Plasma (1)

The restoration and preservation of metallicarchaeological objects made of ferrous materialsrequires the elimination of the chlorinatedcomponents contained in the corrosion products.Otherwise, after excavation of the artefact,corrosion will continue, sometimes at anaccelerated rate. Different treatments may beapplied for that purpose : electrolytic or alkalinesulphite baths, cold hydrogen plasma. This paperdescribes the hydrogen plasma treatment reactorin use at the Archaeological Museum of Val dOise.Results of plasma treatments on artefacts arepresented, together with optical measurementsmade at the reactor during the treatments.

n INTRODUCTION

Excavation of an artefact starts a critical phase of its life, asthe equilibrium between this artefact and its burying envi-ronment, which has been established for years, is then sud-denly broken.

Actually, when buried, the objects are submitted to corro-sion, which results in the formation of both internal andexternal corrosion layers. Internal corrosion layers confinearound the subsisting metallic core, and are mainly made ofFe3O4 magnetite (1), on ferrous artefacts. External layersmainly consist in iron oxides and hydroxides such as-FeOOH, -FeOOH or -Fe2O3 combined with other mine-ral products coming from the soil, among which chlorides,leading to the formation of -FeOOH, for example, whichcontains chloride anions in its chemical structure.

After the artefact has been excavated, chlorinated productsdecompose into chlorides which promote corrosion again. Forexample, -FeOOH becomes -FeOOH, and chloride anionscontained in -FeOOH can then initiate new corrosion phase,named active corrosion. This active corrosion can dramati-cally affect the artefact, up to complete destruction.

Conservation and restoration of metallic artefacts aim to eliminate external corrosion layers, in order to reach the original surface, and also to prevent new corrosion phasesin order to save the object for the future. This implies the eli-mination of chlorides contained in the corrosion layers.

Mainly two methods may be used in order to eliminate thechlorides :

Chemical treatment in alkaline sulphite baths : the object isimmersed in a NaOH-Na2SO3 bath at 50C. SO32- anionslead to the chemical reduction of the corrosion layers andthe extraction of the chlorides. The main disadvantage ofthis method, the most widely used for conservation and res-toration of ferrous artefacts, is the duration of the treatment,which can last over a few months. Moreover, baths have tobe regenerated when saturated in chlorides, which repre-sents quite a heavy maintenance.

Plasma treatment under a high chemically reducing atmos-phere by using hydrogen. The use of a low pressure hydro-gen plasma in order to reduce oxides on corroded objectswas first presented in 1979 (1), and this method was firstdeveloped for ferrous artefacts in the eighties at ZrichUniversity by S. Veprek et al. (2).

In this article, after a brief report about the principle of thistreatment, we will present the plasma reactor used at theArchaeological Museum of Val dOise for this kind of treat-ment. We will report the results of this treatment for someferrous objects, and how this treatment helped.

In order to try to understand the mechanisms of this treat-ment, and also to control it as precisely as possible to pre-vent any degradation of the objects, optical diagnostics byemission spectroscopy were also conducted in the systemmeasurement device, and results will be presented.

As a conclusion, extensions of this plasma treatment toeither other nonferrous metallic artefacts or for industrialgoals such as fine cleaning or surface reductions will be dis-cussed.

La Revue de Mtallurgie-CIT/Science et Gnie des Matriaux Septembre 2001 809

Plasma treatment of artefacts

C. Normand-Chave*, P. Leprince**, F. Dussre**** Materia Systmes, Ormes** Laboratoire de Physique des Gaz et des Plasmas,

Universit Paris-Sud, Orsay*** Muse Archologique Dpartemental du Val dOise,

Guiry-en-Vexin

Manuscrit reu le 20 avril 2001, bon publier le 1er septembre 2001. La Revue de Mtallurgie 2001.

810 La Revue de Mtallurgie-CIT/Science et Gnie des Matriaux Septembre 2001

La restauration et la conservation des objets archolo-giques mtalliques ferreux passe par llimination desproduits chlors contenus dans les produits de corro-sion, partir desquels la corrosion continue se dve-lopper, parfois de manire acclre aprs excavationde lobjet. Pour ce faire, diverses mthodes peuvent treutilises : les bains de sulfite ou lectrolytiques, ou lestraitements par plasma froid dhydrogne. Les premiers,les plus couramment utiliss actuellement, ncessitentplusieurs mois de traitement et la mise en place dunemaintenance parfois lourde. Les seconds, outre le gain de temps quils permettent (traitements allant dequelques heures quelques jours), permettent la miseen vidence et la conservation de certains dcors fra-giles sur les objets.

