Chemical and structural properties of ancient metallic artefacts: multitechnique approach to study of early bronzes

Abstract

The results are presented of a characterisation study of the microstructure and microchemistry of archaeological bronze (Cu–Sn) artefacts from the eighth to the sixth century bc. Metallographic examination, with optical and electron optical microscopy, has been performed on polished sections of early Iron Age studs and bracelets found in incineration tombs of the Necropolis of Chiavari in Italy. A heterogeneous microstructure of the bronze was observed, exhibiting a wide range of grain sizes, and a predominant α-phase solid solution containing α/δ and α/? eutectoid phases decorated with a high density of inclusions. The composition of grain boundary surfaces was determined, using scanning Auger microscopy (SAM), on specimens fractured in vacuo. The extent of tin segregation at the grain boundaries was 3–5 times greater than that in the grain interiors. Copper rich sulphides occasionally containing the oligoelements (iron and lead) were identified as the predominant type of inclusions formed at the grain interfaces and within the grains. Analysis of the corrosion patina at the surface of the bronze artefacts was conducted by combining X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and SAM. The patina was found to exhibit a multilayered structure and a complex chemical composition forming various crystallographic phases including malachite, cuprite, and copper–tin oxide. Corrosion of the underlying bronze matrix proceeded along the grain boundaries, where the sacrificial corrosion of tin reacting with diffused oxygen and chlorine was identified. The results of this study have been used to clarify the metallurgy and manufacturing processes of the examined finds, and to evaluate the state of their degradation.     

Characterization of corroded bronze Ding from the Yin Ruins of China

This paper presents the result of scientific examinations carried out on the soil-buried archaeological bronzes Ding from Yin Ruins of China. Eight of typical fragments from different bronze Ding were selected as researched samples according to their deterioration characteristics. Optical microscopy (OM), scanning electron microscopy coupled with energy (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to understand the corrosive morphological characteristics, to clear the nature of patina and to analyze the elementary composition of bronze Ding. The results indicated that it is not possible to distinguish the original lustrous metallic surface in most samples because of the corrosive crust. The substrate of bronze Ding contains74-86% Cu, 1.1-4.6% Pb, and 10-18% Sn, which is in agreement with the historical investigation in the ritual vessels of Shang time. Copper-containing compounds were the main constituents of natural patina: Cu₂(OH)₃Cl existed as corrosion product in all the powdery or crack surface; Cu₂(OH)₂CO₃ was the main corrosive product in a compact and hard corrosive surface. This study provides useful information for the restoration and protection of bronze Ding in Yin Ruins.     

Corrosion behaviour of bronze in urban environments: influence of tin content

Abstract

In this paper, the influence of the tin content on the corrosion behaviour of copper alloys was investigated. Three different bicomponent bronze alloys were exposed to a natural urban environment. The kinetic corrosion processes and the patina properties were monitored using the open circuit potential, electrochemical impedance spectroscopy and spectrocolorimetry. Results show that the bronze corrosion behaviour improves upon increasing the tin content. In addition, the mechanism of the patina formation is different for the three alloys.     

Characterization of archaeological bronze and evaluation of the benzotriazole efficiency in alkali medium

An analytic study permits us to characterize the altered surface of an archaeological bronze coin and to determine the structure and the composition of the patina covering the whole surface of this artefact. This patina could be interpreted at first sight as a type I patina, with copper product deposits on it, with some punctual and enlarged localized type II corrosion. To achieve the purpose of this work, we investigate the behaviour of an archaeological bronze in the presence of benzotriazole (BTA) in alkali medium with the intention of getting a better passivity while favouring the formation of a polymeric film on the surface of the working electrode. The behaviour of the considered interface is investigated by electrochemical impedance spectroscopy, in the presence and absence of an oxide layer, according to the immersion time. At pH = 9, in the presence of 15 mmol/l of BTA, the optimum percentage of inhibition efficiency (IE%) is 67% obtained after 30 min of immersion. A pre‐polarization of the bronze working electrode is realized in order to accommodate the preservation technique used in museums and to improve the formation of the Cu(I)‐BTA polymeric film. The use of a pre‐polarized electrode for 30 min at ?35 mV/SCE carries an enhancement of the protection versus the non‐polarized electrode. While comparing the result of our investigation with that obtained using the traditional preservation method, we can establish that using a concentration of BTA 15 times lower, important inhibitor efficiencies (%) of 92 and 97.4%, respectively, for 30 min and 96 h of immersion are reached.     

