A methodological approach to estimate soil corrosivity for archaeological copper alloy artefacts

Omid Oudbashi

doi:10.1186/s40494-018-0167-4

Research article
Open Access

A methodological approach to estimate soil corrosivity for archaeological copper alloy artefacts

Omid Oudbashi¹Email author View ORCID ID profile

Heritage Science20186:2

https://doi.org/10.1186/s40494-018-0167-4

Received: 23 August 2017
Accepted: 7 January 2018
Published: 23 January 2018

Abstract

Characterization of soil corrosivity in archaeological sites is an important subject to understand the conservation conditions of archaeological bronze collections and helps conservators to prepare a conservation strategy for long term preservation of bronze objects. In this paper, a research approach is established to identify soil corrosivity in two archaeological sites and to find correlation between corrosion events and soil characterizations. Therefore, an analytical study was carried out to identify different factors of soil environment influencing corrosivity of the soil in two sites. Based on the results, measuring different factors such as chemical composition, pH, texture, soluble salts and water content and SOM displayed different soil environments in two archaeological sites. The results represent correlative relationship between corrosion mechanism and soil characteristics in these archaeological sites.

Keywords

Archaeological conservation
Archaeological copper alloys
Soil corrosivity
Soluble salts
Soil moisture

Introduction

Soil is a complex environment with various parameters, that influences archaeological materials in long term burial and causes to change appearance and the chemical nature of the buried objects. Archaeological metals suffer different corrosion events during burial time that may lead to formation of different corrosion layers on their surface with variable thicknesses and chemical compositions. These corrosion layers may form in different forms from a thin multiple corrosion layer to a thick corrosion crust covering remnants of original metal, and in some cases a completely corroded object without any metallic remnant may be discovered [1, 2]. Different parameters affect the corrosion events in archaeological metals that may be related to the metallurgy of the artefact (e.g. composition and manufacture), or the characteristics of the burial soil (e.g. texture, pH, redox potential, soluble salts) [3–11].

Corrosion of archaeological bronzes in soil environment has been subjected for many researches in last decades. These multidisciplinary studies could be categorized into three main groups:

Studies about corrosion morphology in buried bronzes [12–14],
Studies to identify corrosion compounds and products, phenomena and mechanisms in bronze corrosion [15–18].
Studies on the effect of burial environments and soil conditions on corrosion of archaeological bronzes [19–22].

Usually, archaeological metals are placed in the soil for a long time period and it makes difficult or even impossible to follow up the corrosion processes during burial time. In fact, the conditions of the archaeological metals are the results of interaction between metal/alloy and soil that can be observed only after unearthing the objects. On the other hand, soil environment may alter during the millennia that leads to change the soil conditions during burial time; indeed, the soil that is excavated today, may be different from the soil that objects are abandoned in it at the first time. Thus, study of the soil conditions in archaeological sites and comparing the results with the corrosion mechanism in the excavated materials can lead to better identification of long term corrosion mechanism in archaeological artefacts.

The aim of this research is to understand the effect of soil characteristics on the corrosivity of the burial environment as a medium that influences corrosion in the archaeological bronzes. In this paper, the soil conditions are determined regarding to the corrosivity of the soil environment and the effects of long term burial conditions on the corrosion are studied in some excavated bronze artefacts from two archaeological sites from Iran, the Sangtarashan Iron Age and the Haft Tappeh Elamite sites, by analysis different soil parameters and evaluation of soil corrosivity in the sites by using an integrated soil assessment approach based on the data obtained from soil analysis. Finally, the correlation between soil conditions and corrosion mechanism in the bronze artefacts are discussed.

Materials and methods

Archaeological sites

Two archaeological sites are selected to study on soil corrosivity and soil effects on conservation condition of archaeological bronze objects. These sites are Haft Tappeh Middle Elamite site (second millennium BC) located in southwestern Iran in the Khuzestan plain and the Sangtarashan Iron Age Site (first millennium BC) located in Luristan, western Iran (Fig. 1).

Fig. 1
Map of Iran and location of two archaeological sites studied in this paper

The archaeological soil of Sangtarashan includes two specific parts: the surface soil in black–brown colour with about 20–30 cm depth, apart from the slope of the hill in the northern part of the site that it is about 100 cm in depth, and the red soil under the black–brown one that is the main soil of the area. This second layer is from ca. 30 to 100 cm in depth from the surface of the site and is the burial environment in which the archaeological remains were discovered. The bronze objects also are found in the red soil or the interface of the red and black–brown soils. Of course, a very thin layer in dark brown colour is observed over the black–brown soil that is influenced by recent agricultural activities and is very similar to the black–brown soil (Fig. 2a). Furthermore, the Haft Tappeh site has a different situation, apart from a few centimetres upside the surface that are dated to the Parthian–Sasanian period (beginning of the first millennium AD), all of the burial environment (in some areas more than 200 cm) is dated to the middle Elamite period. The colour of the soil is similar in the surface and in the depth of the Haft Tappeh site (Fig. 2b). The bronze objects are discovered in different depths of the Haft Tappeh soil medium.