Cet article prsente le racteur de traitement par plasmafroid dhydrogne dont sest quip le Muse Archo-logique Dpartemental du Val dOise, ainsi que desrsultats de traitements dobjets et les analyses optiquesralises sur le racteur durant les traitements.

Dot dune enceinte ractionnelle de 0,45 m de dia-mtre et de 1,5 m de longueur (volume de 240 l environ),ce racteur permet la cration dun plasma majoritaire-ment compos dhydrogne (80 90 % typiquement) au moyen dun gnrateur RadioFrquence (27,12 MHz), des pressions pouvant varier de quelques 10-1 quelques mbar (0,5 mbar typiquement), et pour despuissances de lordre de 1000 1500 W.

Dans ces conditions, la dure moyenne de traitementdun objet varie gnralement de 2 10 h, et la temp-rature de cet objet reste stable, infrieure 300C pen-dant la dure du traitement.

Les atomes dhydrogne crs dans ce plasma peuventalors, dune part, rduire certains oxydes des couchesexternes de corrosion, rendant ainsi la surface de lobjetcorrod pulvrulente et friable (et facilitant donc un net-toyage mcanique ultrieur), et, dautre part, rduire lesproduits chlors contenus dans les couches de corro-sion active internes (ces produits, rduits, disparaissentsous forme gazeuse, stoppant et stabilisant ainsi la cor-rosion).

En cas de mise en uvre ultrieure dun traitement parbain de sulfite afin dliminer les dernires traces de pro-duits de corrosion chlors, la dure de ce traitement estrduite quelques jours seulement.

Les traitements raliss sur un certain nombre dobjetsont permis de mettre en vidence la facilit plus grandede nettoyage mcanique au cours de la restauration,ainsi que la prservation de certains dtails de dcorfragiles.

Afin de mieux comprendre les mcanismes intervenantlors de ces traitements par plasma, un dispositif dana-lyse optique a t mis en place sur le racteur de traite-ment du Muse Archologique Dpartemental du ValdOise : il est constitu dun spectromtre optique cou-pl une fibre optique, les longueurs donde analysesallant de 200 900 nm.

Il est principalement noter que lon observe une mis-sion notable, au cours des traitements, dans la rgiondes 310 nm, correspondant lmission de radicaux-OH, et pouvant tre attribue la formation dhy-droxydes au cours de la rduction de certains oxydes,ainsi que, dans une certaine mesure, un dgazagecomplmentaire durant le traitement.

Des analyses complmentaires, en cours sur des objetsplus chargs en produits chlors que les prouvettesanalyses dans un premier temps, devraient permettrela mise en vidence dmissions caractristiques lies la rduction des produits chlors au cours du traitement.

Traitement dobjets archologiquespar plasma avant restauration

C. Normand-Chave*, P. Leprince**, F. Dussre**** Materia Systmes, Ormes** Laboratoire de Physique des Gaz et des Plasmas,

Universit Paris-Sud, Orsay*** Muse Archologique Dpartemental du Val dOise,

Guiry-en-Vexin

EXPERIMENTAL PROCEDURE

Principle of the plasma treatment

Low pressure plasmas, created by means of DC, radio fre-quency, or microwave discharge, are partially ionized (ioni-zation rate between 10-8 and 10-4 depending on the way theplasma is created), and contain electrons, ions and electri-cally neutral species such as atoms, radicals or excitedmolecules, which can be highly chemically active.

For example, low pressure plasmas created in air or oxygencontain oxygen atoms which can enhance surface modifi-cations such as improvement of plastics wettability. Suchplasmas are already used for industrial purposes : plastictreatment before sticking, painting, metallizing, printing (3).

In hydrogen plasmas, hydrogen atoms are created : theirconcentration may reach 80 % depending on operatingconditions (4). These atoms will play a key-role for the chemi-cal reduction of the corrosion products.

The plasma reactor at the ArchaeologicalMuseum of Val dOise

The system is presented figure 1 and the vacuum chamberand control panels are shown figure 2.

The vacuum chamber, made of glass, is 1.5 m in length and0.45 m in diameter (volume 240 l). The objects to be trea-ted are put on a quartz plate of 1.5 m in length.

The pumping device consists in a rotary pump of 80 m3/h,associated with a dust filter, in order to prevent severedamage of the pump due to potential abrasive dust comingfrom the artefact. Hydrogen, argon, nitrogen and methaneare introduced into the vacuum chamber for the treatment ;their flow are controlled by means of flow meters.

The plasma is created at 27.12 MHz by means of a2.8 kW Sairem generator, and the power is coupled tothe gas by means of copper electrodes outside of thevacuum chamber. The impedance adapter is motori-zed, in order to get easy use of the reactor. The gene-rator is water-cooled.