Voltammetric behaviour of an archeaological bronze alloy in aqueous chloride media

The interface archaeological bronze alloy‐chloride media was characterized by using linear sweep rate cyclic voltammetry. The electrochemical behaviour of the Cu‐Sn alloy being different from pure copper and pure tin. In fact, the voltammograms show an anodic peak and two cathodic peaks. Surface examination revealed a compact layer which spread over the ancient material. The influence of different parameters such as scan rate, chloride concentration and repetitive cycling on the voltammetric response of the archaeological bronze was also studied. The results indicate that patina layer formation is a spreading process over the electrode surface leaving only small pores. Diffusion of the halide controls the growing rate of this layer. Two other anodic peaks are evidences when potential cycling increases. The oxidation became more difficult as a result of a constant layer thickness.     

已锈蚀青铜在大气环境中的腐蚀发展及其保护研究

用动电位扫描法和循环伏安实验对青铜文物在模拟大气环境介质0.028 mol/L NaCl+0.01 mol/L Na2SO4+0.016 mol/L NaHCO3中的电化学行为进行了研究,探讨了CuCl、Cu2O、碱式氯化铜、混合锈对裸青铜的保护作用.结果表明：在腐蚀电位较负时,Cu2O、CuCl对基体有一定的保护性,而且保护性能优于碱式氯化铜和混合锈;在腐蚀电位较正时,Cu2O或CuCl对青铜的保护性变差,而此时碱式氯化铜和混合锈对裸青铜的保护性优于Cu2O、CuCl.0.1％MBO(2-巯基苯并恶唑)酒精溶液对裸青铜或被Cu2O、CuCl覆盖的青铜阳极过程有一定的抑制作用,对被碱式氯化铜或混合锈覆盖的青铜阳极过程有优异的抑制作用.       

Corrosion of bronze by acetic and formic acid vapours,sulphur dioxide and sodium chloride particles

This paper studies the corrosion of patinated and unpatinated bronze by acetic and formic acid vapours, sulphur dioxide and sodium chloride salt particles, at 100% relative humidity. Weight loss, X-ray diffraction, infrared and scanning electron microscopy were the techniques used. Acetic and formic acid vapours, sulphur dioxide and sodium chloride produce a high corrosion rate on bronze. In general, no protective effect was found by the patina on bronze. The principal compounds identified were Cu₂O, Cu₂S, Cu₅(SO₄)₂(OH)₆ · 5H₂O, Cu(CH₃CO₂)₂ · XH₂O and Cu(HCO₂)2.     

过量CO2气氛环境下青铜表面生成孔雀石锈蚀产物的模拟研究简

利用一种新型的盐溶液-蒸汽模拟技术,在过量CO2的气氛环境中模拟生成孔雀石等锈蚀产物,并利用光学显微镜、扫描电子显微镜、能谱仪、X射线衍射仪、Raman光谱仪等表征仪器,系统表征了锈蚀产物的微观组织和化学成分等。研究分析了在不同过量CO2的气氛环境下,青铜表面生成锈蚀产物的特征及其生长机理,为古代青铜器的保护提供了科学依据。     

On the corrosion resistance of certain ancient chinese bronze artifacts

Certain bronze mirrors and weapons produced in China during the Chou and Han Periods (400 B.C.–200 A.D.) have successfully resisted corrosion for more than two millenia. These artifacts are characterized by lustrous black (sometimes brown, dark green, or grey) surface coatings that protect the high tin bronze substrate. To assess quantitatively the corrosion resistance of these objects, polished sections of the ancient bronze have been exposed to corrosive atmospheres containing H₂S and OCS in wet air. These gases were chosen for these experiments because they are emitted to the atmosphere by natural processes such as swamps and volcanoes and because they are known to rapidly sulfidize copper and its alloys. Experiments demonstrate that the uncoated bronze is relatively susceptible to sulfidation, being similar in this characteristic to modern bronzes previously studied. In a separate experiment, samples of a modern alloy closely matching the ancient bronze composition (70–75% Cu, 20–25% Sn, 1–5% Pb) were treated with a modern protective coating of known permeability and exposed along with a coated ancient bronze sample to a highly corrosive atmosphere. By comparing the resulting sulfidation, the H₂S permeation constant for the ancient coating is estimated to be < 2 × 10^?4 barrers. This is an extremely low figure even by modern standards and suggests the possibility that the coating, could it be duplicated, might prove useful for the protection of modern materials.     

Comparative study of patinas formed on statuary alloys by means of electrochemical and surface analysis techniques

A comparative evaluation of natural patinas and those formed during acid rain laboratory tests was applied to predict patina stability in corrosive urban environments. SEM-EDX analyses of various statuary alloys after the laboratory tests distinguished areas of correlative intensity of attack and S percentage, depending on the alloy. Moreover, EDX and X-ray diffraction (XRD) determined the chemical composition of different coloured patina carefully scraped from a bronze monument, submitted to 75 years outdoors exposure.To analyse the electrochemical potential evolution with time the in situ Pourbaix technique was applied on the laboratory-exposed samples and over representative areas of the monument, at 2 years interval.     