Fig. 2
a Soil profile in Sangtarashan site and place of finding a bronze object, b an excavation trench, architectural findings and soil profile in the Haft Tappeh archaeological site

Estimation of degree of deterioration (Fdet)

Nord et al. [20, 21] suggested an estimating factor to represent corrosion severity in archaeological copper alloy objects, named as the degree of deterioration (Fdet). They classified the objects in five groups regarding to the volume of corrosion occurred and observed in the cross section of copper alloy objects. In current study, based on the cross section of selected bronze samples from two sites, a new Fdet was designed including six groups of objects based on the volume of corrosion penetration into the metallic microstructure. It helps to characterize the penetration of corrosion in bronze structure in more details and shows that how the bronze objects are affected by soil environment. Although many bronze objects are discovered from two archaeological sites, 26 bronze samples were selected from Haft Tappeh and 22 samples from Sangtarashan. As mentioned above, the corrosion penetration and Fdet were estimated by observing the mounted cross section of the samples [23].

Soil sampling

Six soil samples from each site were selected from three different areas to undertake analytical study on soil and estimate soil corrosivity of two archaeological sites. The Haft Tappeh samples were selected from three trenches during the archaeological excavations in 2012 (trenches 27, 294 and 298) [24]. Three samples were selected from the surface of the trenches (0–30 cm of the surface) and three samples from the depth near the archaeological architectural floor of the Middle Elamite period. The Sangtarashan samples were selected from three trenches (trenches J16, E12 and H18) during 2009 and 2011 excavation seasons [23]. Three samples from black–brown surface soil and three samples of red deep soil were selected. The characteristics of soil samples are presented in Table 1.

Table 1

Characteristics of the soil samples from two archaeological sites of Haft Tappeh and Sangtarashan

Sample	Sample type	Trench	Depth of sampling (cm)	Munsell code	Colour
HT.01	Surface	27	30	10YR 6/4	Dull yellow orange
HT.02	Deep	27	90	10YR 6/4	Dull yellow orange
HT.03	Surface	294	30	10YR 5/3	Dull yellowish brown
HT.04	Deep	294	150	10YR 7/3	Dull yellow orange
HT.05	Surface	298	30	10YR 7/3	Dull yellow orange
HT.06	Deep	298	180	10YR 6/3	Dull yellow orange
ST.01	Surface	J16	30	7.5YR 2.5/2	Brownish black
ST.02	Deep	J16	80	5YR 4/4	Dull reddish brown
ST.03	Surface	E12	30	7.5YR 3/1	Brownish black
ST.04	Deep	E12	90	5YR 3/3	Reddish
ST.05	Surface	H18	30	7.5YR 4/1	Brownish grey
ST.06	Deep	H18	70	5YR 4/2	Greyish brown

Methods of soil analysis

The analytical methods were selected and designed based on two main aspects: identification of soil type and characterization of physico-chemical factors influencing soil corrosivity. Thus, two analytical systems were performed to find different features of the soil in the archaeological sites: instrumental as well as standard and classic analytical methods.

First to remove free water from the soil texture, all samples were dried in the open air [25]. Then the samples were separated into two parts: the first part was used to analytical studies and soil particle size analysis test. The second one was sieved with sieve mesh size of No. 10 (2 mm) [26] and was used for other tests, including characterization of soluble ions (soluble salts), pH measurement, electrical conductivity test (EC), and measuring soil water and soil organic matter.

Soil samples were analysed by X-ray fluorescence (XRF) method [26–28]. For this purpose, 5 g of each sample were selected and prepared for XRF analysis. The samples were analysed by an ARL 8410 WDXRF analyser. The XRF analyses were carried out on the powder pellets. Also, the phase compounds in the soil samples were determined by X-ray diffraction method [26, 28–30]. Also, 4 grams of each sample were powdered and analysed by an X-ray diffractometer MPD 3000 model manufacture by GNR company.

Soil particle size distribution was measured by the sieve and hydrometer method in samples of two archaeological sites based on the standard method ASTM D422-63 [31].

Water and soil organic matter (SOM) contents of soil samples were calculated by heating the sieved samples in an oven and then burning off in a furnace (L.O.I) and were measured based on the weight loss during these processes. Five grams of each sample were heat 12 h in an oven in 105 °C and then water content was measured in percent based on loss of weight after heating [26, 32, 33]. There are different methods and variable temperatures to determine soil organic matter in the soil, but to void the effect of high temperature on different soil constituents, heating in 360 °C was selected [34–36]. Thus, the samples were heated and dried in oven then were heated in a furnace in 360 °C for 4 h. The weight change after heating was considered in percent as SOM content of the soil samples [34].

The pH of soil samples was analysed by a pH-meter Metrohm 744 model based on ASTM standard test [37].

The soil electrical conductivity (EC) may be an important factor showing soil salinity and consequently its corrosivity because it may present the soluble salt content in the soil [38–40]. For this purpose, the dried and sieved soil samples were selected and an extract was prepared from each sample with 1/2 proportion of soil and water respectively. This extract proportion was prepared based on the water content measured in the samples [26]. Then, the EC of soil samples was determined in the extracts by a conductometer InoLab Terminal 740 manufactured by WTW Company.

