Bartuli C., Angelucci S., Lanuti S., polarization resistancemeasurements for themonìtoring ofthe corrosion rate of

protected copper alloy sculptures.

ESTRATTO DA

Proceedings of thè 6th International Conference on Non-Destructive Testing and microanalysis for thè Diagnostics and

Conservation of thè Cultural and Environmental Heritage,Rom 17-20 May 1999, ICR e AIPnD, 1999, pp. 1343 - 1360.

POLARIZATION RESISI ANCE MEASUREMENTS FOR THE MONITORINGOF THE CORROSION RATE OF PROTECTED COPPER ALLOY SCULPTURES

Cecilia BartùliDepartment of Chemical Engineering and Materials Science, University "La Sapienza"

Via Eudossiana 18, 00184 Rome, Italy

Sergio Angelucci and Stefano LanutiSculptures Conservators, Piazza Oderico da Pordenone 1, 00145 Roma

The use of polarization resistance measurements for thè evaluation of thè surface corrosionrate of a meta! is proposed as a versatile and reliable tool for thè continuous monitoring of thècondition of bronze and other copper alloy monuments.An articulate and detailed project was conceived in thè frame of a strict collaboration with thèdirection of thè Peggy Guggenheim Collection in Venice, one of thè most importantCoìlections of Modern Art, both for thè wideness and completeness of thè intemationalartistic outline and for thè extraordinary quality of its works of art.The method, totally non-destructive and non-invasive, is successfully applied to ninescuiptures, selected as representati ve of various states of conservation, various types andthickness of patina, particular conservation histories, various materials and different exposuresites. Both outdoor and indoor exposed scuiptures were considered.The method seems to suit thè demands of a modera and efficient pian of ordinarymaintenance and programmed restoration. In particular, from thè continuous monitoring ofthè corrosion rate of thè selected scuiptures we expect to draw important short or long terniinformation, concerning:

thè degradation of a metallic surface, in stable and Constant exposure conditions withtime;thè effect of restoration or ordinary maintenance on thè corrosion rate of thè treatedsurface;thè durability of a protective coating;thè comparison between different protective coatings or different coating procedures;thè comparison between different exposure sites and, consequently, thè choice of thèbest location for each work, also considering thè constituent materials;fmally, and most importantly, thè distribution of thè values of corrosion rate on thèsurface of a sculpture, with thè identification of possible weak points, suchincompatibility between artifact and support, differential aeration zones, etc..

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IntroductionConservation and restoration are aspects of a common problem, arising from thè

"categorica! imperative of preservation " that, according to Cesare Brandi [I], comes from thèmora! obligation to hand down any object, once it is recognized to be a work of art.

A differentiation between thè two concepts - conservation and restoration - can befirstly found at thè end of thè 18th Century in Edwards' writings. expressed as a distinctgraduality of conservative procedures [2].More recently, however, a clear technica! distinction was precisely motivated andscientifically founded in thè operative Pilot Pian for Programmed Conservation of Cultura!Herìtage in Umbria, formulated by Giovanni Urbani, Istituto Centrale del Restauro, in 1975[3]. Conservation, thè inherent aim o f restoration, together with thè re-establishment of thèaesthetical "potential unity" of thè work. assumed from that moment a new identity and a veryspecial relevance.

In a state of emergency and crisis for thè conservation of thè entire Cultural Heritage,determined by both thè impact with an increasingly adverse environment and thè massive anddiffuse exploitation, thè possibility to distinguish conservation from restoration allows one toforesee and prevent thè decay, constantly enjoying thè fui! communicative potential of thèwork of art, which is also thè ultimate goal of any restoration.

A ftindamental difference between conservation and restoration is thè frequency ofoperation: necessarily periodic for conservation, hopefully exceptional for restoration.

Unfortunately, however, these fondamenta! features are often disregarded, and thè usuaitrend is to restore without maintaining, thus omitting thè essential instrument of conservation.

Conservation can consist of indirect procedures, like thè continuous monitoring andcontrol of dangerous environmental factors (relative humidity, temperature, light, etc.) as wellas direct treatments of thè work, from thè very simple (like dusting and water cleaning) to thèmore complex, like thè coating of thè surface with protective layers, acting as a barrier againstthè negative effects of an unsuitable or polluted environment, with no damage for thèconstituent material of thè work.

Unfortunately there are Museums where correct conservation procedures are notapplied, where simpJe dustings are too seldom performed, and where outdoor exposedsculptures are often practically forgotten and abandoned to their fate.

