1. INTRODUCTION

From the inception of metallurgy, craftsmen have tak-en at great pains to decorate and to embellish the surfaceof objects of different nature with thin or thick metalliccoatings or to vary their appearance or external naturewith also fraudulent intent to produce forgeries [1-5].

Different manufacturing methods have been used forapplying an Au or Ag leaf, foil or thin film onto the sur-face of less precious substrates like metals, ceramics, mar-bles and wood to produce coated jewels, statues, coins,amulets for creating the impression of a solid silver orgold artefacts or for creating a fascinating aspect by usingthe decorative effect of the noble metals in combinationwith other materials.

The main effort of the ancient gilder was to developcoating methods for obtaining an Au or Ag layer as thinas possible in order to save precious metals, as well as toproduce artefacts completely or partially coated with a re-sistant and adherent noble metal layer able to resistagainst wear [1-4].

The case histories presented in this paper have beenaccumulated over a time period spanning a few years asthe result of micro-chemical and micro-structural exami-nations finalised to identify the ancient coating tech-

niques and the corrosion agents and mechanisms to beused for the selection of reliable conservation materialsand methods.

Indeed, the micro-chemical and micro-structural in-vestigations were carried out to ensure the chemical-physical stability to these important witnesses of our an-cient art as well as to obtain information on the techno-logical competence reached by ancient metallurgists andon the ancient manufacturing processes.

2. EXPERIMENTAL DETAILS

The micro-chemical structure of the coated artefactshas been studied by the combined use of optical mi-croscopy (OM), scanning electron microscopy (SEM)combined with energy dispersive spectrometry (EDS).

Both SEM and EDS characterisations were carriedout by using a Cambridge 360 scanning electron micro-scope equipped with a LaB6 filament and a four sectorsback scattered electron detector.

The surface morphology of the coins has been alsoobserved by using an optical microscope Leica MZ FLIIIequipped with a digital camera. Metallurgical featureshave been investigated by using a Leica MEF 4 micro-scope equipped with a digital camera.

9

Microchemical Investigationof Ancient Silver and Gold Plated Objects:

Coating Techniques and Degradation Mechanisms

Gabriel Maria Ingo1, Giuseppina Padeletti1, Tilde de Caro1, Cristina Riccucci1, Giuseppe Guida2, Emma Angelini3 and Sabrina Grassini3

1Istituto per lo Studio dei Materiali Nanostrutturati (ISMN-CNR)2Istituto Centrale per il Restauro

3Dipartimento Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino

via Salaria km 29.5,Monterotondo Stazione, Rome, 00016

ItalyPhone: +390690672336, Fax: +390690672821

e-mail: gabriel.ingo@ismn.cnr.it

The manufacturing techniques used in ancient times for coating the copper or silver core of Greek, Roman,Barbarian, Dark and Middle Age artefacts with a thin or thick layer of gold or silver have been studied bymeans of the combined use of different analytical techniques in order to elucidate some aspects of themanufacturing processes. The results indicate that several methods were used including the simple mechanicalapplication of a thin malleable gold foil or the most complex and sophisticated method based on the use ofmercury i.e. the so called fire gilding. The results of the study of golden altar of S. Ambrogio (825 AD) inMilan (Italy) are also presented with the description of the manufacturing processes of this fascinatingexample of large gilded artefact. Finally, corrosion mechanisms acting during the long-term archaeologicalburial has been investigated in order to identify degrading agents and mechanisms.

Keywords: plating, corrosion, gilding

AP°YPO¶OY§OY TELOS 30-01-08 11:00 ™ÂÏ›‰·9

G.M. Ingo et al.

X-ray diffraction (XRD) patterns were recorded di-rectly on the objects or their fragments, by multiple scan-ning using an automated Seifert XRD-3000 diffractome-ter. The identification of the species was carried out byusing a Seifert 3000 Software Index I.

Some fragments of the coated objects were embeddedin epoxy resin with a setting time of 24 hours and sec-tioned by using a diamond saw in order to preserve asmuch as possible the corrosion products and the surfacefeatures. The sections were then polished with silicon car-bide papers until 1200 grit and the final polishing was per-formed with diamond pastes up to 1/4 Ìm in order to havemirror like surfaces.

