grassini, s. et al. plasma treatment for cleaning and protecting metal artefacts. 2007

8/7/2019 Grassini, S. Et Al. Plasma Treatment for Cleaning and Protecting Metal Artefacts. 2007

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1. INTRODUCTION

One of the main problems for the preservation of metallic archaeological objects excavated and stored in mu-seum is their long-term conservation. Not only is corrosion

is a never-ending problem, conservator-restorers (C-R)must also clean the artefact from dirt to reveal its shape andhidden decorations, as well as stabilize it. C-Rs must choosethe correct strategy to slow down surface degradation phe-nomena so as to avoid further damages.

Conservation must use protection methods which arereversible as well as respecting the integrity of the objects,such as the aesthetic appearance.

Low pressure plasmas (the so-called cold plasma) ex-hibit the following major advantages, which suit perfectlytheir application in the field of metals conservation:— dry processes easily performed at room temperature in

order to preserve the metallurgical features;— application can be made directly on the exterior or the

interior of complex shapes of objects;— plasma etching can be controllable and selective at

nanoscale and the surface ion bombardement can becarefully controlled in order to avoid any damage;

— plasma deposition can produce very thin coatings withcustomizable structure having a minimal effect on theappearance of the surface, along with very good corro-sion protection.Cold plasmas fed with organosilicon compounds ob-

tain one of the most investigated classes of PECVD trans-parent thin films [1]. Among them, SiCxHyOz pinhole freethin films have been successfully employed for the corrosionprotection of different metals. Moreover, plasma depositioncan be preceded by surface treatments with inert or reactivegases, which play a fundamental role in enhancing the adhe-sion of the coating to the underlying substrate [2, 3].

The aim of this study, carried out under the frameworkof PROMET project (financially supported by the Euro-pean Commission, VI FP, INCOMED contract no 509126)is to develop a non-destructive and reversible procedure forcleaning and protecting precious archaeological objects,mainly Cu and Ag-based alloys, in low pressure plasma.

Silver objects are usually alloyed with copper, and evenif there is 50/50 Cu/Ag, the appearance of the metal is stillwhite. When silver is tarnished, the removal of tarnishingfilm requires the use of Table 1 – Experimental conditionsof Plasma Treatments

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Advanced Plasma Treatment for Cleaning and Protecting

Precious Metal ArtefactsSabrina Grassini1, Emma Angelini1, Riccardo d’Agostino2, Fabio Palumbo3, Gabriel M. Ingo4

1Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Italy2Dipartimento di Chimica, Università di Bari, Italy

3Istituto per le Metodologie Inorganiche e dei Plasmi, IMIP-CNR, Bari, Italy4Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR, Roma, Italy

Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di TorinoCorso Duca degli Abruzzi 24, 10129 Torino, Italy

Phone: +390110904642Fax: +390110904699

e-mail: [email protected]

Low pressure plasmas can be successfully carried out to develop a reversible procedure for cleaning and pro-tecting precious metal artefacts. In this study, cold plasma treatments were applied on Ag-based alloys withmicrochemical and microstructural features similar to ancient artefacts, buried for six months in the archaeo-logical site of Tharros (Sardinia, Italy). Dry etching in H2 plasma allows to completely remove the patina with-out affecting the bulk properties, while SiOx thin films, deposited in a glow discharge fed with TEOS, Ar andO2, exhibit good corrosion protection.

Keywords: low pressure plasma, protective coatings, dry etching, metallic artefacts, corrosion

Table 1 – Experimental conditions of Plasma Treatments

Step Feed composition Pressure Power Treatment time Thickness(sccm) (mbar) (W) (min) (Ìm)

