Al-Saad, Z. y Bani, M. Corrosion and Preservation Islamic Silver Coins. 2007

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1. INTRODUCTION

Coins are important historical documents as they pro-vide valuable information about the various aspects of an-cient civilization used them. Coins are usually found inlarge numbers in archaeological excavations, because theyhave high survival rate when compared to other types of metal artefacts.

The analysis of elemental composition of ancient coinshas generated a lot of interest in recent years as it can pro-vide valuable information on various aspects of humanpast civilizations particularly the political socio-economic

and technical aspects. [1].These important historical records, especially those

made of copper and silver, can suffer from various formsof corrosion caused by various natural and human fac-tors. Corrosion may in some cases be so extensive that itmay lead to complete mineralization of coins with theconsequent loss of important historical information.Identifying and understanding the causes and mecha-nisms of corrosion is a key step toward designing and im-plementing effective conservation measures to preservethese coins.

This paper focuses on the study of the corrosion be-haviour of silver coins. A collection of Islamic silver alloycoins from the collection of the Museum of JordanianHeritage at Yarmouk University is used for this purpose.

Corrosion behavior of silver alloy coinsIn general silver can be found in a native state com-

bined with gold, tin, copper, and platinum. It is completelystable in aqueous solutions of most pHs as long as oxidiz-ing agents or complexing substances are not present. Inaddition, silver is not appreciably affected by dry or moistair that is free from ozone, halogens, ammonia, and sulfurcompounds [2]. Silver is readily tarnished by hydrogen sul-fide, which is an atmospheric contaminant: a thin layer of dark silver sulfide (Ag 2S) is formed. Occasionally silver

sulfide forms thick black layers of patina. The presence of chlorides in solids where silver is buried causes the forma-tion of stable, gray silver chloride (AgCl) which may,through the great expansion accompanying the changefrom metal to mineral, cause severe deformation of theoriginal object [3]. In the case of silver alloys with signifi-cant amount of copper, the copper corrodes preferentiallyforming cuprous oxide, cupric carbonate, and cuprouschloride [4].

Silver corrosion processes vary according to the origi-nal composition of the silver alloy and the conditions towhich it has been subjected. Therefore, corrosion behav-

ior of silver is a function of the surrounding environmentalconditions.

All metals undergo corrosion, but the chances forpropagation of the corrosion process are determined bythe structure of the oxide layer growing on the metal con-cerned. With silver, the oxide is formed by insertion of oxygen into the octants of the face centered cubic silverlattice, and the resulting film is oriented with respect tothe metal. Silver oxide should, therefore, be highly protec-tive. But despite this compatibility of silver oxide, silverbecomes easily tarnished. The causes of tarnishing are dis-cussed below [5].

In the moist air, the adsorption of oxygen on silver isaccompanied by simultaneous adsorption of water mole-cules. This combined adsorption gives rise to hydrogenperoxide which, in its turn, generates and also promoteslattice defects in the thus formed silver oxide. This thenleads to a non-stoichiometric model of metal oxide. Thepassages pierced thus through the oxide film are open tothe silver cations, which then drawn by the electrostaticpotential already set up across the oxide film, move fromthe metal phase, through the oxide phase, to the inter-phase oxide/atmosphere. A subsequent contact of the sil-ver cations with the sulfide ions from the polluted atmos-phere air produces the insoluble black silver sulfide,which is responsible for the tarnishing of silver [5, p.184].

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Corrosion behavior and preservation of Islamic Silver Alloy Coins

Ziad Al-Saad and Manar Bani HaniFaculty of Archaeology and Anthropology, Yarmouk University

Yarmouk UniversityIrbid-Jordan

e-mail: [email protected]

The corrosion behaviour of a collection of Islamic silver-based coins from Museum of Jordanian Heritage atYarmouk University was investigated by using a combination of non-destructive analytical techniques includ-ing scanning electron microscope with energy dispersive X-rays (SEM-EDX) and Energy Dispersive X-rayFluorescence Analysis (ED-XRF). The results of this study have been used to devise the proper conservationapproaches and methods that should be adopted to preserve these important cultural heritage materials.

Keywords: Islamic Silver Coins, Corrosion, Conservation

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Z. Al-Saad and M. Bani Hani

The reactions between silver and hydrogen sulfide (H 2S),and silver and carbonyl sulfide (OCS) in the atmosphereare simplified by the following overall reactions [6]:

2 Ag + H 2S + 1/ 2 O2 Ag2S + H 2O (1)2Ag + OCS Ag2S +CO (2)An aggravating factor in the tarnishing of silver is that

the already formed film of silver sulfide upon the film of silver oxide has a type of lattice in which the arrangementof the sulfide anions is perfect, while the silver cations aredisarranged and also free to move through the film. Inaddition, silver sulfide is a better electrical conductorthan silver oxide and its cations diffuse with ease throughthe corrosion film, which under these conditions growsfollowing a parabolic relationship with time. Such disor-ders contribute eventually to the growth of spongy silversulfide [5].

Silver embrittlementSilver is normally malleable, ductile and easily fabri-cated metal. However, some archaeological silver objectscan be brittle, as a long-term consequence of corrosionand micro-structural changes [6].