The atmospheric corrosion of quaternary bronzes: The action of stagnant rain water

The corrosion behaviour of a quaternary bronze UNS C83600 exposed to stagnant acid rain was examined through wet-dry tests. During the tests, parallel monitoring was performed to determine the evolution of both the bronze surface and the weathering solution composition. The results show that the kinetics of bronze oxidation is governed by diffusion through a two-layer patina: an inner Sn-rich layer and an external Cu and Pb-rich layer. The corrosion rate of the alloy decreases with time, but the dissolution of individual metals (Cu, Zn and Pb) in the environment increases with different trends, showing progressive patina destabilisation.     

Evolution of patinas on copper exposed in a suburban area

Copper plates were exposed under sheltered outdoor conditions for up to one year, starting in September 2001 in Musashino City, Tokyo, a suburban area. Following various periods of exposure, the patinas on the plates were characterized to investigate their evolution by using X-ray fluorescence analysis, X-ray diffraction, field emission scanning electron microscopy, and glow discharge optical emission spectroscopy. The difference in the roles of sulfur and chlorine in the early stages of copper patination were identified by analyzing the depth profiles of these two elements. Sulfur was found on top of the patina as cupric sulfates such as posnjakite (Cu₄SO₄(OH)₆ · H₂O) or brochantite (Cu₄SO₄(OH)₆). Brochantite appeared only after 12 months of exposure. In contrast, chlorine was found on the surface after only one month of exposure. It gradually penetrated the patina as the exposure period lengthened, forming copper chloride complexes. Chloride ions accumulated at the patina/copper interface, forming nantokite (CuCl), which promoted corrosion.     

MOLTEN SALT AND SLAG IN ANCIENT CHINA

This paper deals with molten salt and slag in ancient China. The following fields are discussed: copper smelting.bronze melting and casting, bronze weapons manufacturing. iron smelting. glaze and glass making. Some important archaeological diseoveries are also describled.     

Papers to be presented at Eurocor '77

Abstract

Despite numerous studies on atmospheric corrosion of copper and copper based alloys, the corrosion induced release processes of individual alloy constituents suffer from significant knowledge gaps. This investigation comprises metal release rate measurements of copper, zinc and tin from some copper based alloys including brass (20 wt-%Zn) and bronze (6 wt-%Sn), and their pure alloying metals, copper, zinc and tin. Data have been generated during a 2·5 year urban field exposure in Stockholm, Sweden and parallel laboratory investigations in a specially designed rain chamber using artificial rain. Brass shows significantly lower annual release rates of both copper and zinc compared to pure metal sheets of its alloy constituents. Zinc is preferentially released compared to copper. Dezincification of brass occurs both at field and laboratory conditions, a process influenced by rain characteristics. Alloying with tin does not largely reduce the release rate of copper from bronze compared to pure copper. No measurable amount of tin is released from the bronze surface.     

Characterisation of anodic layers on Cu-10Sn bronze (RDE) in aerated NaCl solution

The anodic surfaces formed on Cu-10Sn (wt.%) alloy (α-bronze) are investigated in aerated 0.1 M aqueous chloride solution, using electrochemical reduction and characterisation methods such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the whole anodic domain, investigations performed on a bronze rotating disk electrode (RDE) reveal the systematic formation of a uniform oxidation layer. It is evidenced that the chemical composition of the layer varies with the applied anodic potential, but also that the latter always exhibits a poorly crystallised (probably nanocrystalline) hydrated and hydroxylated nature. Close to E_oc, the compounds are mainly (hydroxide) oxides of tin and copper, incorporating very low amounts of chlorides. At intermediate oxidation potentials corresponding to the active-passive transition, the first oxidation peak corresponds to the formation of hydrated tin oxyhydroxide chloride species which transforms in a more stable one - probably related to the Sn(II) → Sn(IV) oxidation. At higher anodic potential, on the current plateau, the layer contains hydrated tin (IV) oxyhydroxide and copper chloride (mainly CuCl). However, XRD and XPS results reveal that the barrier layer has a complex nature, including unidentified products and different spatially distributed charged surface zones. The corrosion mechanism involves an internal oxidation of the alloy linked to a preferential dissolution of copper, namely a decuprification. A decuprification factor f_Cu is defined and calculated. Both f_Cu and the layer thickness increase with the applied potential. We show unambiguously that the tin compounds remain in the corrosion layer, acting as stabilizing species. It is suggested that the tin species promote the formation of a network as for tin oxide xerogel, through which copper ions and anions migrate. Both the layer microstructure and the decuprification factor (f_Cu) are in agreement with those found in Type I patina of ancient bronzes.     