The 1/2 extract of soil/water was also used to determine soluble anions in the soil samples [26, 39]. A preliminary qualitative test was done on the soil samples to detect the soluble anions present in the samples, including HCO₃⁻, CO₃²⁻, SO₄²⁻, NO₃⁻, PO₄³⁻, Cl⁻ [41]. Based on the results of qualitative tests, soluble sulphate, carbonate, nitrate and chloride were measured in the soil samples. To identify the soluble chloride content, titration (Mohr) method was used based on ASTM standard [26, 39, 42]. Soluble carbonate and bicarbonate content also was determined by titration method with sulphuric acid and based on ASTM standard [39, 43]. Soluble sulphate content was determined by turbidity measurement using spectrophotometry method with a SPECTRONIC 20 spectrophotometer manufactured by BAUSCH & LOMB company [39]. Finally, soluble nitrate was measured by ion chromatography method [44].

The soluble cations including Na⁺, Mg²⁺, K⁺ and Ca²⁺ were measured by flame atomic absorption spectroscopy (FAAS) method [39] on the 1/2 extract of soil/water by using a Perkin-Elmer 2380 Atomic Absorption Spectrometer.

Results

Previous results on the corrosion of the objects

Corrosion mechanism and morphology in the bronze objects of these archaeological sites are studied previously. The result showed that the main corrosion mechanism in the Haft Tappeh bronze collection is active corrosion (bronze disease). The main corrosion products identified in the Haft Tappeh bronzes are copper oxides (cuprite, tenorite) and copper trihydroxychlorides (atacamite, paratacamite); while uniform corrosion with noble patina is observed in Sangtarashan bronze objects with cuprite, malachite and azurite as the main corrosion compounds [45, 46].

Nevertheless, some details should be explained here to find relationship between soil condition and corrosion in archaeological copper alloy objects. Haft Tappeh objects are made of impure copper and tin bronze alloy and are corroded heavily, as many of the objects are corroded completely and no metallic structure is retained after long term burial in the soil environment. As mentioned above, the main corrosion mechanism in these objects is bronze disease or active corrosion that is reaction of copper with chloride resulting to form basic copper chlorides and copper oxides [47]. The corrosion morphology of Haft Tappeh objects includes various red, green, white–grey and black corrosion layers in partially and completely corroded objects [46]. The Sangtarashan objects are corroded partially by forming a smooth noble patina on the surface of bronze objects as well as the formation of an external corrosion layer consisting of basic copper carbonates so-called as type I corrosion [12]. This corrosion morphology is occurred in Sangtarashan bronzes by internal oxidation of tin and selective dissolution of copper [45].

Soil appearance

The appearance of Haft Tappeh soil samples consist of a fine grained soil with many white particles that may be the remnants of gypsum used as the binder or plaster in the Elamite buildings of Haft Tappeh [48, 49]. Some very fine black particles are also visible in the soil samples that may be charcoal particles from burning the site at the ancient time [50]. Soil samples of Sangtarashan are including three dark samples from the surface and three red samples from the depth of the site. Some large white particles are visible in the Sangtarashan soil that may be small pieces of broken rocks that are used to build the sanctuary during the Iron Age [51]. Table 1 also presents the colour characteristics of soil samples from two archaeological sites. The colour of soil samples is specified by Munsell soil chart method [52–54].

Chemical composition

Results of XRF analysis of soil samples are presented in Table 2. Based on the results SiO₂, CaO, Al₂O₃, Fe₂O₃, MgO and K₂O are the main constituents in the composition of all samples. Haft Tappeh soil samples contain SO₃ as major and Na₂O, and Cl as minor constituents while metallic contaminations are detected in Sangtarashan soil as minor/trace constituents (Co₃O₄, NiO, MnO, CuO, ZnO). It is evident that the soil samples are similar in each site, although some variations are visible in their composition such as high amounts of SO₃ in sample HT.01 in Haft Tappeh or different main composition in sample ST.06 in Sangtarashan.

Table 2

Results of XRF analysis of soil samples from two archeological sites (wt%)

