Hajjaji, N. Et Al. Conservation Problems an a New Method Protection Bronzes. 2007

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1. INTRODUCTION

Since the end of the 19th century, the excavations car-ried out in Morocco have produced a large number of ar-chaeological objects made of copper alloys such as bronze.The most spectacular and important bronze objects are ex-hibited at the Archaeological Museum of Rabat.

Most of these objects have been discovered in Volubilis,a prosperous Roman city between 1st century BC and 3rd

Century AD, but many objects were also found in Banasa,Thamusida, Sala, Lixus or Tamuda, and represent one of the most important and famous collections of Romanbronzes from North Africa and the Mediterranean basin [1].

The museum collection is representative of differentperiods of the ancient history of Morocco and expressesthe first influence evidences of the Mediterranean civi-lization in Mauritania Tingitana, which corresponds to theRoman period in Morocco. Some of the objects are ondisplay to the general public, but many are stored away inthe museum in uncontrolled conditions, which favoursactive corrosion for the bronze objects.

To deal with the problems of preserving this impor-tant collection, scientific, analyses, conservation treat-ment were carried out in Morocco and abroad to find abest solution to preserve them.

The PROMET project is very important in this effortto preserve this museum collection by helping our effortto preserve such a metal collection, and to strengthen ourinternational cooperation with leading scientists in thisfield to determine the best practice for our country. Tenbronze artefacts were selected to test new corrosion in-hibitors after initial testing using bronze coupons. The re-sults of this investigation will allow for new knowledge asto the best way to protect such artefacts.

2. THE EXHIBITION CONDITIONS

The bronze collection encounter great problems re-lated to the conditions of their exhibition at the museum,which are divided into two factors: natural and humanones. As far as the natural factors are concerned, uncon-trolled environment (temperature, light and relative hu-midity), insects and dust are the leading threats. Lessqualified technical staff and a lack of resources is alsocontributing to the continued corrosion of these collec-tions. In 2003, the renovation of the museum’s buildingshelped to improve the environmental conditions.

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Conservation problems and a new method of protection:

The case of bronzes at the Archaeological Museum of Rabat, MoroccoNajat Hajjaji1, Fatima Zohra El Harrif 2, Choumicha Kaouane3, Abdellah Srhiri4,

Hisasi Taknouti5 and Kamal Rahmouni5,1Laboratory of Electrochemistry, Corrosion and Environment, Faculty of Science

2National Institute of Science of Archaeology and Patrimony, Morocco3Direction of Patrimony, No 17, Morocco

4SERVICHIM SARL society of corrosion inhibitors production, Morocco5LISE - UPR 15, CNRS, Paris VI-France

Faculty of sciences BP 133Kenitra city, MOROCCO

Phone: (00212)61109025Fax: (00212)37379682e-mail: [email protected]

The paper outlines the general conservation problems and needs for the archaeological bronzes at the museumin Rabat Morocco. It also describes the preventive conservation measures that were taken in order to improvethe environmental conditions of the bronzes on display in the exhibition room of the museum. However, themain aim of this study was to test a new corrosion inhibitor able to protect archaeological bronzes against corro-sion. The authors have synthesized an organic product named 3-phenyl 1,2,4- triazole-5 thione noted PTS. Thiscompound acts as good corrosion inhibitor to protect bronze in neutral chloride medium. So as to facilitate itsuse in application to bronze artefacts, a special formulation known as FPTS was prepared. The inhibiting effectof FPTS was studied using transient electrochemical technique and surface analysis. The results indicate thatFPTS has good inhibiting effect and this inhibitor formulation acts on both anodic and cathodic processes.

Keywords: archaeological bronze, museum, preventive conservation, corrosion inhibitors

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Conservation problems and a new method of protection: Bronzes at the Archaeological Museum of Rabat

2.1 Exhibition Room

Between 1952 and 2003, the bronze collection was ex-hibited in an oval room in a surface area of 162m2. Thisroom has 24 windows situated in the roof and permanent-ly closed to avoid the entrance of the natural light Howev-er, some of them can remain opened for a long time, es-pecially when broken and aerosol salts from the AtlanticOcean can easily contaminate the room especially by thewind and the salty fog. This room was accessible by alarge door leading directly to the outside. The museum islocated in a narrow street and only approximately 200mfrom a main avenue, which exposes the collections to thepollution created by the heavy traffic. Such conditionswill also be enhanced by the climate of Rabat, the hydro-gen sulphide and the sulphur dioxide as well as the carbondioxide, which is producing carbonic acid.