Etching of the patina H2: 20 1.3.10-2 50 20-60 -TEOS: 2

SiOx deposition O2: 45 1.33 250 90 0.6-1.5Ar: 22

A P ° Y P O ¶ O Y § O Y T E L O S 3 0 - 0 1 - 0 8 1 1 : 0 2 ™ Â Ï › ‰ · 1 2 7



S. Grassini et al.

chemicals, abrasives, or electrochemical reduction to re-

move the silver sulphide. The restored surface still has high

susceptibility to re-tarnish, when re-exposed to an environ-

ment with sulphur containing materials. In this case, the

silver objects have to be cleaned frequently, and each time

the tarnish is removed so is some silver [4, 5]. The best wayto avoid frequent cleaning is to protect the silver objects in

a controlled environment, if possible. However, it may be

impossible for curators to maintain such a microclimate

free of sulphur, and for this reason, some conservators pre-

fer to coat silver objects for their protection against tar-

nishing. However, lacquering or waxing is not recommend-

ed for silver, because of the difficulties in obtaining an even

coating, so that if there is uneven coating when the object

tarnishes again, the end result may be worse than if no

coating is applied at all. For this reason, the suitability of

PECVD transparent thin films for protecting such objects

made of cultural property need to be investigated further.

2. EXPERIMENT

The experimental apparatus used for this study is a ca-

pacitively coupled parallel plate reactor with an asymmet-

ric electrodes configuration. It consists of a vacuum cham-

ber made of stainless steel, a powered electrode connected

to a RF power supply (13.56 MHz) through an impedance

matching unit and a ground electrode. Gas and organosili-

con vapour flow rates are controlled by mass-flow and

vapour source controllers, respectively, while a turbomole-

cular pump backed by a rotary pump, a throttle valve, and a

pressure gauge allow to keep the pressure fixed at the se-

lected value.Plasma treatments have been applied on two silver/

copper reference alloys with microchemical and mi-

crostructural features similar to ancient artefacts. Their

chemical compositions are as following:

— Alloy A: Ag 97.0, Cu 1.5, Pb 1.5 (wt%)

— Alloy B: Ag 96.5, Cu 3.5 (wt%).

In order to evaluate the possible application of the

plasma treatment on real artefacts, the reference alloy

specimens were buried at the archaeological site of Thar-

ros (Sardinia, Italy) for a period of 6 months. For acceler-

ating the corrosion degradation 5wt% of NaCl has been

added to the soil. This addition is justified considering the

role played by chlorides on the degradation of silver alloysduring burial.

After 6 months, the aged specimens were gentlycleaned in ultrasonic bath using iso-propylic alcohol for 15

minutes to remove all the traces of soil.

Dry etching of the patina has been performed in low

pressure hydrogen plasma, while protective SiOx thin films

have been deposited in plasma fed with tetraethoxysilane

(TEOS), Ar and O2 (seeTable 1).

The chemical and morphological characterisation of the

patina, before and after the H2 glow discharge, was per-

formed by means of X-Ray diffraction (Philips X-Pert pow-

der diffractometer ) and Field Emission Scanning Electron

Microscopy (FEG-SEM Supra40, Zeiss, Microscope

equipped with a EDAX 9900, Energy Dispersion Microprobe).The protective effectiveness of the SiOx film was as-

sessed by means of Electrochemical Impedance Spec-

troscopy (EIS). Electrochemical impedance measure-ments were performed in aerated 0.1M NaCl solution atthe laboratory temperature of 25±1oC, using a frequencyresponse analyzer EIS300 (Gamry Instruments). Imped-ance spectra were recorded at the open circuit potential af-ter 1h of exposure to the aggressive environment, by apply-ing a sinusoidal signal of 10mV amplitude in the frequencyrange 100 kHz–10 mHz with five points per decade.

3. RESULTS AND DISCUSSION

3.1 Characterisation of the Silver Alloys

The silver reference alloys were characterised by het-erogeneous microchemical structure, chemical composi-tion and metallurgical features similar to those of the an-cient alloys.

The SEM-EDS results (Fig.1) disclose the presence of small copper islands scattered in the silver alloys and theoccurrence of a phase separation i.e., discontinuous pre-cipitation typical of the copper-silver alloys used in antiqui-ty. It is well known that this phenomenon, as well as thepresence of Pb along grain boundaries, could be related tothe age-embrittlement of ancient silver based artefacts.