	SiO₂	CaO	Al₂O₃	Fe₂O₃	MgO	K₂O	SO₃	P₂O₅	TiO₂	Cl	Na₂O	Co₃O₄	NiO	MnO	CuO	ZnO	L.O.I
HT.01	21.8	25.7	5.1	3.8	3.0	1.8	13.6	0.22	0.36	1.2	0.88	–	–	–	–	–	22.48
HT.02	31.1	26.5	6.8	4.2	3.9	2.1	2.2	0.11	0.40	0.14	0.33	–	–	–	–	–	22.40
HT.03	35.7	26.7	7.5	4.7	4.3	2.6	3.1	0.21	0.46	–	0.58	–	–	–	–	–	14.16
HT.04	32.0	21.5	6.4	3.9	4.0	2.6	0.74	0.43	0.38	0.85	0.74	–	–	–	–	–	26.60
HT.05	28.9	23.7	6.2	4.2	3.6	2.6	4.7	0.21	0.39	0.71	0.58	–	–	–	–	–	24.15
HT.06	29.7	24.9	7.0	5.8	4.2	3.0	2.8	0.79	0.51	0.66	0.72	–	–	–	–	–	20.00
ST.01	30.5	23.5	8.8	7.6	4.0	2.2	0.13	0.58	0.72	0.082	–	0.035	0.067	0.10	0.091	0.064	21.56
ST.02	33.1	20.1	9.1	7.9	4.3	2.1	0.074	0.25	0.77	0.079	0.93	0.037	0.068	0.097	0.094	0.065	20.78
ST.03	26.0	24.5	9.7	9.1	4.4	2.4	0.085	0.46	0.85	0.057	–	0.034	0.079	0.091	0.093	0.066	22.10
ST.04	30.5	23.2	7.7	7.6	4.5	1.7	0.082	0.37	0.83	0.053	–	0.027	0.074	0.085	1.8	0.062	21.25
ST.05	32.3	22.9	8.6	7.5	3.7	2.1	0.075	0.48	0.67	0.035	–	–	–	0.041	–	–	21.60
ST.06	55.7	3.0	14.5	11.1	4.9	3.4	0.054	0.26	1.0	0.035	–	–	0.0055	0.12	0.0038	–	5.98

Results of XRD analysis are presented in Table 3. The main compounds in soils are quartz, calcite, dolomite and illite that are detected in all samples. Other commonplace phases of soil such as albite, microcline, hematite, clinochlore and nantronite are identified as soil components in some samples. Significant feature of Haft Tappeh and Sangtarashan samples in presence of different types of calcium sulphate minerals, including gypsum, basanite and anhydrite, especially in Haft Tappeh soil samples. Also sylvite (KCl) is detected as a minor phase in one sample of Haft Tappeh.

Table 3

Results of XRD phase analysis of soil samples

	Quartz	Calcite	Dolomite	Illite	Albite-Calcian	Clinochlore	Gypsum	Bassanite	Anhydrite	Microcline	Albite	Nontronite	Hematite	Sylvite
	SiO₂	CaCO₃	CaMg(CO₃)₂	K(Al,Fe)₂AlSi₃O₁₀(OH)₂·H₂O	(Na,Ca)Al(Si,Al)₃O₈	Mg–Fe–Fe–Al–Si–O–OH	CaSO₄·2H₂O	CaSO₄·0.5H₂O	CaSO₄	KAlSi₃O₈	NaAlSi₃O₈	Na_0.33Fe₂₊₃(Si,Al)₄O₁₀(OH)₂·xH₂O	Fe₂O₃	KCl
HT.01	■	■	■	■	–	–	■	–	–	■	■	–	■	–
HT.02	■	■	■	■	■	–	–	■	–	–	–	■	–	–
HT.03	■	■	■	■	■	–	■	–	■	–	–	–	■	–
HT.04	■	■	■	■	■	–	–	–	–	–	–	–	–	–
HT.05	■	■	■	■	■	–	■	–	–	–	–	–	–	–
HT.06	■	■	■	■	–	–	–	■	–	–	■	–	■	■
ST.01	■	■	■	■	■	–	–	–	■	–	–	–	–	–
ST.02	■	■	■	■	■	■	–	–	–	–	–	–	–	–
ST.03	■	■	■	■	■	■	■	–	–	■	–	–	–	–
ST.04	■	■	■	■	■	■	–	–	■	–	–	–	–	–
ST.05	■	■	■	■	■	■	■	–	–	■	–	–	–	–
ST.06	■	■	■	■	–	■	–	–	–	–	–	–	■	–

Measurement of corrosivity factors

Different factors affecting to the corrosivity of soil environment in archaeological sites were measured to understand the correlation between the soil environment and corrosion events occurred in bronze objects of two archaeological sites. The results are presented in Table 4.

Table 4

Results of determining some factors including soil texture, electrical conductivity (EC) and pH in the studied soil samples

Sample	Soil texture (%)			L.O.I (%)		Soil conductivity		pH
Sample	Sand	Silt	Clay	Water	SOM	EC (µs/cm)	T (°C)	Soil	Distilled water
HT.01	19	72	9	5.27	2.58	7750	20.3	7.41	6.72
HT.02	14	67	19	2.01	1.71	3780	20.3	7.86
HT.03	19	63	18	1.69	3.47	393	19.2	7.94
HT.04	17	67	16	1.70	1.28	3730	20.3	7.9
HT.05	19	62	19	1.77	2.08	7440	19.9	7.88
HT.06	20	65	15	2.26	1.14	3660	19.9	7.66
ST.01	17	55	28	3.99	7.18	594	20.6	6.94	6.89
ST.02	22	47	31	3.38	2.57	348	20.0	7.04
ST.03	21	45	34	4.02	6.78	345	19.3	7.02
ST.04	33	39	28	3.52	5.79	267	19.2	7.02
ST.05	18	54	28	4.14	8.19	405	19.3	6.88
ST.06	8	46	46	5.17	6.80	357	18.7	6.59

Results of particle size analysis showed that the soils of two sites are different based on three main constituents characterizing the soil types: sand, silt and clay. In Table 4 it is visible that sand amount in surface samples of Haft Tappeh is similar while in the deep samples is partially variable, but sand amount is variable in all samples of Sangtarashan (between 8 and 33%). Nevertheless, all samples could be classified as fine grained soils because of low amount of the sand in their texture regarding to the Unified Soil Classification System (USCS) [55]. Figure 3 shows the soil classification ternary diagram of sand–silt–clay. The measured amount of these main factors in the soil of two archaeological sites are marked on the standard diagram of USDA classification [56]. Based on the diagram, it is visible that the Haft Tappeh soil texture in all samples is more similar than Sangtarashan samples. The Haft Tappeh soil can be classified as silt loam, while in Sangtarashan the soil texture varies from silty clay to silty clay loam and clay loam.

