Surface investigationofsomemedievalsilvercoinscleanedinhigh-frequency cold plasma

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Please cite this article in press as: E.G. Ioanid, et al., Surface investigation of some medieval silver coins cleaned in high-frequency coldplasma, Journal of Cultural Heritage (2010), doi:10.1016/j.culher.2010.09.004

ARTICLE IN PRESSG Model

CULHER-2477; No.of Pages7

Journal of Cultural Heritage xxx (2010) xxxxxx

Case study

Surface investigation of some medieval silver coins cleaned in high-frequencycold plasma

Emil Ghiocel Ioanid a, Aurelia Ioanid a, Dorina Emilia Rusu b,, Florica Doroftei a

a Romanian Academy Petru Poni Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, 700487, Iasi, Romaniab Moldova National Museum Complex, 1, Stefan cel Mare si Sfant, 700028, Iasi, Romania

a r t i c l e i n f o

Article history:

Received 29 April 2010Accepted 10 September 2010

Available online xxx

Keywords:

High-frequency cold plasma

Cleaning

Silver coins

Scanning electron microscopyenergy

dispersive X-ray microanalysis (SEMEDX)

a b s t r a c t

Processing in cold plasma (cleaning and/or decontamination) represents an ecological alternative for

applications in various domains of a diverse range of materials. Considering the advantages it presents,high-frequency cold plasma hasbeen employed to removethe corrosion products found on thesurface of

somesilvercoins pertainingto a Polish medieval numismatic collection. The effects of plasma treatment

havebeen evaluatedthrough the investigation of the coinsurface before and afterthe treatment,by means

of different analytical techniques: scanning electron microscopyenergy dispersive X-ray microanalysis

(SEMEDX), X-ray diffraction (XRD), FTIR spectroscopy and colorimetric measurements.

2010 Elsevier Masson SAS. All rights reserved.

1. Introduction and research aim

The deterioration of ancient metals, especially coins, represents

the undesired effect of some physico-chemical processes, such asthose involved in uncontrolled evolution corrosion which grad-

ually alter their aspect, shape,nature andresistance,up to thestagewhich makes impossible their use as historical evidence of humancivilizations or even up to complete destruction of the items [1].

The coins containing non-noble metals (copper, magnesium)

show some punctiform spots or more extended corroded zones, onthe surface, constituted of chlorides, carbonates, sulfates or metaloxides. Notall of thesecorrosionproducts are harmful.For instance,copperoxides found on thesurface generate an inert characteristic,

which is non-active and should not be removed. However, silver orcopper chlorides and sulfides have a permanent action on metal,causing serious deterioration of the items, therefore their removalis imperative.

In the most recent twenty years, scientists from differentresearch domains have directed their attention to the applicationof unconventional cleaning methods of metal artifacts high-frequency cold plasma has come to be considered as a viable,

non-destructive and ecological method [2,3]. Thus, the effect ofO2/Ar sau H2/Ar plasma on metals and some of their corrosion

Corresponding author. Tel.: +40 232 213 324; fax: +40 232 218 383.

E-mailaddresses: ioanid [email protected] (E.G.Ioanid), ioanid [email protected]

(A. Ioanid), [email protected] (D.E. Rusu), [email protected]

(F. Doroftei).

products [4,5], as well as the effect of H2 plasma on silver [6,7],

has been studied. Also, it has been proved that hydrogen plasmais efficient in reducing silver chlorides and sulfides on metal silver

[8,9].Our study aims to evaluate the effects obtained by high-

frequency non-thermal cleaning treatment applied on the surfaceof some Polish medieval silver coins, chemically and microstruc-turally.

2. Experimental

2.1. Coins under study

The studied coins belong to a thesaurus, containing 82 coinsmarked as M1M82, discovered at Cordareni, Botosani County,

Romania [10]. Two medieval silver coins were selected forstudy Polish 3 grosz minted during the reign of Sigismund III

Wasa. Thecoins, presentedin Fig.5 ad, wereobtainedfrom Vilnius(M6) and Riga(M9) cities where, it is supposed, they were minted

[11] and were identified according to Gumowskis catalogue [12].Their weightranges between 1.86 and2.37g anddiameter between20 and 22 mm. The coins present specific corrosion, as well as anumber of organic deposits.

2.2. Plasma cleaning treatment

For the plasma cleaning treatment of the coins, dependingon the goal pursued, a luminescent discharge is performed in a

gas or in a mixture of gases. Thus, gases with oxidant character

1296-2074/$ see front matter 2010 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.culher.2010.09.004
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2 E.G. Ioanid et al. / Journal of Cultural Heritage xxx (2010) xxxxxx

Fig. 1. SEM images of the coin surfaces before plasma treatment: a, b: details of M6 surface; c, d: details of M9 surface.