3.2 Plasma Etching of the Patina

All Ag-alloy specimens buried in the archaeological siteof Tharros had on both sides, a uniform dark brown patina

on its surface. Soil traces (carbonates, etc.) are present in thewhite areas. The results of the chemical and microstructuralcharacterisation of the surface confirm that the artificialpatinas are similar to the archaeological ones because themain corrosion products detected are chloroargyrite (AgCl)and paratacamite ((Cu,Zn)2(OH)3Cl) as observed on silverarchaeological artefacts (Fig. 2).

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Figure 1 - Back scattered electrons images that show themetallurgical features of the reference alloy B (a) and of aRoman Republican silver coin (b)




Advanced Plasma Treatment for Cleaning and Protecting Precious Metal Artefacts

Figure 2 – XRD spectra recorded on the reference alloy A,before (black spectrum) and after the H2 plasma treatment(red spectrum)

During burial the silver alloys specimens were subject-ed to the main degradation phenomena found on ancient

silver artefacts:— silver corrosion with formation of patina mainly madeup of chloroargyrite

— the copper degradation commonly called “bronze dis-ease”, since copper exists in the copper rich phases of the silver alloy matrix.

shaped area of the surface, that appear black in the BSE im-age; while high Ag concentration is detected in the white ar-eas (Fig. 3). A glow discharge in a low pressure hydrogenplasma produces H atoms as well as ions. Though not stud-

ied in detail yet, it can be assumed that hydrogen atoms di-rectly react with the patina and remove the dangerous cop-per chlorides by the formation of volatile HCl. However therole of the impinging ions cannot be ruled out: very likely thereactivity of the surface is enhanced by the striking ions. Fur-thermore, the glow discharge reduces silver ions (Ag+) backto silver (Ag0). These two effects of the H2 discharge areconfirmed by the XRD and EDS analyses: the peaks of chloroargyrite disappear in the XRD spectrum (Fig. 2),

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Figure 3 – SE and BSE images of the patina of the alloy A (a); EDS maps of: Ag (b), Cu (c), Cl (d) and O (e)

The back scattered electron images of the patina andEDS data show high concentration of Cu in small, circle-

Figure 4 – SE and BSE images of the surface of the alloy A

after H2 discharge (a); EDS maps of: Ag (b), Cu (c), Cl (d)and O (e)

while the EDS maps show evidence of a noticeable reduc-tion of the chloride concentration on the surface treated inH2 plasma (Fig. 4). Consequently, the surface was convert-ed again into bright metallic silver without affecting thebulk properties (Fig. 5).

Depending on the treatment time, the H2 plasma wasable to significantly reduce the thickness of the corrosionlayer. Fig. 6, the thickness of the patina grown during burialon the silver reference alloy B is about 13.4 Ìm. After aplasma treatment of 30 min, it is reduced to about 6.4 Ìm.




S. Grassini et al.

Figure 7 – Bode plots recorded on silver alloy B before and

after the deposition of the SiOx film

Figure 7 shows the Bode plots recorded on the silver al-

loy B before and after the deposition of the protective SiOx

layer. Notwithstanding the low thickness (about 0.9 Ìm) of

the deposited coating, a noticeable inhibiting action to-

ward the corrosion process occurring on the metal surface

was observed, as underlined by the increase of the value of the low frequency complex impedance |Z| that may be esti-

mated from the plateau region on the Bode plot in the fre-

quency range 0.001-0.01 Hz.

The utilization of the Bode plot has the advantage to

avoid the complex and time-consuming data fitting pro-

cedures normally used to define the system’s equivalent

electric circuit for fitting the impedance spectra. The

value of |Z| indicates the extent of the electrolytic con-

duction paths at the metal/coating interface: high |Z|

values are indicative of a good protective effectiveness

of the coating [7].

The increase in the value of the impedance modulus

(|Z|) can be correlated to the barrier effect of the SiOx film

against the aggressive environment and also to its high con-

formability to the surface, even if it is already coated by a

thin layer of corrosion products. The SiOx film smoothes

the cavities present in the patina, avoiding the penetration

of the corrosive agent through the corrosion layer till the

metal surface.