Fig. 3
Ternary diagram of soil texture based on Unified Soil Classification System (USCS), showing the type of soil in two archaeological sites

Water content of the soil or water holding capacity is the resistivity of soil in losing water in the dry state and may be present in three forms: free ground water, capillary water and gravitational water [57]. Water content in Haft Tappeh soil samples is variable from 1.69 to 5.27%, while it is more similar in the Sangtarashan samples. Soil organic matter (SOM) is apparently higher in Sangtarashan samples in comparison with Haft Tappeh ones. Also, it is visible that SOM is higher in surface samples of both sites. Figure 4a presents columnar diagram showing water and SOM content in all soil samples. Based on the diagram, it is apparent that the Sangtarashan soil has high water holding capacity and also more SOM.

Fig. 4
Columnar diagrams of soil characteristics measured in the soil samples from archaeological sites, a water and SOM content, b electrical conductivity (EC) of soil samples, c pH of the soil samples

Electrical conductivity (EC) of soil extract shows the presence of soluble salts and salinity of soil. Electrical conductivity of soil extract was measured in the samples (Table 4). High measured EC in Haft Tappeh samples shows that the soil of this site is highly saline in comparison with Sangtarashan (despite of sample HT.03). It is visible that deep samples of Haft Tappeh have similar EC, while it has been measured more variable in the surface samples. Also the extract of black–brown (surface) samples of Sangtarashan are more conductive in comparison with the red (deep) samples (Fig. 4b).

pH of soil samples also was measured and the results showed that the pH range is slightly alkaline in Haft Tappeh samples while in Sangtarashan is more neutral (near 7) (Table 4, Fig. 4c).

Soluble anions and cations were determined by different methods explained in “Materials and Methods” section. The results are presented in Table 5. The amount of three soluble anions (chloride, sulphate and bicarbonate) in Haft Tappeh are significantly higher than the Sangtarashan soil while soluble nitrate is detected only in Sangtarashan samples. The high amount of sulphate of Haft Tappeh samples may be related to the presence of high amount of SO₃ determined in XRF analysis and calcium sulphate compounds that were detected by XRD method. Soluble nitrate was only detected in Sangtarashan samples; although it has been measured in low amount, but it is higher in the surface sample (black–brown samples). Also the amount of soluble cations (Ca, K, Na and Mg) was determined by FAAS method showing high amount of calcium, potassium and sodium in the Haft Tappeh samples (Table 5). Also calcium concentration is detected more than other cations in Sangtarashan samples. It can be explained that high amount of soluble salts in the Haft Tappeh soil can be correlated with higher EC measured in the soil samples while it has been determined in Sangtarashan samples in very lower level.

Table 5

Results of quantitative analysis of soluble anions and cations carried out on the extract of the soil samples (mg/kg)

	Na⁺	K⁺	Mg²⁺	Ca²⁺	Cl⁻	HCO₃⁻	SO₄²⁻	NO₃⁻
HT.01	226.8	52.2	23.86	559.2	3404.84	117.12	16309	n.d.
HT.02	128.2	54.8	18	569.2	650.52	146.4	15350	n.d.
HT.03	108.8	177.6	7.2	614.2	377.16	170.8	16218	n.d.
HT.04	155.8	231.6	13.2	185.8	1460.2	131.76	6866	n.d.
HT.05	160.8	258.2	14	647.2	3653.98	122	16366	n.d.
HT.06	111.2	263.2	13.9	539.2	636.68	136.64	15491	n.d.
ST.01	10.3	14.13	25.16	80.66	17.32	5.24	5.2	96
ST.02	5.51	5.54	15.42	72.64	8.1	4.76	3.12	28
ST.03	7.1	7.39	15.54	48.74	16.4	4.68	3.8	19.2
ST.04	5.85	11.14	11.94	35.69	8.8	5.16	3	3.8
ST.05	10.18	8.37	15.88	57.44	16.93	5.24	4.4	31.2
ST.06	3.98	7.83	24.7	44.44	16.6	5.04	8.8	11.2

Discussion

Soil corrosivity

The results of soil analysis in studied archaeological sites from Iran (Haft Tappeh and Sangtarashan) apparently presents that the soil environment is significantly different in two sites. In fact, these different burial environments make different effects on buried archaeological copper alloy objects during long-term burial leading to occur different corrosion mechanisms and morphologies in objects.