It appears however more and more difficult to think in terms of conservation in a timewhere thè rapid increase of thè economical ratio between labour and materials makes it easierto substitute than repair and maintain, also considering thè rapid obsolescence of thè products.

Moreover, in thè fìeld of Cultural Heritage more attention is paid to thè issues ofknowledge and exploitation of works of art than to thè problems of thè management, andtherefore conservation of artistic works, which results in a heavy penalty for both culturalidentity and economical resources. Research programs fruit of collaborations between thèworlds of Science and Cultura! Heritage, centered on thè problems of conservation andrestoration, are scarce and often qualitatively inadequate. This fìeld, suffers in particular, fromthè lack of control procedures which can establish and assess, for example, thè actualtechnical benefit of an act of conservation, or estimate with non-empirical meters thè real stateof conservation of a work of art. The explicit cali that exponents of thè world of Culture andArts bave made to thè scientific and technical community only produced, up to date, meagerreplies [4, 5].

This, however, does not mean that it is impossible to operate successfully inconservation and restoration; it means, instead, that decisions about wherfsad how to operatefor thè reinstatement of thè optimal conservation conditions of single works of art are foundedon estimations based on thè past experience acquired in thè field by qualified operators, that ison thè basis of valuable but mostly subjective and non-reproducible parameters.

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In thè present paper we wish lo propose thè application of a widely used industriaitechnique, employed for thè measurement of thè corrosion velocity of metallic structures (forexample steel rebars in reinforced concrete) to thè field of conservation of works of art,especially bronze and other copper alloys sculptures.

This project was conceived in thè frame of a strict collaboration with thè direction of thèPeggy Guggenheim Collection of Venice, one of thè most importarli Collections of ModernArt, both for thè wideness and completeness of thè international artistic culline and for thèextraordinary quality of its works of art.

A detailed and comprehensive classìfication of ali pieces according to thè constituentmaterials, thè main morphological features, Ihe present and past exposure sites and thèconservalion and restoration history was drawn recently, and is published in [6].

These data can be completed, starting from now, by objective evaluations of thè stale ofconservation of every single bronze or brass artifact, or any pari of il, based on Ihe reliableand repealable measuremenl of precise physical-chemical and electrical parameters, andsupported by thè same aesthetical and personal evaluations that were able to guarantee, duringthè lasi fìfteen years, thè very good stale of conservalion of ali sculptures exposed in IheColleclion.

Polarizalion resislance measuremenls were already carried out in Ihe pasl on thè MarcusAurelius equestrian monument, on thè statues of thè Riace Warriors as well as on thè outdoorgates of Pisa Baptistery [7-9]. In Ihose cases, relevanl informalion was drawn concerning Ihemagnitude and Ihe homogeneity of Ihe corrosion rate in differenl areas of Ihe same bronzesurfaces. A clear barrier effecl of Ihe palina againsl Ihe progress of corrosion was Ihenevidenced, showing higher values of corrosion rale in Ihe washed out areas. The formalion ofgalvanic coupling in Ihe presence of residuai gilding, with consequent increase of thè metallicalloy aclivily was also ascertained in thè case of thè Marcus Aurelius statue.

The sculptures of thè Peggy Guggenheim CollectionThe seventy-three sculplures belonging lo Ihe Peggy Guggenheim Colleclion, ali daled

Ihe presenl century, represenl brillianl and hislorically imporlanl examples of Ihe modernconceplion of Ihe Avant-garde Sculptural Art. The seleclion of Ihe constituenl malerials,ranging from copper and copper alloys lo iron, wood and lead, passing Ihrough paper, silver,glass and cement, is an immediale indicalion of thè greal originalily of Ihe exposed works (see[6] for a delailed classifìcation),

Of thè nine pieces selected for thè present experimenlal campaign, six are made ofbronze, and are exposed outdoors, in Ihe Sculplures Garden (Amphora-fruit, by Jean Arp,1946(7), Walking Woman, by Alberto Giacomelti, 1932, In thè Streets of Athens, by MaxErnst, 1960, The Morse, by Raymond Duchamp-Villon, 1914, Three Standing Figures, byHenry Moore, 1953, and Two Figures, by Luciano Minguzzi, 1950-52) or on thè terracefacing thè Grand Canai (The Angeì of thè City, by Marino Marini, 1948). Two are made ofpolished brass and are exposed indoors (Bird in Space, 1932-40, and Maiastra, 1912, byConslanlin Brancusi). The pian of Palazzo Venier dei Leoni, seal of Ihe Peggy GuggenheimColleclion, with location of thè mentioned sculplures is shown in Fig.l.