Salt efflorescence traces were located in the roof of

this room, which is a sign of water infiltration. Further-more, the walls were covered by low density wooden pan-els, which absorb the humidity during winters and releaseit in the summers.

The exhibition room of the bronze collection was dis-played in a long showcase along the wall and totallyopened at the top. The objects were organized accordingto the themes and laid out on wooden panels. The statueswere put on marble or rock plinth without any more pro-tection, and could be easily touched by the visitors. Thelighting was issued from fluorescent tubes and the naturallight from the door and the 24 windows.

Apart from the curator, the technical and scientificstaff do not have sufficient training in conservation to

deal with the preservation of such large collections.

2.2 New exhibition conditions

During the summer 2003, the museum was completelyrenovated. The large glass case in the wall was replacedby several mobile showcases with filtered and incorporat-ed lighting. Such conditions largely decreased the pene-tration of the insects, dust and airborne pollutants. As faras the water-tightness of the new display cases no waterinfiltration is possible.

Lighting was also improved by installing a new electri-cal network to avoid natural light by closing completelythe 24 windows of the room and the door leading to the

outside. The showroom became accessible by anotherlarge room for the exhibition of the marble collection.Between 2000 and 2001, the International Centre for

the Study of the Preservation and Restoration of CulturalProperty (ICCROM) organized a course of conservation-restoration of the cultural heritage collections of the mu-seums called “Rabat Course” with the aim to strengthenthe experience of scholars from the Maghreb countries.Thus the metal collections (bronze and iron) benefitedfrom several studies related to the conditions of their ex-hibition and storage rooms. Our paper provides a sum-mary of these studies [2]:

a. Lighting: two luxmeters were placed in the open show-case. One of them is located near the door and another in-side the room. On average 250 lux was recorded during theyear with a large peak of 900 lux in July and August between6 and 8a.m. in the morning coinciding with the morning sun-shine. Whereas, those placed near the door recorded on av-erage more than 350 lux with a peak in October equivalentto 500 lux recorded between 8 and 10a.m. in the morning.

b. Climate: sensors were placed at the entry and the bottomof the room to control the thermo-hygrometric variationsrecorded in the room. The overall results have still not beengiven to the museum. However, initial results show that theRH recorded during three days in November oscillated be-tween 73% and 92% and the temperature between 18 and21oC with a marked increase between 12 a.m. and 5 p.m.

2.3 New dataDuring July and August 2006, new measurementswere carried out by the Moroccan members of thePROMET project inside the showroom. The results (Fig-ures 1, 2 and 3) do not reveal big fluctuations in the tem-perature nor in the RH during three days: the 3rd and 15th

July and the 3rd August. However, the RH is alwaysgreater than 60%, and in the presence of salt aerosolsgreatly increases the corrosion rate of metals.

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Figure 1 - Variations of temperature in Bronze roomat different time of the day

Figure 2 - variations of RH in Bronze room at differenthours of the day




N. Hajjaji et al.

Figure 3 - variations of the temperature in bronzeroom and reserve at different days

3. NEW ORIENTATION TO PROTECT THE AR-

CHAEOLOGICAL BRONZES

The PROMET project aims at establishing a commonvision for the problems of museums and a practical ap-proach for resolving conservation problems and lookingfor better and efficient method to protect and preservetheir metal artefacts.

We have chosen a “phalere” from the ten bronzes se-lected in PROMET project for further analysis and to testthe new corrosion inhibitor using some electrochemicaland spectroscopic techniques. First the phalere’s compo-sition and patina layer were determined using scientificanalysis. Then, a new organic compound, 3-phenyl 1,2,4-triazole 5-thione (PTS) as corrosion inhibitor was tested

on bronze coupons before applying this new protectionmethod to this archaeological object. The bronzecoupons were prepared by author’s partner in PROMETproject.

3.1 The “phalera”

The photo illustrates in Figure 4 the original form of this bronze. The patina layer formed is a green to greycolour.

3.2 Inhibitor

The product tested is an organic corrosion inhibitor,PTS, which was synthesized in our laboratory and testedas a formulation noted FPTS [3].