Phase composition of soil in Haft Tappeh shows presence of calcite and dolomite (carbonate) as main compounds and also the pH of soil is alkaline. Regarding that, it is possible to classify the Haft Tappeh soil as calcareous [58]. Also, calcium sulphate compounds are detected in the soil of Haft Tappeh beside chloride phase (sylvite). These phases may lead to the high salinity of Haft Tappeh soil that was also proved by EC measurement. On the other hand, the presence of high amount of soluble Cl⁻ and SO₄²⁻ in soil states that this burial environment is a corrosive medium in which severe corrosion may occur in the long-term burial period. In fact, the presence of high amount of soluble salt results in increasing conductivity of the soil and accelerating in electrochemical reactions leading to corrosion of archaeological copper alloys [2].

As noted above, soluble sulphate is the main soluble anion was measured in the Haft Tappeh soil samples. It may be due to presence of calcium sulphate phases (and high amount of SO₃) in the composition of soil because of gypsum used as a binder or plaster in the architecture of Haft Tappeh. Usually the presence of sulphide (metallic sulphide) and its oxidation leads to form sulphuric acid and acidifying the soil as well as decreasing pH [59], but presence of calcium sulphate phases doesn’t change the pH of the soil [26]. In fact, slightly alkaline pH of Haft Tappeh soil is more due to presence of magnesium and calcium carbonates and soluble bicarbonate anion leading to pH about 8. Although, calcium sulphate may also coexist with carbonates in this pH range [26, 60, 61].

Silt is the main constituent of soil texture in Haft Tappeh that leads to limit absorption of water and air in the soil and also the ability of soil drying is decreased in time. In fact, higher amount of sand may cause increasing aeration and the drainage ability of the soil [22]. The water holding capacity in the Haft Tappeh may be significant due to its texture and regarding to the depth of placement metal objects as well as ground water table (low depth) on this site, it could be an important factor in corrosivity of the soil [62]. Of course, this factor should be considered beside the presence of high amount of corrosive anions in the soil.

The soil environment of Sangtarashan site is significantly different with Haft Tappeh one. Type and concentration of soluble salts are different in Sangtarashan and NO₃⁻ is detected as the main soluble salt in the soil. Of course, the concentration of all soluble anions is very low in comparison with Haft Tappeh soil. Presence of nitrate in the soil is in correlation with high SOM determined in the Sangtarashan soil and may be due to agricultural activities during the time. Also, EC is measured as low amount in Sangtarashan soil samples that is related to low amount of soluble salts in soil. pH of Sangtarashan soil is neutral that may be due to low concentration of soluble salts as well as the high amount of SOM [26]. The neutral pH can lead to decrease the rate of corrosion in the soil environment [2]. The texture of soil in Sangtarashan is variable with different percentage of sand and high percentage of clay. Clay may lead to decrease aeration of soil, but sand may lead to penetrate more water and air as well as decreasing water holding capacity and increasing rate of drainage in the soil. Regarding to the depth of placement objects, pH, concentration of soluble salts and texture of the soil in Sangtarashan site, the soil environment could be considered as a non-corrosive or partially-corrosive medium for the archaeological copper alloy object.

Application of a numerical model is a very useful approach to estimate soil corrosivity and the effect of soil environment in archaeological metal objects based on different parameters measured in the soil [22, 63]. There are different standard models to estimate soil corrosivity that are used in industrial studies in which the main factors are selected and considered for specific modern metals or alloys. Some important factors for archaeological copper alloys are not considered to design these standard models [64]. In fact, these models don’t consider the conditions of long-term effect of soil and important factors in corrosion of copper alloy artefacts. For example, some models are considered to sulphide concentration as the important soluble anion while it has no effect on archaeological objects in the aerobic soil environment, or type and concentration of soluble salts are avoided in some corrosivity estimation models [65, 66].

Thus, a useful model to estimate corrosivity of soil regarding the archaeological copper alloys was considered in this research. It was designed by incorporating some different exist models to make an appropriate corrosivity estimation model useful for archaeological copper alloys [65–70]. The factors considered in this model are including:

Soil pH (A),
Soil organic matter (SOM) (B),
Concentration of soluble chloride anion (Cl) (C),
Total soluble salts (TSS) measured in the soil (D),
Water content in the soil (based on water holding capacity and the place of objects in relation with ground water table) (E),
Soil aeration (soil texture) (F),
Electrical conductivity (EC) of extract of the soil (G).