The nine sculplures were selected as representative of various slates of conservation,different alloys and thickness of palina, parlicular conservalion hislories and differenlexposure siles. Bolh ouldoor and indoor exposed sculplures were considered, in order lo beable lo compare dala proceding from a severe environmenlal situation wilh dala pertinent to aconslanlly conlrolled environment (lemperalure = 18-20°C. relalive humidily - 50%. totallighl inlensily = 150 lux) as indoors sculptures enjoy. We-wish bere lo recali, on ihis subjecl,ihal Ihe greal originalily of Ihe Peggy Guggenheim Colleclion is just to be found in thèimmediate and intimate contaci belween observer and work of art, in thè parlicular spiril of a

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private collection, to be "used" continuously and without any kind of obstacle or barrier. Theallocation of sculptures and paintings inside protective cases or beyond defending screens istherefore avoided as much as possible.

Outdoor environmental conditions that characterized thè city of Venice in thè pastdecades. on thè other end, were particularly severe, both for thè characteristics of naturaiclimate (high salinity of thè atmosphere together with high values of relative humidity, withan average of 78% calculated on 30 years [10]) and for thè consequences of thè heavyindustriai pollution of thè adjacent areas (maximum values of 0.258 mg/m3 of SO2 and of 0.05mg nr of HC1 were recorded in thè city in thè past years [11]). The atmospheric pollution isbenefically affected today by thè deep crisis of thè industriai production in thè mentionedareas. bui thè effects of thè aggression of thè past years are stili present and well visible ontoany exposed metallic surface.

The surfaces examined were carefully maintained in thè last fifteen years in protectedconditions, in most cases coated with a doublé layer consisting of an acrylic resin coating(Incralac) containing benzotriazole, BTA (corrosion inhibitor) covered by a layer ofmicrocrystalline wax (Soler LS or LC), also containing BTA.

The ordinary mainlenance of most sculptures exposed outdoors consisls of frequenldusling, periodic washing wilh currenl water to eliminate vegelal residuals or particulale, andrenewaì of Ihe wax layer when necessary. The frequency of this last operation has beenevaluated up to present on thè basis of a caretul visual inspeclion, accounling for Ihe colour,ihe brilliance and thè texture of thè surface, and for Ihe conlaci angle of freshly sprayed waterdroplets.

When a chromatic variation and/or a funclional degradalion of thè Incralac layer isevidenced, thè acrylic varnish is also removed. This slep is always necessary for sculpturescharaclerized by a very smooth and therefore delicate surface, where thè presence of any traceof degraded film or uneven coating would be aeslhelically unacceplable (see for exampleAmphora-Fntit). The complete removal of thè prolective coaling is also unavoidable for Ihepolished brass sculptures of Conslanlin Brancusi, exposed indoors, whose only possibleproleclion, by thè aestelical poinl of view, consisls of an extremely thin {1-2 um) Incralaclayer.

From thè continuous monitoring of thè corrosion rate of thè selected sculptures weexpeci lo draw important information either on Ihe short or on Ihe long lerm, concerning:

Ihe degradalion of a metallic surface, in slable and Constant exposure conditions withlime;thè effecl of resloralion or ordinary mainlenance on thè corrosion rate of thc treatedsurface;Ihe durabilily of a proleclive coaling;Ihe comparison between differenl proteclive coaltngs or differenl coaling procedures;me comparison belween different exposure sites and, consequently, thè choice of Ihebesl localion for each work, also considering thè consliluenl malerials;fmally, and mosl importanlly, me dislribulion of thè values of corrosion rate on thèsurface of a sculpture, wilh Ihe idenlification of possible weak poinls, suchincompatibilily belween artifacl and supporl, differenlial aeralion zones, eie..

ExperimentalPolarization ResistancePolarization resislance (Rp) is a represenlalion of thè resistance of Ihe inlerface of Ihe

corrosion process in progress on a melallic surface. Il is therefore intrinsically correlated lo itsactual instanlaneous corrosion rale, and is sirongly dependent on thè composition of thè metalor thè alloy and on its physical-chemical inleraclion wilh me environmenl.