There are many types of embrittlement. Corrosion-in-duced embrittlement is a consequence of selective corro-sion that penetrates the metal and eventually fragments it.This occurs whether or not the object is acted upon by ex-ternal loads or forces, although they may accelerate theprocess. Micro-structurally induced embrittlement causesapparently pristine metal to crack and fracture under theaction of external loads or forces. However, it is importantto note that corrosion-induced and micro-structurally in-duced embrittlement can act synergistically, and that mi-cro-structural features are involved in both kinds of em-brittlement [6].

Recognition and determination of corrosion-inducedand micro-structurally-induced embrittlement, and alsotheir synergy, are important for conservation of ancientand historic silver alloys [7].

2. SAMPLE AND EXPERIMNTAL PROCEDURES

2.1 The SampleTen silver-based coins that are dated to the Islamic pe-

riod (Abbasid) are selected for the purpose of this study.The coins are part of Islamic coins collections of the Mu-seum of Jordanian Heritage. The selected coins wereminted in a short time span (43 years) in the period: A.D.769 A.D.812 (A.H. 152 - A.H.196) during the reigns of five prominent Abbasid caliphs: Mansur, Mehedi, Hadi,Harun er-Rashid, and Amin. The collection is a typical ex-ample of the Islamic coinage traditions during the Ab-basid period. A selected example from the study collectionis shown in Figure 1.

Figure1 - The obverse (left) and reverse (right)of three coins selected from the study sample.

2.2 Previous treatment of coinsThe selected coins were previously chemically cleaned

using thiourea and formic acid. However, mild silver dipsolution that consists of 5 percent thiourea and 5 percentformic acid was prepared to remove outer silver tarnishand copper corrosion products. The solution was appliedlocally using cotton swab in order to avoid future risks andto keep the cleaning process under control. After clean-ing, the coins were thoroughly washed using distilled wa-ter in order to remove the remains of the added chemicalsand the soluble products that they carry. After that, thecoins were dried using acetone. No coating were appliedto protect them, they were displayed in the museum show-cases with no environmental control.

2.3 Condition AssessmentVisual examination combined with optical microscopic

and SEM investigation was employed to assess and evalu-ate the general preservation condition of the coins.

2.4 Chemical AnalysisChemical analysis was carried out to identify the

chemical composition of both the metal core and the cor-rosion products formed. This enables the determinationof the extent of damage of the artifact and the under-standing of the corrosion causes and mechanisms. Theseare essential prerequisites for the development and imple-mentation of effective and suitable conservation treat-ments [8].

Chemical analysis of corrosion products was made us-ing two analytical techniques: XRF (XRF: PANalytical,MiniPal 2) and SEM-EDX . Energy dispersive X-ray Fluo-

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Coin 1

= 23.84 nm

Coin 2 = 24.49 nm

Coin 1 = 27.86 nm

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Corrosion behaviour and preservation of Islamic silver alloy coins

rescence Analysis (ED-XRF) in order to determine theAg content as well as minor constituents (Cu, Pb) andtrace elements. In addition, the presence of chloride orsulfide ions gives an indication about the presence of cor-rosion, its type and extent within the surface of the metalartifact.

Energy dispersive X-ray microanalysis with a scanningelectron microscope was used to assess the extent and na-ture of the corrosion of the coins. The first EDX-SEManalysis was done in four different but neighboring pointson coin 3 as shown in Figure 2, as well as the results of theanalysis in Table 1. Further analysis was carried out on acorrosion area for coin 3 as shown in Figure 3 and analyti-cal results given in Table 2 .

Table 1 - Chemical Compositions of Points on Coin 3

Using SEM-EDX Analysis (percent error = 0.01%).

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Figure 2 - The Four Analyzed Pointson the Surface of Coin 3.

Figure 3 - Corrosion Point Analyzed.

Element wt.%

O 29.9Zn 0.6Na 0.3Mg 0.8Al 4.4Si 24.2P 0.4

Au 0.9Hg 0.6S 0.5Cl 8.0Ag 27.4K 4.1

Total 100

Table 2 - Chemical Compositions of a Corrosion Areaon Coin 3 Using SEM-EDX Analysis

(percent error = 0.01%).

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Table 3 - Corrosion Forms using different techniques of analysis for the coins.

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3. RESULTS AND DISCUSSION

3.1 Visual and Microscopic ExaminationThe results of the visual and microscopic investiga-

tions are given in Table 3. The results show that the coinsare generally in good condition. However, all the coinshave a thin general corrosion layer formed. Some of thecoins suffer from localized corrosion and some others

have slight embrittlement problems.

3.2 XRF AnalysisThe results of the XRF analysis are presented in Table

4. The results show that all the coins have a high silvercontent which indicates the high quality control of themanufacturing process of these Islamic coins.