Table 6 shows the factors and the quantity specified for each factor (between 0 and 10). The corrosivity of soil is quantified based on the following equation:

$${\text{Soil corrosivity index}}\left( {{\text{SCI}}} \right) = {\text{A}} + {\text{B}} + {\text{C}} + {\text{D}} + {\text{E}} + {\text{F}} + {\text{G}}$$

Table 6

The factors used to estimate soil corrosivity index (SCI) in the archaeological soil environment

Factor	Point	Value	Measurement unit	Corrosive factor
A	3	9 <	–	pH
	0	5.5–9
	2	4–5.5
	4	2–4
	5	2 >
B	0	No SOM	Weight percent	Soil organic matter (SOM)
	1	5 >
	2	5 <
C	1	10 >	mg/kg	Soluble chloride (Cl⁻)
	2	10–50
	3	50–100
	6	100–1000
	10	1000 <
D	1	100 >	mg/kg	Total soluble salts
	2	100–200
	3	200–500
	5	500–1000
	7	1000–2500
	9	2500–5000
	10	5000 <
E	10	Under GWT	Ground water table (GWT) + water holding capacity (%)	Water content
	8	Around GWT
	4	Upper than GWT-Wet soil (> 20%)
	2	Upper than GWT-Wet soil (< 20%)
F	6	60	Percent of sand	Soil aeration
	4	40
	3	20
	0	0
G	0	50 >	µs/cm	Soil conductivity
	1	50–100
	2	100–200
	4	200–333
	6	33–1000
	10	1000 <

Total point	Soil corrosivity index (SCI)
0–10	No corrosive
11–20	Partially corrosive
21–30	Corrosive
31–40	Severely corrosive

Then the soil corrosivity is classified into four groups: non-corrosive (0–10), partially corrosive (11–20), corrosive (21–30) and severely corrosive (31–40). Figure 5 shows the columnar diagram of soil corrosivity index of the soil samples analysed in two archaeological sites. It is obvious that the SCI for Haft Tappeh soil samples varies from 27 to 35 and is less than 30 only in one sample. Thus, the Haft Tappeh soil could be classified as severely corrosive soil for archaeological copper alloy objects. On the other hand, SCI is calculated between 14 and 18 in Sangtarashan soil samples showing that the soil environment in this archaeological site is partially corrosive.

Fig. 5
Diagram showing Soil Corrosivity Index (SCI) in the soil samples analysed from two archaeological sites. The soil of Haft Tappeh is identified as severely corrosive while the Sangtarashan soil is detected as partially corrosive soil

The Haft Tappeh archaeological site is located in the alluvial plain of Khuzestan that includes a southeastern extension of the Mesopotamian plain and a part of the Zagros Mountains to the northeast. The Zagros mountain is drained by several rivers, that have built up large alluvial fans and partially saline mud flats in this plain at the north of the Persian Gulf [71]. On the other hand, the Sangtarashan site is located in the Zagros Highlands with different geological system from Khuzestan plain and it is the most important reason for difference between soil environment in these sites [23].

Soil environment and corrosion of archaeological objects

Table 7 shows the factors used to explain Fdet in archaeological objects based on the morphology and corrosion penetration on the cross section of selected samples. Figure 6 also shows the columnar diagram indicating Fdet in objects of two sites. The diagram states that Haft Tappeh objects are corroded heavily and about 75% of studied objects are corroded completely and all metal has been transformed to corrosion products while in Sangtarashan samples, less than 50% of metal has been altered during burial time. Although in some samples of Haft Tappeh, Fdet is determined as groups G-1 to G-3 (5 samples) but it is obviously visible that the corrosion is different in two sites.

Table 7

Characteristics used to estimate degree of deterioration (Fdet) of different groups of copper alloy objects in this study

Corrosion type	Characteristics	Corrosion percentage	Group
Almost no corrosion	Almost all of metal is retained	< 10	G-1
Minor corrosion	Thin internal corrosion layer is exist	10–30	G-2
Considerable corrosion	Considerable internal corrosion layer is exist	30–50	G-3
Almost severe corrosion	Internal corrosion layer more than half of cross section	50–70	G-4
Major corrosion	Small part of metal is retained	70–90	G-5
Almost no metal core	Almost all metal has been altered to corrosion products	> 90	G-6

Fig. 6
Degree of deterioration (*Fdet*) estimated in the copper alloy objects from two archaeological sites. It is obviously visible that the corrosion rate and volume in Haft Tappeh objects is higher than the Sangtarashan bronze objects

The significant finding in the analysis of soil in Haft Tappeh was high concentration of soluble sulphate ion even more than chloride. Nevertheless, the main corrosion products of Haft Tappeh objects are copper trihydroxychlorides and there is no evidence of formation of copper sulphates among corrosion layers [46]. Copper sulphates are the main corrosion products observed in the copper sculptures and monuments placed in open-air urban environments [72, 73], although they may be formed in water containing soluble sulphate [74, 75]. These compounds are rare corrosion products in corroded objects buried in the soil, although there are some reports about identification of copper sulphates as corrosion products in buried objects [76–79]. The basic copper sulphates are stable in acidic conditions. By changing the pH of environment to an alkaline condition, they will transform to more stable compounds such as tenorite (CuO) in the open-air environments [80]. On the other hand, these products will transform to copper trihydroxychlorides (basic copper chlorides) in the presence of high concentration of soluble chloride ions [57, 81]. In fact, the main factor of formation of basic copper sulphates in open-air environments is SO₂ [81, 82], but because of absence of SO₂ in soil environment, it is not probable to form basic copper sulphates as corrosion products in buried objects in the soil of Haft Tappeh.