3.3 Corrosive medium

The bronze coupons were exposed to atmosphericcorrosion. The electrochemical test was carried out in 3%NaCl solution, prepared with distilled water and thereagent grade chemical products.

3.4 Techniques

The electrochemical tests was carried out using aEG&G apparatus 6310 with a classical three electrodecell. A platinum grid was used as counter electrode,Ag/AgCl as a reference electrode, and bronze coupon asa working electrode. The analysed frequency range was

100 KHz to 1 mHz, 5 points per decade and 5mv as ampli-tude of the sinusoidal signal used.The SEM EDX analysis was carried out using HV 16

KV with ZAF quantification.

4. RESULTS AND DISCUSSION

4.1 SEM/EDX observations of phalera fragment

A small thin section of the phalera was taken and isshown in Figure 5.

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Figure 4 - General view of phalere

This phalere measuring: L= 8,2cm and l= 5,9cm wasmanufactured locally during the Roman period.

Figure 5 - SEM photograph of Phalere cross section

The SEM observations revealed various zones on thesample area as shown in Figures 5 and 6.

Figure 6 - Cross-section of Phalere




Conservation problems and a new method of protection: Bronzes at the Archaeological Museum of Rabat

The patina layer in grey colour at the surface was fewtens of micrometers. This patina layer analysis permits toclass it in type II as described in previous work [1]. EDXanalysis was also carried out on the cross section to deter-mine the metal’s chemical composition. The correspond-ing results are summarised in Table 1.

Table 1 shows that oxygen is the major constituent of patina layer, and Si, Cu, Zn, Sn, Pb, Cl and P were alsodetected.

The results at the substrate show that the bronze isquaternary Cu-Pb-Sn-Zn alloy.

Table 1 - Bronze composition of phalere

The values of R (resistance) and C (capacitance) as-sociated to high frequencies semi circle, are given inTable 2. The associated characteristic frequency Fc corre-sponds to the apex of the capacitive loop.

Table 2 - Fc, C and R values issued from Figure 7

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External part of patina is rich in exogenous elementslike O, P, C, Mg, K and poor in principal constituents of substrate like Cu, Sn, Pb, and Zn. The first elementscome from soil during patina formation.

It is interesting to note that the P is present only in thesuperficial layer, and this element may come from humanmanipulations.

We can also note that the Cu/Sn ratio is higher in thecorrosion layer rather than the metal substrate. This mayinterpreted by selective dissolution of copper named asdecuprification in accord with previous paper [4].

4.2 Electrochemical measurments with bronze coupons

Electrochemical measurements were carried out withthe PROMET bronze coupon, which have roughly a samecomposition as the archaeological artefacts.

The electrochemical impedance spectra were collect-ed at the open-circuit potential. The diagrams obtainedwithout inhibitor are illustrated in Figure 7. The shape of these diagrams is not modified by immersion time, andone capacitive loop followed by an initiation at low fre-quencies of a beginning of second time constant as shownin this figure.

Figure 7 - Effect of immersion time of bronze couponin 3% NaCl solution

I’immersion Frequency Capacitance Resistanceperiod (h) (Hz) (ÌF) (kø)

2h 1 106 1.5024h 0.398 110 3.6148h 0.251 123 5.1872h 0.158 269 3.75

The resistance value associated to this high frequencyloop, and obtained by intersection of obtained semi circlewith real axis. The obtained value increases with an in-crease of immersion time. Simultaneously, the capaci-tance is raised. This may correspond to a roughening andthe accumulation of corrosion products at the surface,which can result from the hydrolysis of patina layer or theformation of corrosion products. The patina layer may re-sult in a volume increase. The application of surfaceanalysis techniques indicate that the electrical propertiesof patina layer are modified.

The impedance diagrams in presence of inhibitor arepresented in Figure 8, which shows that an increase of im-mersion time modifies the behaviour of the bronzecoupon.

The polarisation resistance increases dramatically whenthe immersion time increases from 2 to 144 hours. This re-sistance increase is accompanied by an apparition of twotime constants. The second one seems to have a higher val-ue than 8 kohms. The inhibiting effect may be reinforcedwith immersion time and in comparison with blank test, afrequency shift was noted to higher values in presence of in-hibitor. It passes for example from 1Hz to 250 Hz after 2hours of immersion time. The low frequency loop is verymarked by the presence of the inhibitor and at high immer-sion time values. The associated resistance values seem todiverge to the infinite [2]. The polarisation resistance is like-ly very high with inhibitor addition compared to blank es-say. The values of dielectric parameters for the two capaci-tive loops are displayed in Table 3.

