archaeological bronzes and M3 could be compared with type II. The M2 morphology deviates significantly from the two previously explained morphologies. The main deviations in the Deh Dumen corrosion morphology are the presence of an external cuprite layer formed over the original surface in all identified morphologies, and the preserved original surface even in entirely corroded samples. The thickness of internal corrosion layers in M2 morphology is another deviation, that may be due to significant corrosivity of the soil environment in some burial areas.

Consequently, the corroded tin bronze (and arsenical copper) vessels of Deh Dumen show generally similar morphologies to previously established type I and II corrosion morphologies, but they are different in some important details, that may lead to developing a new corrosion morphology in the future. These variations are likely due to the presence of chloride anions in the soil environment as well as high water levels due to very low distance between the site and the Khersan river. Generally, this study revealed some new characteristics of long-term corrosion of archaeological tin bronze in the soil environment.

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

The authors are grateful of Dr. Julia G. Bakker-Arkema from the Metropolitan Museum of Art for reading and editing the paper, and Dr. Federico Carò, Dr. Atefeh Shekofteh and Dr. Marco Leona from the Metropolitan Museum of Art for their supports and helps to perform SEM-EDS and Raman spectroscopy analyses.

Funding

This research has been supported by University of Zabol with the Grant Number: UOZ-GR-9618–100.

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  1. Department of Conservation, University of Gothenburg, Medicinaregatan 7 B, 41390, Gothenburg, Sweden

    Omid Oudbashi

  2. Department of Archaeology, University of Zabol, Bonjar Road, Zabol, Iran

    Reza Naseri

  3. Department of Conservation of Cultural and Historical Properties, Art University of Isfahan, Sangtarashha Alley, Hakim Nezami Street, Isfahan, Iran

    Parnia Asadi Hasanvand

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OO contributed to the sampling and analytical work and interpretation of the analytical data as well as preparing the draft of the manuscript; RN contributed to the sampling and interpretation of the analytical data as well as editing the manuscript; PAH contributed to the sampling and analytical works. All authors read and confirmed the final version of the manuscript.

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Oudbashi, O., Naseri, R. & Asadi Hasanvand, P. Long-term corrosion of copper alloys in the soil: new aspects of corrosion morphology in archaeological vessels from south-western Iran. Herit Sci 12, 73 (2024). https://doi.org/10.1186/s40494-024-01176-7

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