(oxygen), gases with reductive character (hydrogen) or theirdiluted mixtures with argon can be used [13]. The plasma and thetreated object are in a stationary interaction (in physical contact,but electrically isolated), the plasma being extended over the

whole surface of the object. The cleaning process is achieved under

the action of the active species from plasma, through chemicaland mechanical interactions [14]. The treatment is achieved atsufficiently low temperatures (up to 60 C), in order to avoid met-allographic modifications (recrystallized atoms are created with a

specific function in the chemical reduction of corrosion products).

Table 1

Mass percentage values of the elements found on the surface of coins, determined by EDXa.

Coins Elements

C O Al Mg Si S Cl Ag Cu Ca

(wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %)

M6obverse

Before treatment 23.00 22.27 1.00 0.64 32.65 20.45 After O2 treatment 8,11 13,42 1,13 1,89 0,51 0,38 72,34 2,23

After H2 treatment 4.83 7.40 1.57 80.50 5.70

M6 reverse

Before treatment 18.98 27.02 0.89 2.47 - 3.29 30.59 14.97 1.79

After O2 treatment

After H2 treatment 2.35 90.30 7.36

M9 obverse

Before treatment 21.45 37.43 0.80 1.04 20.04 18.17 1.06

After O2 treatment

After H2 treatment 7.82 9.90 0.57 1.98 72.49 7.23

M9 reverse

Before treatment 16.69 23.28 1.46 0.65 0.51 2.46 54.95

After O2 treatment 12.82 32.85 1.81 8.93 0.19 0.14 31.78 10.91 0.56

After H2 treatment 5.85 6.06 85.87 2.22

a

The results listed in the table were obtained for the spots marked by arrows in Fig. 5 (ad).
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E.G. Ioanid et al. / Journal of Cultural Heritage xxx (2010) xxxxxx 3

Fig. 2. FTIR spectrum of corrosion products.

For performing the cleaning treatment, an installation con-ceived and built by the authors in the laboratory [15,16] was

used. The device consists of a bell-shaped Pyrex reactor, fixed ontoa stainless steel support. The reactor is coupled capacitively, bymeans of external electrodes, to a high-frequency generator, whichgeneratesand maintainsthe electricaldischargein plasma. The coin

is fixed by a Teflon clamp on the electrical axis of a micro-motor,

producinga rotation movement with a speedof 2 rot/min. Thereac-tor is connected to a vacuumpumpand the gas is admitted into thesystem by means of a needle valve.

The experiments have been carried out under the follow-ing working conditions: maximum temperature 60 C, frequency13.5 MHz, electric field intensity 2030 V/cm, glow power 100 W,gas used oxygen and/or hydrogen. The vessel was vacuumed at

an initial pressure of 5102 mbar, and then the working gas wasintroduced with the pressure of 3.5101 mbar for oxygen, and of7.5101 mbar for hydrogen.

To avoid the overheating of the coins, the treatment was per-

formed in steps. The optimum duration of a step was establishedexperimentally. For this purpose, a silver disk was manufacturedof the same size and shape as the coins, and a thermocouple was

introduced into it. The temperature values obtained as a func-tion of the treatment duration, nature and pressure of gas wererecorded. Hence, it was estimated that for oxygen, the maximumduration of the treatment is 20 minutes, while for hydrogen it is30 minutes.

The samples were placed directly in the plasma region. Theduration of the treatments was varied depending on the corro-sion type and penetration depth; the duration of the treatmentincreased (up to 56 hours), with increasing penetration depth of

corrosion. For a rapid and complete elimination of the corrosionproducts, the treatments were carried out in several steps. In thefirst step, the coins were treated in oxygen plasma for removingthe contaminants of organic nature. After the treatment, the sur-

face of the object wasbrushed with a brush of natural hair (squirrelhair) to remove the impurities that were not eliminated by the vac-

uum system. In the second step, pure hydrogen was used for thetreatment.

2.3. Method of characterization

2.3.1. SEM/EDX surface analysis

The effect of plasma cleaning was assessed by examining the

surface of thecoins before andafterthe plasmatreatment, by usinga non-destructive method of surface analysis scanning electronmicroscopy (SEM) in conjunction with microscopy energy dis-

persive X-ray microanalysis (EDX). The analyses were carried outusingQUANTA 200 scanning electronic microscope withintegratedEDS system, GENESIS XM 2i EDAX with SUTW detector. Scanningelectron microscopy was used in the secondary and backscattered

imaging mode, by using an ETDdetectorand a BSED detector.EDAX

Fig. 3. XRD spectrum of corrosion product: 1: Ag2S (silver sulfide, pattern 00-011-

0688);2: Cu2S (copper sulfide, pattern 01-070-9135); 3: CaCO3 (calcium carbonate,

pattern 00-003-0893); 4: CuCO3Cu(OH)2 (basic copper carbonate, pattern 00-001-

0959).