Comparaison between archaeological and artificially aged bronze interfaces

This paper establishes a comparison between the interface exhibited by an archaeological bronze alloy when previously submitted to a burial environment and a chloride electrolyte. The SEM‐EDAX analysis shows that the archaeological surface exhibited a three layered structure characterized by a tin enrichment at the inner layer, the succeeding layer being only by copper species, the outer layer is relevant of the burial environment. This corrosion structure is to be regarded as Type I morphology described in the literature for bronzes exhumed in the northern side of the Mediterranean Sea. When submitted to chloride solution, the open circuit potential of material tends to constant values after five days of immersion time. The evolution of the resulting interface was characterized using EIS and the spectra obtained were tentatively fitted using a simple electrical equivalent circuit. When, the interface attained the steady state, the interface shows a patina with two layered morphology similar to the archaeological one.     

Electrochemical and spectroscopic investigation of synergestic effects in corrosion inhibition of aluminium bronze Part 1 – In pure HCl

Abstract

The synergistic inhibition of aluminium bronze corrosion by 4-phenyl semicarbazide hydrochloride (PSC) and the iodide ion in HCl of pH (1·8-2) at 60°C has been assessed by polarisation studies, Tafel plots, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The addition of 0·01M KI to 0·01M PSC improved the inhibition efficiency from 86 to 94% owing to the formation of a CuI-PSC complex which is more compact, protective and hard than the film formed in the absence of iodide. The inner layer of this protective film contains a small amount of aluminium oxide, which increases the protective nature of the film and retards the dissolution of the underlying aluminium bronze.     

Morphological study of 16-year patinas formed on copper in a wide range of atmospheric exposures

Much information is available on the atmospheric corrosion of copper and patina formation mechanisms in the short, mid and even long term. However, studies of the structure and morphology of patina layers are less abundant and mostly deal with patinas formed in the atmosphere over a small number of years. The present study concentrates on the structure and morphology of corrosion product films formed on copper after long-term atmospheric exposure (13-16 years) in five Spanish atmospheres of different types: rural, urban, industrial and marine (mild and severe). Characterisation has been performed by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term copper corrosion is higher in industrial and marine atmospheres and lower in rural and urban atmospheres. In all cases a decrease in the corrosion rate with exposure time is observed. The formation of antlerite [Cu₃SO₄(OH)₄] is seen in more acidic conditions and in specimen areas subject to a high time of wetness. The presence of nantokite (CuCl), which is not generally mentioned in field studies, has been detected under the cuprite layer very close to the base copper.     

Copper patinas formed in the atmosphere—III. A semi-quantitative assessment of rates and constraints in the greater New York metropolitan area

Because of the ability of copper to interact with a variety of atmospheric species and to retain the signatures of those interactions in a stable patina layer, copper is a particularly appropriate material for a study of the rates of corrosion processes. The rate of formation of copper patinas in a specific geographical location is dependent upon the atmospheric concentrations of corrosive species, their degree of interaction with the copper surface, and the mechanisms and rates of the processes that govern interaction. A semi-quantitative model of patina growth is developed for samples exposed for up to several decades to the atmosphere in the greater New York City metropolitan area. In contrast to previous studies of patina chemistry, which have dealt with equilibrium conditions, the present work treats the problem from a kinetic standpoint. Under modern atmospheric circumstances, it is shown that neither the supply of atmospheric water, the supply of incorporated corrosive species from the atmosphere, nor the rate of oxidation chemistry are limiting factors in patina growth. Rather, the growth is controlled by the rate of cementation of patina components at early stages of the growth process and by the supply of diffusing copper ions at later stages. To assess patina formation on a historical basis, estimates are made of atmospheric and precipitation chemistry in the greater New York City metropolitan area over the past century. The rapid formation of modern patinas, compared with those formed a few decades ago, is shown to be a consequence of increased atmospheric levels of strong inorganic acids, particularly H₂SO₄, perhaps in combination with increased concentrations of atmospheric oxidizing species and organic compounds.     

碱钨青铜粉体的溶胶-凝胶法合成与表征

采用溶胶-凝胶法合成了碱钨青铜(M_xWO_3,M=Cs,Rb中的1种或2种)粉体,探究了铯、铷分别单掺杂和铯、铷共掺杂对钨青铜粉体晶相、形貌的影响。利用X射线衍射仪(XRD)、X射线能谱仪(EDS)、扫描电子显微镜(SEM)、X射线光电子能谱仪(XPS)、紫外可见近红外光谱(UV-Vis-NIR)、隔热膜温度测试仪对所合成碱钨青铜粉体的物相、形貌、光学性能和隔热性能进行了表征。结果表明:溶胶-凝胶法所合成的碱钨青铜粉体结晶性较好、粒径较小;Cs、Rb共掺杂的透明绝热指数K=157.34比Rb单掺杂提高了23.92,改善了Cs单掺杂在1076 nm处透过率突变的情况;Cs、Rb共掺杂的温差(与起始温度相比)为4.9℃,比Rb单掺杂的温差下降了5.7℃。