The high degree of adaptability to any surface rough-

ness is another key factor to meet the requirements of cu-

rators involved in the conservation of archaeological sil-

ver artefacts characterised by very complex decorations.

The advantage of the EIS technique over other labora-

tory techniques for the assessment of the corrosion behav-

130

Figure 6 – SEM images of the patina of alloy B (a) before(13,4 Ìm thick) and (b) after (6.4 Ìm thick) the H2 plasmadischarge

Figure 5 – Aged alloy A, before and after the H2 plasmatreatment

The plasma cleaning treatment restored the originalsurface of the object and it is important to underline thatthe H2 discharge can be performed also for cleaning gildedartefacts without damaging surface decorations. In thiscase the experimental parameters must be optimised by re-ducing the discharge input power, and consequently reduc-ing the surface ion bombardment.

3.3 Plasma deposition of the protective layer

The high versatility of plasma processes allow to per-form, after the cleaning treatment, a deposition of a thinprotective coating without exposing the object to atmos-pheric contaminants.

The SiOx film was deposited in the experimental condi-tions optimised for silver substrates in a previous study.Here, the high barrier properties against sulphide, oxygenand water vapour can be achieved by depositing the film at

high input power in oxygen rich plasma [6].The SiOx coating is really effective in the protection of

the silver surface against aggressive environments as estab-lished by the impedance measurements.




Advanced Plasma Treatment for Cleaning and Protecting Precious Metal Artefacts

iour of coated systems is the possibility of using very smallamplitude signals (10 mV) without significantly disturbingthe properties being measured.

This approach can be applied by curators and C-Rs not

skilled in electrochemistry; as a matter of fact C-Rs caneasily obtain an indication of the protective effectiveness of the coatings without affecting the conservation state of theartefact.

9. CONCLUSIONS

The conservation of Cultural Heritage requires a mul-tidisciplinary approach in order to develop tailored conser-vation strategies for cleaning and protecting preciousworks of art of archaeological and historical interest.

The preliminary results, discussed in this paper, showthe feasibility of the application of low pressure plasmatreatments in the conservation of movable ancient silverartefacts.

Further investigations should be considered in order tooptimise the cleaning procedure as a function of the chem-ical composition of the patina and to optimise the deposi-tion of plasma coatings as a function of the samples surfacecondition.

REFERENCES

[1] Yasuda H. Bumgarner, M. O., Marsh, H. C., Mo-rosoff, N.: “Plasma polymerization of some organic

compounds and properties of the polymers”, Journalof Polymer Science: Polymer Chemistry, 14, pp. 195-224 (1976).

[2] Angelini, E., Fracassi, F., d’Agostino, R., Grassini, S.,

Rosalbino, F.: Trends in electrochemistry and corro-sion at the beginning of the 12th century, Costa J.M.,Brillas Enric, Cabot, Pere-Lluìs ed. IV. Col-lecciò,Publicacios UB, pp. 979-999 (2004).

[3] Angelini, E., Grassini, S., Rosalbino, F., Fracassi, F.,Laera, S., Palumbo, F.: “PECVD coatings: Analysisof the Interface with the Metallic Substrate”, Surfaceand Interface Analysis, 38, pp. 248-251 (2006).

[4] Organ, R.M.: “The current status of the treatmentof corroded metal artefacts”, National Bureau of Standards Special Publication 479, Proc. of a Semi-nar, Corrosion and Metal Artifacts, A DialogueBetween Conservators and Archaeologists andCorrosion Scientists held at the National Bureauod Standards, Gaithersburg, Maryland, March 17-18 (1976).

[5] Lins, A., McMahon, N.: “Current Problem in theConservation of Metal Antiquities” (1993).

[6] d’Agostino, R., Fracassi, F., Palumbo, F., Angelini,E., Grassini, S., Rosalbino, F.: “Protection of Silver-based Alloys from Tarnishing by means of Plasma-Enhanced Chemical Vapor Deposition”, PlasmaProcesses and Polymers, pp.91-96 (2005).

[7] McDonald, J. R.: “Impedance Spectroscopy”, JohnWiley & Sons, New York (1987).

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