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was used for identifying the elemental composition in the areas ofinterest of the images obtained in BSED mode.

2.3.2. X-ray diffraction

The quantitative characterization of the coin was carried outusing X-ray diffraction (XRD) (D8 Advance Bruker AXS). The X-rayswere generated using a Cu Ka source, with an emission currentof 36 mA and a voltage of 30 kV. Scans were collected over the

2= 270 range using a step size of 0.01

and a count time of0.5 s/step. The diffractograms were studied with an EVA soft (fromDiffracPlus evaluation package).

2.3.3. FTIR Spectroscopy

The FTIR analysis was done using fine dispersed samples asKBr pellets. The FTIR spectra were recorded in the range of3500428cm1, using a BRUKER VERTEX 70 Spectrophotometer,

in the FTIRATR regime, at the resolution of 2 cm1 and 64 scans.

2.3.4. Color measurement

Color modifications of the coins were measured by means of a

Pocketspect Color QATM device, illuminant D 65. The surface colordifference was calculated using the CIE L*a*b* system. According tothis color representation, L* is lightness, a* is the red-green compo-

Fig. 4. SEM images of the M6 coin surface before and after plasma treatment: ac: details before treatment; df: details after treatment.


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nent, b* is the yellow-blue component. RGB values provided by thePocketspec Color QATM device were further processed with Easy

RGB database and the L*a*b* were obtained and then used for cal-culating the color difference,E*, according to the relationship (1)[17]:

E* = (L*2 +a*2 +b*2)1/2 (1)

2.3.5. Gloss measurement

The gloss variation on the surface of the samples was measuredwithHoriba IG-320 Gloss-Checker apparatus.The glossvalues weredetermined comparing the intensity of the luminous reflection on

the sample surface with the value recorded for standard surface(polished black glass, an accessory of the apparatus) [18].

3. Results and discussion

3.1. SEM/EDX analyses

The investigated coins were first cast from coinage silver ofhigh purity. EDX spectrum, obtained for coin M6 (internal layer),reveals the composition of the alloy: 90.30 wt% high-grade silver,

low content of copper (7.36 wt%) and magnesium (2.36 wt%).

The images presentedin Fig.4 revealthe initial lowconservationdegree of the coinscorroded surfaces (Fig. 4d), and the presenceof a layer of dark brown color, which diminishes the natural shine

of silver (Fig. 4ac), as well as some green zones that have beenremarked especially on the reverse of coin M6.

The coins were examined directly by SEM and the obtainedimages are presented in Fig. 1ad. The microphotographs realized

evidence rugoussurfaces with numerous burrs dueto manufactureand some corrosion products both on the obverse (Fig. 1a, c) andon the reverse (Fig. 1b, d).

The weight percentage values of the elements identified on the

surface of the coins by EDX, (Table 1), evidence a lower grade of sil-ver than the original one, still existent in the coin body, unaffectedby corrosion. Additionally, a high content of Cu has been remarkedon the corroded areas, in comparison with the one identified in

the coin body. EDX spectrum indicates a compositional enrichmentin copper, reaching 54.95%. It may be observed that both on theobverse and reverse, but also on different spots from the same sideof the coin, the superficial chemical composition of the monetary

alloy is non-homogeneous. Besides Ag,Cu andMg, EDX spectra evi-dence the presence of Cl, S, Si, Ca, in lower percentages, and higherpercentages of C and O.

The results obtained by observing the EDX spectra lead to the

conclusion that the coins show (Ag2O) deposits on the surface,identified in the FTIR spectrum, showing an absorption band at561cm1 (Fig. 2). There have been identified corrosion products

as crusts, containing (CuCO3CU(OH)2) and (CaCO3). These contam-inants areidentifiedin theFTIR spectrum,which present absorptionbands at 871 cm1 and at 1402cm1 and are also evidenced by theXRD results (Fig. 3). EDX spectra are characterized by the presence

of corrosion species, such as S and Cl. Extremely fine deposits of(Ag2S) and (Cu2S) covering different areas of the coins have beenevidenced by the XRD.

In the firststepof cleaning processaimed atthe reduction ofthe

organic compounds, was followed by using oxygen plasma, whichis very efficient in removing organic deposits. For this purpose, amixture of oxygen/argon was used (25/75). Oxygen dissociated inplasma interacts with organic compounds through oxidation reac-

tions, thus resulting in volatile products (CO, CO2, H2O), whichare evacuated by the evacuation system of the pump. After a 20-minute treatment, the results obtained were assessed by the EDXmicroanalysis of different zones (Table 1). EDX spectra evidence a

decrease in carbon and oxygen, while other contaminants are stillpresent.