3. RESULTS AND DISCUSSION

In Fig. 1, the morphological details and the chemicalcomposition of the corrosion products grown on a silvercoated coin struck in Southern Italy (IV cent. BC) areshown.

The coin is broken because in ancient times, the coinappeared as solid silver coin and only the partial removalof silver or a notch could reveal the coin fraudulent nature.

In this case the coin was cut in one or more pieces or"signed" in order to withdraw it from circulation.

The results reveal that the thickness of the silver layeris about 150 Ìm and that the core is constituted by a near-ly pure copper. The ED spectra show that copper and sil-ver chlorides have been formed during the long-term ar-chaeological burial thus forming chloro-argyrite, Cu (I)chloride and Cu (II) compounds.

The manufacturing process used to produce these sil-ver plated coins was carried out by wrapping a copperblank with a silver fine foil on both sides and heating thewrapped coin to weld the metals together by diffusionbonding [5]. This method enhanced the adherence of thenoble metal layer to the substrate via a metallurgical ad-hesion. The silver coated copper core was then struck toproduce the coin.

This methods was widely used in early times; in fact,as the silver content and the size of the coins was reduced,the profit decreased accordingly and therefore, thismethod for coating an object with a silver layer was aban-doned in favour of other cheaper methods.

In more recent times, when the techniques of purifyingthe naturally occurring gold or the silver obtained via cu-pelation became available, the silver or gold foils as thick200-300 Ìm were gradually replaced by the thinner leavesthus ensuring economic and aesthetic advantages [5-8].

The above described method was also used by the Ro-mans for producing mainly coins.

Indeed, in order to produce coated coins, the Romanspreferred the temperature diffusion bonding processes(self-soldering or brazing) during the Republican period[9], in some cases the mercury amalgam-based methods(silvering or gilding) [5] and also the thermo-chemicalprocesses in the latest periods.

Figure 1 - Morphological details and micro-chemicalcomposition of the corrosion products grown on a silvercoated coin struck in Southern Italy (IV cent. BC).

10

AP°YPO¶OY§OY TELOS 30-01-08 11:00 ™ÂÏ›‰·10

Microchemical Investigation of Ancient Silver and Gold Plated Objects

According to this latter method, a copper core with alow amount of silver was Ag-enriched onto the surface bymeans of repeated cycles of thermal treatments inducinga selective copper oxidation and corrosion using picklingsolutions for removing copper oxides.

By using the above listed methods the Romans wereable to produce a large number of gold or silver plated ob-jects and in particular, coins with generally an excellent styleand sturdy enough to resist the wear of the coin circulation.

Some examples of coins coated with a very thin film ofsilver and gold by using mercury-based amalgam have beenalready recently been published and have precisely alloweddating the first use of mercury in the Roman world [5].

An example of Roman Republican silver coated coin(denarius) produced by diffusion bonding is shown in Fig.2. In this latter figure, the cross-sectioned micro-chemicalstructure of the silver layer and the interface Ag-Cu coreis shown with some ED spectra.

The optical image (OM) indicates the bad conserva-tion state of the coin with the presence of large corrodedareas and the BSE-SEM micrograph shows the thicknessof the Ag layer that varies from 130 to about 200 Ìm.

Furthermore, the ED spectra reveal the chemicalcomposition and micro-chemical structure of the interfa-cial region where diffusion of copper in silver occurred,thus forming a layer of an Ag-Cu eutectic.

As a consequence of the joining of different metals,the behaviour and the rate of corrosion are remarkablyinfluenced by the intimate contact between metals withdifferent electrochemical potential.

This contact induces the less noble metal to becomeanodic in a couple strongly conductive to corrosion.

Indeed, when two different metals or alloys are elec-trically connected by an electrolyte, current will flow fromthe more noble (cathodic) metal to the less noble (anod-ic) metal thus inducing preferential corrosion in the lessnoble anodic areas.

The amount of current which flows and therefore, theextent of corrosion depends on many variables, amongwhich are the chemical-physical parameters of the burialcontext, the difference in potential of the two metals, thepresence and nature of the electrolyte and the micro-chemical structure of the alloy.