Two pressure gauges allow precise control of the pres-sure before (outgassing) and during the treatment. Athermocouple has also been added in order to controlthe temperature of the object during the treatment. Infact, this parameter has to be carefully controlled, andoperating conditions have to be adjusted to avoidimportant temperature increase of the artefact, in orderto prevent degradation due to metallurgical phasetransformations which may occur with increased tem-perature.

In order to prevent exhaust of toxic products, gases atthe exhaust of the pump are cracked in a furnacebefore coming out at atmospheric pressure.

The whole system is associated with an hydrogendetector and a general control panel, in order to allowcompletely safe operation.

traitement plasma


Figure 1 Schematic of plasma reactor.Figure 1 Schma de principe du racteur

de traitement par plasma.

Figure 2 Plasma reactor. Vacuum chamber and control panels.

Figure 2 Racteur plasma : vue densemble.

n RESULTS AND DISCUSSION

Treatment of ferrous artefacts

Typical operating conditions are : pressure 0.5 mbar, plasmacontaining either 90 % H2, 8 % N2 and 2 % Ar, or 80 % H2,20 % Ar, radio frequency power between 1000 and 1500 W,treatment time approximately 2 to 10 h.

Under these conditions, temperature of an object remainsstable under 300C during the treatment.

Figure 3 (5) shows a Merovingian damascened buckle after plasma treatment. Reduced corrosion products afterthe treatment are shown, with the restored original damas-cened surface.

Figure 4 (5) shows other Merovingian objects after plasmatreatment. The plasma treatment here helped to revealsome decorative details that had not never been seen pre-viously (fig. 5).

Figure 6 also shows claps on which some textile pieces havebeen preserved after the plasma treatment.

From these few examples, it appears that the main advan-tages of the plasma treatment for ferrous artefacts are : corrosion products become dusty and friable, allowing

easier work for the restorer and preservation of somedecorative details that are damaged when using othermethods ;

traitement plasma


Figure 4 Other Merovingian objects after plasma treatment and restoration.

Figure 4 Fibules mrovingiennes aprs traitement par plasma et restauration.

Figure 3 Merovigian damascened buckle after plasma treatmentand partially restored.

Figure 3 Boucle mrovingienne damasquine aprs traitementpar plasma et restauration partielle.

Figure 5 Detail of figure 4.Figure 5 Dtail de la figure 4.

Figure 6 Claps with textile pieces after plasma treatment and restoration.

Figure 6 Boucle daumnire ayant conserv des traces de textile, aprs traitement par plasma et

diffrents stades de restauration.

plasma treatment lasts over a few hours against a fewmonths for sulphite bath treatments ;

after plasma treatment, artefacts also are submitted tosulphite bath treatment, in order to eliminate residualchloride traces. In this case, the sulphite bath treatmentonly lasts a few days or weeks, and is easier to conduct.

For all these reasons, plasma treatment is a very promisingmethod for the treatment of artefacts before restoration andconservation.

At the moment, most of the experiments conducted at theArchaeological Museum of Val dOise concern ferrous arte-facts, but other scientists over Europe and Egypt are alsoworking with plasma devices on nonferrous artefacts, suchas copper, copper alloys, silver, tin (4). They point out thatplasma treatment is also suitable for these materials, underappropriate operating conditions.

These preliminary studies at the Archaeological Museum ofVal dOise and by other scientists in Europe have demonstra-ted the feasibility and interest of plasma treatment.

A purpose now is to optimize this way of treatment, and todetermine whether it is possible to set real-time control of it. For this purpose, the plasma reactor at ArchaeologicalMuseum of Val dOise has been equipped with optical dia-gnostics.

Emission spectroscopy

As the plasma naturally emits some light, emission spectro-scopy is widely used to identify plasma species, to measurethem, and to control industrial plasma processes.

The experimental setup for spectroscopic analysis is pre-sented figure 7.

An optic fibre is placed 0.5 cm away from the vacuum cham-ber, so that the light emitted by the plasma around the objectduring the treatment can be analysed. The signal is trans-mitted by the optic fibre to a Jobin Yvon-Sofie spectrometer,which is able to accumulate spectra in order to minimize the noise and get the most precise information possible.Spectra are recorded between 200 and 900 nm, but as thevacuum chamber is made of glass, transmission of wave-lengths between 200 and 300 nm is very poor.

Figure 8 shows the spectrum of a gas mixture containinghydrogen, nitrogen and argon, without any artefact in theplasma.