All of the coins contain copper, and most of them haveappreciable amounts of lead which indicates the deliber-ate additions of these two metals in the alloy used for themanufacturing of the coins. Copper could have been de-liberately added to debase the silver coins and save themore expensive silver. In addition, there is a technical rea-son for the addition of copper to silver. Pure silver is nor-mally considered too soft to be used for coinage and couldrelatively easily be worn in circulation. The addition of copper considerably increased the strength, hardness andwear-resistance of silver without leaving a deleterious ef-fect on its ductility and formability. The chemical analysisresults show an appreciable amount of lead present in thecoins. This indicates that the silver used in the coins wasproduced by the cupelation process. The complete re-moval of lead in this process seems never to have beenachieved. Silver coins can be embrittled in the course of time due to the remains of lead left from cuppelation. Ithas been found that it is particularly bad in the presence

of copper. Condition assessment of the coins shows clearcorrelation between lead content and degree of embrittle-ment of these coins.

The analysis of the corrosion products reveals a rela-tively high chloride content in some of the coins with verylow content of sulfide. This indicates that silver chloriderather than silver sulfide is the major corrosion product of these coins. It can be concluded that the coins were exca-

vated from a chloride rich burial environment.

3.3 SEM-EDX AnalysisExamination using SEM revealed corrosion and em-

brittlement phenomenon that are present in most of thestudied coins. SEM investigation shows localized corro-sion that penetrates the silver and enables embrittlementdue to cracking along the corrosion paths as seen clearlyin Figure 4.

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Table 4 - Chemical Composition of the Coins Using XRF Analysis. (Percent error =0.1% ).

Figure 4 - Corrosion Induced Embrittlement in Coin 1(SEM image)

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In addition, visually undetected scratches probablycaused by mishandling or improper cleaning were seen inthe surface of many coins (as depicted in Figure 5)

Figure 7 - Lost Part from the Rim of Coin 1(SEM image)

4. CONSERVATION CONSIDERATIONS

The conservation requirements of the selected coinsvary to large degree based on the nature and state of cor-rosion and on the nature of the contaminants. Some of thecoins need cleaning to remove the localized disfiguringcorrosion at the rims. Some of them need only cosmeticcleaning to remove some of the copper corrosion that ob-structs some of the details and for aesthetic reasons.Chlorides need to be removed to stabilize the coins and toreduce their damaging effect on the copper component of base silver alloys [4].

The crucial factor guiding decisions about cleaningand/or consolidation of an object is its stability. Priorityshould be given to stabilization and only later can questionsabout appearance and legibility, which are generally moresubjective, be addressed. In some cases, the preservation of details such as evidence of manufacturing techniques mayguide the selection of methods of treatment [9].

5. CONCLUSIONS

Microscopic and chemical analysis enables the deter-mination and understanding of the corrosion behaviour of silver coins under various surrounding conditions. Effec-tive conservation and stabilization treatments of such ob-jects are only possible when this behaviour is well under-stood.

REFERENCES

[1] Al-Saad, Z.: Chemical Analysis of some UmayyadDirhems Minted at Wasit, Journal of Economicsand Social History, Vol. 42, No. 3, pp. 351-363(1999).

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Figure 5 - Scratches in the Surface of Coin 1(SEM image)

Using SEM we were able to relate uncorroded regionswith the non-inscribed regions whereas it appears thatcorrosion and embrittlement are concentrated in the in-scribed regions where the corrosive agent are trapped asshown in Figure 6. In addition, the coins greatly sufferfrom the rims with some parts totally lost. This may be aresult from the synergistic action of plane of weaknessthat results from the continuous downfall on the rim andcorrosion that increase the weakness of this part of thecoin as shown in Figure 7.

Figure 6 - Corrosion and Embrittlementin the Inscriptions Region of Coin 1 (SEM image)

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[2] Plenderleith, J. and Werner, A., The Conservationof Antiquities and Works of Arts, London: OxfordUniversity, pp. 239 (1971).

[3] Goffer,Z., Archaeological Chemistry. New York:John Wiley and Sons.Gold and silver mines (1980).

[4] Hamilton, D.: Conservation of Metal Objects fromUnderwater Sites: a Study in Methods, Austin:The Texas Memorial Museum, pp.15 (1976).

[5] Stambolov, T.: The Corrosion and conservation of Metallic Antiquities and Works of Arts, Amster-dam: CL Publication, pp. 182-184 (1985).

[6] Wanhill, R., et al.: Damage Assessment and Preser-vation of an Egyptian Silver Vase (300-200 BC), Ar-

chaeometry, Vol. 40, No. 1, pp. 123-137 (1998).[7] Wanhill, R.: Brittle Archaeological Silver: a Frac-

ture Mechanisms and Mechanics Assessment, Ar-chaeometry, Vol. 45, No.4. pp. 625-636(2003).

[8] Linke, R., Schreiner, M., and Demortier, G.: TheApplication of Photon, Electron and Proton InducedX-ray Analysis for the Identification and Characteri-zation of Medieval Silver Coins, Nuclear Instru-ments and Methods in Physics Research B, Vol. 226,pp. 172178(2004).

[9] Costa, V.: The Deterioration of silver Alloys andsome Aspects of their Conservation, Reviews inConservation, Vol. 2, pp. 19-35 (2001).

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Al-Saad, Z. y Bani, M. Corrosion and Preservation Islamic Silver Coins. 2007