As clearly shown by the above reported results, thecontact between silver and copper in the silver coatedcoins has enhanced the corrosion phenomenon whosemain agent is the chloride anion coming from the soil [5].

Indeed, the SEM, EDS, XRD (not shown) and OMresults show that silver corrosion occurred with formationof patina mainly composed by chloro-argyrite and con-taining also soil elements such as Si, Al, Ca, Fe and P thusdemonstrating the interaction between soil componentsand corrosion products.

The results show also that different copper compoundsincluding the dangerous and unstable Cu (I)-chloride areformed during the long-term archaeological burial, thusforming a complex internal and external corrosion structure.

Figure 2 - OM and SEM images with ED spectra of across-sectioned Roman Republican silver plated coin.

11

AP°YPO¶OY§OY TELOS 30-01-08 11:00 ™ÂÏ›‰·11

G.M. Ingo et al.

Another example of a Roman Republican silver plat-ed coin is shown in Fig. 3 where SEM and OM images ofthe cross-sectioned coin are reported.

The images disclose that the thickness of the externalsilver-based layer varies from 100 Ìm to about 200 Ìmand is composed by copper islands surrounded by a silvernetwork on a copper core. SEM and OM images show al-so that the more external copper islands are corroded andcompletely transformed in Cu (I) and Cu (II) compoundsas well as that the copper core is corroded and partiallytransformed in cuprite (Cu2O).

Furthermore, the altar silver panels have been gildedin order to evidence some part of the figures and to createa fascinating decoration combining gold and silver asshown in Fig.s 4 and 5.

The altar was given to the holy Ambrose’s Basilica byAngilbertus II, archbishop of Milan (Italy) from 829 up to859 AD and was manufactured by a goldsmith calledVolvinius who is unknown to us.

The silver, gold and silver coated panels have beenstudied in order to identify the technology used to pro-duce the silver and gold plates and the ancient methodol-ogy for coating the silver plates with gold as well as to de-termine the conservation state of the altar.

The SEM+EDS results identify the coating techniquethat was based on the use of mercury gilding.

12

Figure 3 - SEM and OM images of a cross-sectioned RomanRepublican silver plated coin by using the brazing method.

This information indicates the use of a brazingmethod, and of a hard silver-copper solder that allowedsaving a relevant amount of the precious metal [9]. Afterthe thermal treatment (i.e. the brazing), the silver surfaceamount could be enhanced via a pickling process to artifi-cially enrich the surface [9].

The last example of gold coated ancient artefact is thegolden altar of S.Ambrogio in Milan (Italy). The altar isconsidered one of the most important goldsmith’s workof art never realised and is constituted by large panels ofgold and silver decorated by precious enamels and manydifferent gems, such as emerald, topaz, amethyst, agateand carnelian.

Figure 4 - A panel of the golden altar of S. Ambogio in Mi-lan (Italy) manufactured by a goldsmith called Volvinius(829-859 AD).

In particular, the micro-chemical structure and mor-phology of the gold layer, the nature of the interface withthis latter and the silver base metal and the oxidation/degradation phenomena have been identified and theseinformation are used to ascertain some technological as-pects of the mercury gilding technique as well as the ori-gin of the altar degradation that has been observed insome areas of the gold plated silver panels.

The SEM image and the ED spectra shown in Fig. 5disclose the presence of mercury in the thin external goldlayer thus demonstrating the use of a mercury-gold amal-gam that was first spread by Volvinius onto the surface tocoat the silver plates [1,6-7,10-11]. Then, a thermal treat-ment was carried out at about 250ÆC-300ÆC for a fewminutes causing the mercury to evaporate thus forming agold enriched region on the silver surface thick less that12 Ìm.

Finally, the micro-chemical and micro-structural re-sults indicate that the cracks and the loss of the thin goldlayer was induced by repeated cycles of mechanical ex-pansion and contractions of the wood case where the sil-ver plates are anchored likely induced by repeatedchanges of environmental conditions.

AP°YPO¶OY§OY TELOS 30-01-08 11:00 ™ÂÏ›‰·12

Microchemical Investigation of Ancient Silver and Gold Plated Objects

Figure 5 - Back scattered electron image and ED spectrafor a cross sectioned gilded silver panel.