Figure 8 - Effect of immersion time of bronze couponin 3% NaCl solution + 0.07% of inhibitor FPTS




N. Hajjaji et al.

Table 3 - Values of parameters associated to impedancespectra presented in Figure 8

were developed in the Laboratory of Electrochemistry, Cor-rosion and Environment in the Faculty of Sciences in Kenitra(Morocco). The inhibiting performance was tested on bronzecoupons. The FPTS shows good inhibition efficiency, and it

will be very interesting to test it on real artefacts for protec-tion especially in a marine environment. Currently, the effectof the corrosion inhibitor on the phalera fragment is beingtested, and this inhibitor will be tested other bronze artefactsselected from our collection from the Rabat museum.

ACKNOWLEDGEMENTS

This work was realized in the framework of “PRO-MET” project and the authors gratefully acknowledge thefinancial support. We also would thank the Direction of Patrimony for giving agreement for this study, and the LISELaboratory (Paris VI-France) for its collaboration throughCNRS – CNRST project PICS no3388.

REFERENCES

[1] Boube-Piccot, Ch.: ‘‘Les bronzes antiques du Maroc”,I la statuaire, Etude et Travaux d’Archéologie Maro-caine IV, (1969).

[2] Serarfi, S., Annane, S.: ‘‘La conservation des objetsde bronze en exposition”, Mémoire de fin d’étudespour l’obtention du diplôme d’étude supérieure spé-cialisées en conservation-restauration, «Cours deRabat», (2000-2001).

[3] Dermaj, A., Chebabe, D., Hajjaji, N., Joiret, S., Rah-mouni, K., Srhiri, A., Takenouti, H., Vivier,V.: ‘‘Étude

de la corrosion de bronze Cu-8%Sn et l’effet inhibi-teur de 3-phenyl-1.2.4- triazole-5-thione”. Acte du17ème Forum d’impédance, électrochimique, éditépar C. Gabrielli, Paris 257-268(2005).

[4] Rahmouni, K.: ‘‘Corrosion et protection des bronzesrecouverts de patine: étude électrochimique spectro-scopique de la surface d’objets archéologique et syn-thèse d’une patine sur un bronze B66”. Thèse soutenuel’Univesité Ibn Tofail à Kénitra (Maroc) le 16 Decem-ber 2005.

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Time RHF RBF CHF CBF FHF FBF

(h) (kΩ) (kΩ) (ÌF) (ÌF) (kHz) (Hz)2 0.088 0.661 0.450 240 3.98 1.0024 0.104 1.67 0.380 378 3.98 0.25148 0.161 2.73 0.240 584 3.98 0.10072 0.677 3.89 0.230 648 1.00 0.063120 1.85 »8 0.216 - 0.398 -144 2.95 »8 0.210 - 0.251 -

For the surface area of 1cm2

The summary of this table suggests that the low fre-quency loop has the same physical origin to the loop ob-served in absence of inhibitor. The values of RHF associ-

ated to high frequencies semi circle are lower than thoseobtained. Thus, the attribution of this loop to chargetransfer leads to an increase of the rate of corrosion. Thiswould be contradictory with previous analyses confirminginhibiting effect of FPTS to protect bronze. The high fre-quency loop may be therefore associated with a thin in-hibitor film formed on bronze surface.

In accordance with this interpretation, the low fre-quency tail observed after long immersion may be attrib-uted to the Faradic process being held on the level of thesites where the film is missing.

The estimated values of polarisation resistance arehigher than those obtained for blank tests the inhibitoracts as a barrier to protect the substrate bronze. Simulta-

neously, the FPTS may be linked with the patina layer de-posited on bronze coupon and reinforce its stability [4].

5. CONCLUSIONS

No one doubts the importance of archaeological bronzeobjects at the Rabat museum in terms of their great historicaland cultural value. However, for preserving and safeguardingthis rich and diversified inheritance, new corrosion inhibitors


Hajjaji, N. Et Al. Conservation Problems an a New Method Protection Bronzes. 2007

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