Characteristic spectral bands of molecular excited nitrogenappear in the range of 300-430 nm, and between 550 and800 nm. Characteristic emission lines of hydrogen are alsoseen at 486.3 and 65.5 nm.

traitement plasma


Figure 7 Experimental setup for spectroscopic analysis.Figure 7 Dispositif exprimental pour la ralisation

des analyses spectroscopiques.

Figure 8 Spectrum between 200 and 900 nm of a plasma in a mixture of H2-N2-Ar. No artefact within the plasma.Figure 8 Spectre entre 200 et 900 nm dun plasma

dans un mlange de H2-N2-Ar.

Figure 9 Spectrum between 200 and 900 nm of a plasmacreated in a H2-Ar mixture. No artefact within the plasma.

Figure 9 Spectre entre 200 et 900 nm dun plasma plac dans un mlange de H2-Ar.

(656,2

7 mm)

(656,2

7 mm)

RF generator

Plasma reactor Spectrometer Photomultiplier

Acquisitionsystem

Computer

Optic fiber

Gas

iner

t

To pumping diviceand furnance

Figure 9 shows the spectrum of the hydrogen-argon mixturewithout any artefact. Actually, nitrogen mainly acts as a pas-sivating agent for the surface. As artefacts after plasmatreatment are restored, the passivation layer is mechani-cally eliminated by the restoration. So some experimentswere conducted without any addition of nitrogen in the gasmixture. Yet, considering the limit pressure of the system,some nitrogen always remains trapped on the chamberwalls, which explains the emission of NH, and the existenceof some bands between 300 and 430 nm which may be attri-buted to excited N2.

Figure 10 presents recorded spectra at various times duringthe plasma treatment of an artefact. It has to be noticed thatan intense emission appears at the beginning of the treat-ment, in the region of 310 nm, which may be attributed to-OH bonds. This emission might be due either to the out-gassing of-OH radicals (water vapour, for example) from the

object during the treatment, or to the chemical reduction ofhydroxides into oxides. This has to be confirmed by furtherexperiments.

The main expectations related to spectroscopy concern theexistence of characteristic emission of chlorinated productsduring the treatment. Unfortunately, it was not possible toidentify any one.

Yet, it has to be noticed that quantitative analysis on cur-rently treated objects by the Archaeological Museum of ValdOise indicate that chlorine concentrations in these arte-facts is very low, which may explain the lack of optical detec-tion of these species. This has to be confirmed by plasmatreatments on artificially highly chlorinated samples. Thiswork is in progress.

traitement plasma


Figure 10 Evolution of the plasma spectrum during plasma treatment of artefacts.Figure 10 volution du spectre du plasma durant le traitement par plasma des objets.

a) b)

c) d)

e) f)

n CONCLUSION

Plasma treatment of artefacts seems to be a promisingmethod for preparing objects to be restored and conserved,shorter than classical sulphite bath treatment, leading toeasier work for the restorer, and less aggressive than othermethods, as it helps in the preservation of some ornamentaldetails which had previously never been observed.

Yet, this method still has to be investigated, for example inorder to treat nonferrous objects, or, for ferrous objects, toget a precise control of the object temperature, preventingmetallurgical modifications of the structure.

Precise knowledge of the mechanisms of the treatment andthe way to control it also have to be investigated.

As the principle of this method is a chemical reduction ofproducts under plasma conditions, this technique may beused for fine cleaning of other non-archaeological objects :preliminary experiments have been conducted in a laboratorydevice, which prove the feasibility of the method.

As it also has been proved with the plasma reactor at theArchaeological Museum of Val dOise that this kind of plasmais stable, repetitive, easy to start, and that the starting andcoupling may easily be achieved with automatic coupling sys-tems, industrial use of this method might be considered.

Acknowledgements

The authors wish to thank the Archaeological Museum of ValdOise for its help and photographs.

references(1) DANIELS (V.D.), PASCOE (M.W.), HOLLAND (L.) Studies

in Conservation, 24 (1979), p. 85-92.(2) VEPREK (S.), ELMER (J.T.), ECKMANN (C.), JURCIK-

RAJMAN (M.) J. of the Electrochemical Society, 134(10)(1987), p. 2398-2405.

(3) NORMAND-CHAVE (C.) Le Vide : Les couches minces.273 (1994), p. 333-340.

(4) ROUSSEAU (A.), GRANIER (A.), GOUSSET (G.), LEPRINCE(P.) J. of Physics D, Applied Physics, 27 (1994), p. 1412.

(5) Archaeological Museum of Val dOise la recherche dumtal perdu, Errance, ed. (1999), p. 135-155.

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