4. CONCLUSION

In conclusion, by means of the combined use of differ-ent analytical techniques, the micro-chemical and micro-structural study of some gold and silver coated ancientartefacts has been carried out in order to identify the dif-ferent used plating techniques and elucidate some as-pects of the manufacturing techniques.

The results have indicated that the precious metal lay-ers, whose thickness could range from several hundredsto few microns, were obtained by using different methodsthat varied greatly from time to time and place to placeand in many cases were closely guarded secrets.

These manufacturing methods could be the simplemechanical application of a thin malleable gold foil up tothe most complex and sophisticated method based on theuse of mercury amalgam.

The information about the technological aspects evi-dence that the production of the gold and silver coatedobjects constitutes an important document for the historyof the art as well as of the metallurgical technology.

Finally, SEM-EDS, XRD and OM analytical tech-niques have been also used to investigate the nature ofcorrosion products grown during the long-term burial andto identify the degradation mechanisms. The main degrad-

ing agent is chlorine coming from the soil, which inducesthe formation of silver and Cu (I) chloride that could giverise to the copper cyclic reaction that continues to corrodethe copper when exposed to oxygen and humidity.

ACKNOWLEDGEMENTS

This work has been financially supported by the Euro-pean Commission (contract nÆ 509126, project acronymPROMET). The authors would express also their sinceregratitude to Dr.ssa Silvana Balbi de Caro (Director of theRoman National Museum of Rome, Italy) and to Dr, pao-lo Bernardini of the Soprintendenza Archeologica alleProvincie di Cagliari ed Oristano (Cagliari, Sardinia, Italy)for the helpful discussions and for the studied samples.

REFERENCES

[1] Oddy, A.: "Gilding through the Ages" Gold Bullet-tin, Vol 14, pp. 75-79 (1981).

[2] Raub, Ch.J: "The Development of Gilding from An-tiquity to the Middle Ages" Materials Australasia,Vol. November/December pp. 7-11 (1986).

[3] Oddy, A.: "Gilding: an outline of the technologicalhistory of the plating of gold on to silver or copper inthe Old World" Endeavour, Vol. 15, pp. 29-33 (1990).

[4] Anheuser, K.: "The practise and characterisation ofhistoric fire gilding techniques" Journal of Metals,Vol. of November, pp. 58-62 (1997).

[5] Ingo, G.M., Angelini, E.,Bultrini, G.,de Caro,T.:"Combined use of surface and micro analytical tech-niques for the study of ancient coins" Appl. Phys. A,Vol. 79, pp. 171-176 (2004).

[6] Lins, P.A., Oddy, A.: "The origin of mercury gilding"J.Arch.Sci., Vol. 2, pp. 365-373 (1975).

[7] Anheuser, K.: "Cold and hot mercury gilding of met-alwork in Antiquity" Bullettin of the Metals Muse-um, Vol. 26, pp. 48-52 (1998).

[8] Reiff, F., Bartels, M., Gastel, M., Ortner, H. M.: "In-vestigation of contemporary gilded forgeries of an-cient coins" Fresenius J. of Analytical Chemistry,Vol. 371, pp. 1146-1153 (2001).

[9] Zwicker, U., Oddy, A., La Niece, S.: "Roman Tech-niques of manufacturing silver-plated coins" Romantechniques of manufacturing silver-plated coins in Met-al Plating and Patination (ed.s P.T. Craddock and S. LaNiece, Butterworth, 1993) paper nÆ 20, pp. 223-246.

[10] Bunker, E., Chase, T., Northover, P., Salter, C.:"Some early Chinese examples of mercury gildingand silvering" Outils et ateliers d’orfèvres des tempsanciens (ed. C. Elvère, St. German-en-Laye, Museedes Antiquites Nationales, 1993) pp. 55-66.

[11] Ingo, G. M., de Caro, T., Padeletti, G., Chiozzini,G.: "Combined use of GDOES, SEM+EDS, XRDand OM for the microchemical study of the corro-sion products on archaeological bronzes" Appl.Phy.A,Vol.79, pp. 319-325 (2004).

13

AP°YPO¶OY§OY TELOS 30-01-08 11:00 ™ÂÏ›‰·13