In the second step, the coins were treated in hydrogen plasmafor 1 hr and then they were brushed with a squirrel hairbrush. This

procedurewasrepeatedfourtimes.Visualexaminationofthecoins,on both the obverse and reverse sides, evidences a cleaning of thesurface, confirmed by the electron microscopy images of the areas

observed (Fig. 4). Both at a low magnification scale and at a mag-nification of 1000, no modification of the coin details has beenremarked. Notably, the crusts have been removed after plasmatreatment; the crusts became fragile and have been removed byrepeated brushings.

Corrosion products, due to sulfides, and, partially, the greencorrosion products (carbonates) have also been removed. The pos-sibility of the total removal of the patina layer comprising sulphuris thus ascertained.

3.2. Decolouration measurement

CIEL*a*b* parameters and color difference calculated accordingto relation 1 forthe two coins,before treatment, after 20minutes of

treatment in oxygen plasma and after 4 hours in hydrogen plasma,are presented in Table 2.

The increase in luminosity on both sides of coin M 6 in the firststep of the treatment can be attributed to the removal of organic

deposits an action that was evidenced by the analyses performed.The remaining corrosion products are eliminatedin the secondstepof hydrogen plasma treatment. Consequently, after the treatment,luminosity increases significantly, reaching values of 91.318 on the

obverse of the coin and 76.532 on the reverse from the initial valueof 36.155. Thecolordifference,calculated accordingto equation 1 ishigher for the obverse ofthe coinas comparedto the reverse, asthe

Table 2

Mass percentage values of the elements found on the surface of coins, determined by EDX.

Coin Treatment stage CIE L *a*b* parameters E*

L* a* b*

M6 obverse Before treatment 36.155 3.431 13.722

After oxygen treatment 56.475 8.861 17.909 21.19

After hydrogen treatment 91.318 20.787 29.341 59.99

M6 reverse Before treatment 61.866 8.675 18.771

After oxygen treatment 70.201 9.235 20.764 8,58


M9 obverse Before treatment 87.328 13.538 24.426



M9 reverse Before treatment 45.030 15.263 15.623




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surface on the obverse has been more deeply affected by harmfulproducts.

Coin M9 behaves similarly with coin M6, except for the factthat its obverse, which was much better conserved, shows a slight

decrease in luminosity in the first working step due to surfaceoxidation an effect that was suppressed after the hydrogen treat-ment.

3.3. Gloss measurement

Gloss values measured before and after HF plasma treatment

of the two coins are presented in Table 3. An increase in gloss isremarked dueto theremoval, toa greatextent, ofthe deposits men-tionedpreviously.The lowvalue of gloss recordedfor theobverse of

Table 3

Variation of gloss on both sides of the coins.

Coin Gloss values

Before treatment After O2 treatment After H2 treatment

M6 obverse 21.60 55.9 59.73

M6 reverse 32.1 44.00 61.05

M9 obverse 51.06 45.12 59.28

M9 reverse 34.70 69.10 83.86

coin M9, after treatment under oxygen, is determined by the effectexplained above for the decrease in luminosity.

As a consequence, through the removal of mixed organic andinorganic deposits, under oxygen and hydrogen, silver partiallyrecovers its gloss.

Fig. 5. Photographicimages of thecoinsbefore (ad) andafter(eh); HF plasmacleaning: a, e/b,: obverse/reversecoin M6 (Vilnius);c, g/d, h: obverse/reverse coin M9 (Riga).


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Please cite this article in press as: E.G. Ioanid, et al., Surface investigation of some medieval silver coins cleaned in high-frequency coldplasma Journal of Cultural Heritage (2010) doi:10 1016/j culher 2010 09 004




Photographic images showing the aspect (initial and final) ofthe coins are presented in Fig. 5 dg. These support the analysesperformedbefore andafter the treatment, confirming the effects ofHF plasma in partially removing the corrosion products identified

and the degradative agents of organic nature.If the qualitative and quantitative analyses reveal the structural

and compositional modifications of the surface of coins, the pho-tographic images offer the possibility for an aesthetic appreciation

of the results obtained an aspect that can not be neglected in theconservation of heritage objects.

4. Conclusions

By the application of plasma treatment, we succeeded in pre-serving the original patina to a great extent, achieving a uniformaspect of the surface, revealing some coin reliefs and stopping thecorrosion process.

The coins have partially recovered the specific gloss of silver,which does not however confer the aspect of new, and hence,the principle of the minimum intervention on patrimony objects isrespected.

The utilization of gases with reductive character in the HFplasma cleaning processes of silver patrimony pieces permits to

remove selectively the harmful corrosion products.The method is ecological, non-invasive, easy to apply and

is in accordance with the European norms recommending thereduction of using solvents and toxic substances wheneverpossible.

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Surface investigationofsomemedievalsilvercoinscleanedinhigh-frequency cold plasma

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