Furthermore, the corrosion products identified in Sangtarashan objects are basic copper carbonates (malachite and azurite) [45]. As noted earlier, soluble nitrate was detected as the high concentration soluble anion in the soil samples of this site. Nevertheless, no evidence of copper nitrate compounds was observed in the analysis of corroded objects in this site. It is due to the high solubility of copper nitrate compounds that make them unstable and they are extremely rare as corrosion products in archaeological copper alloys [1]. Furthermore, sulphate compounds are found in the soil of Sangtarashan site, but no copper sulphate is detected in corrosion products of Sangtarashan bronzes. Apart from conditions of formation of copper sulphates that are explained earlier, it is also may be due to low amount of soluble sulphate ions in water of the soil. Unlike Haft Tappeh soil, sulphur is detected as minor/trace amount in the soil of Sangtarashan site (as SO₃ in Table 2) and also soluble sulphate ion is determined in very low level (as SO₄²⁻, Table 5). Regarding very low amount of sulphate in Sangtarashan soil, it is predictable that copper sulphate is not possible corrosion products in Sangtarashan bronzes and commonplace copper carbonates and oxides are found as corrosion products in archaeological bronzes [1, 2, 45].

Based on the results and their explanation, it could be obviously deduced that the main corrosive factors in the aerobic soil environments are the presence of chloride ion and water in long-term burial period. It has been noted in literature that water capacity of soil (including soil texture and soil moisture) and anions (soil composition) such as chloride are very important factors to make burial environment as corrosive media (e.g. [1, 2, 19, 20, 22]). Of course, many factors are considered as factors influencing the corrosivity of soil but chloride and moisture are noted as the main factors that also influence others in the soil. As noted above, the active corrosion has been occurred in the Haft Tappeh objects due to high concentration of soluble chloride in the soil medium while there are other soluble salts in its soil. In fact, the presence of free chloride ion and water is caused to occur severely corrosion in archaeological copper alloy objects, while because of no significant amount of chloride salts (ions) in Sangtarashan soil, only internal oxidation and low rate copper dissolution were occurred in the Iron Age bronze objects in this site. Of course, it is worthy to note that other factors such as pH are very important, but the presence of soluble alkaline earth metals and alkali metals as well as soluble bicarbonate in soil water are led to make pH between 7 and 8 in two sites. On the other hand, other soluble anions such as sulphate and nitrate don’t play an important role in the corrosion of archaeological objects. Of course, it should be considered that factors such as total soluble salts and pH may influence the corrosion rate in the buried objects. Furthermore, it should be noted that although the dating of the two sites (and their soil deposit) differs about 600-700 years, but the time has no role in corrosion of archaeological metal objects and is not an effective factor in degree of deterioration in archaeological metals. It is observed that the thickness of patina formed on the surface of modern buried bronzes in soil is similar to archaeological buried bronzes [6, 12, 20]. Therefore, soil conditions are the most important factor influencing long-term corrosion of archaeological bronzes.

Conclusion

Study on the corrosivity of soil environment in archaeological sites was carried out to understand important corrosion factors influencing copper alloy objects in long-term burial period. For this purpose, two archaeological sites from Iran were selected. The corrosion mechanism in copper alloy objects was studied earlier and it made opportunity to find correlation between corrosion mechanism occurred in the objects and factors influencing soil conditions for buried artefact. Based on the results, the soil environment of two studied sites is completely different based on the corrosive factors and Haft Tappeh soil could be classified as severely corrosive while Sangtarashan soil is a partially corrosive burial environment. The main corrosive factors identified in the Haft Tappeh soil are including soluble salts, especially chloride and sulphate anions and soil texture (water holding capacity) while the soil of Sangtarashan has no significant amounts of soluble anions. Regarding to the results of corrosivity of the soil and corrosion mechanism occurred in the buried copper alloy objects, it could be concluded that soluble chloride and water content are the main corrosive factors that influence long-term corrosion of archaeological copper alloy objects in burial environments. Also, application of a numerical approach to estimate soil corrosivity based on different soil factors is very useful to understand soil conditions in archaeological sites to make decisions about conservation of unearthed bronze collections in situ and in-laboratory during and after archaeological excavations.

Declarations

Acknowledgements

The author is grateful to thanks Prof. Parviz Davami, Sharif University of Technology, Mehri Ghobadi, Dr. Atefeh Shekofteh, Dr. Hossein Ahmadi and Dr. Seyed Mohammadamin Emami, Art University of Isfahan, Dr. Behzad Mofidi-Nasrabadi, Archaeological Campaign of Haft Tappeh, Ata Hasanpour and Dr. Mehrdad Malekzadeh, Archaeological Campaign of Sangtarashan and Mozaffar Farhadpour, Choghazanbil and Haft Tappeh World Heritage Site, for their helps to undertake analyses and their comments to improve the result interpretations and the text.

Competing interests

The author declares that no competing interests.

Availability of data and materials

The dataset(s) supporting the conclusions of this article is (are) available in the article.

Consent for publication

Not applicable.

Ethics approval and consent to participate

Not applicable.

Funding

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Authors’ Affiliations

(1)

Department of Conservation of Historic and Cultural Properties, Faculty of Conservation, Art University of Isfahan, Isfahan, Iran

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