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Composition (stability and solubility), morphology (compactness and porosity) and totalthickness (function of exposure time and of environment aggressiveness) of thè solid layer ofcorrosion products grown onto thè metallic surface (patina) nave a strong influence on thècorrosion velocity of thè underlying metal and therefore on thè measured values of Rp.

In particular, polarization resistance and corrosion rate (c.r.. expressed as um/year) areinversely proportional quantities, according to thè expression:

c.r. = k / R p (1)

where, being Rp expressed as O cm?, thè value of thè Constant k is assumed from thè literature[12] equal to about 587500 for a 6% Sn copper alloy (with acceptable errors in thè range ofsome percent points in thè cases here examined).

To evaluate thè polarization resistance of a surface, thè electrical current is measurednecessary for thè polarization of thè metal to a known value of potential (10 mV).

The reference electrode (a bronze element) must be electrically connected to thè surface(working electrode), and it is therefore necessary to wet thè surface to be examined withwater. Finally, a counter-electrode (a stainless steel ring), connected by means of a currentgenerator to thè surface, must be placed within thè same electrolyte. The source of current is agalvanostatic circuii, ensuring thè effeclive flow of thè imposed current, no matter thè ohmicresistance of thè circuii or thè type of employed malerials.

To avoid polarization of thè reference eleclrode, ine measurement of thè potentialbetween thè reference and thè working electrodes is carried oul by means of a high inlelimpedance eleclromelric circuii.

Experìmental procedurePolarization resistance measurements were carried oul on thè above menlioned

sculptures in a maximum of three conseculive campaigns, corresponding to thè followingconditions:

1. Unprotected surface: in thè course of thè complete procedure of extraordinarymaintenance, some of Ihe sculptures were temporarily broughl lo Ihe originai barecondition, and a first sei of measuremenls was carried oul direclly on thè metallicsurface (in Ihe case ofMaiastra and Bird in Space) or on thè nalural palina (for TheMorse andAmphora-Fruii).

2. New protection: Ihe second sei of measuremenls was carried out 70 tn 60 Hayc afwIhe coaiing of thè sculptures wilh new proleclive layers: new Incralac and Soler forAmphora-Fmit and The Horse, new Soler for The Auge! of thè City, Tliree standingFigures, In thè Streets of Athens and Walking Woman, new Incralc for Maiastra,Bird in Space and Two Figures.

3. 2 years protection: ali sculptures were measured after aboul two years of exposure,in which only dusling and washing operalions were conducled wilh thè usuaifrequency (weekly-monlhly for dusling, about twice a year for washing), and noexiraordinary mainlenance was necessary.

Schemalic representations of Ihe employed apparalus and of Ihe designed probe used forin sim measuremenls are shown in Figs.2 and 3, respeclively.

The corrosion meter is a commerciai ATEL AP 101, whose filli scale was modified loiower values of corrosion rale (thè Ipwest deteclion limil of c.r. is 0.01 um/y).

Ali corrosion rale determinalions were carried oul onto wel surfaces, soaked wilh walerai leasl 10 minutes before starting thè Rp measurement. The counter-electrode was put ine>cirical conlaci wilh Ihe surface of Ihe sculplure by means of a wet lining, conslanlly freshly-_—pregnaied wilh waler. The working eleclrode was connected lo Ihe corrosion meler by

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means of a flat-ended wire, put in physical contact with a non-protected portion of thèsurface.

Sculptures were not cleaned before measurement, in order to include in thè evaluationof thè actual corrosion rate any possible effect of impurities (particulate, chlorides and othersalts).

Ali corrosion rate measurements were accompanied by measures of thickness of thèelectrically insulating layer covering thè metallic surface, intended as thè sum of thèprotective artifìcial layer and thè naturai patina. Thickness measurements were carried outwith an induced current Fisher Isoscope.

Results and discussimiResults, in terms of average values of corrosion rate (um/year) and insulating thickness

< u m ) are summarized in Figs.4 and 6. Each value is calculated as an average of at least 5measurements for corrosion rate and at least 15 measurements for thè insulating thickness.Standard deviation values range from 10 to 150 % of thè corrosion rate absolute values andfrom 5 to 30 % of thè thickness values.

It must be clearly stressed that thè values of corrosion rate, as measured with thèpolarization resistance technique, are aiways over-estimated, since they reflect an all-time-wetsurface condition that is never strictly corresponding to thè real exposure environment. Inparticular, some of thè sculptures examined are kept indoors, and are therefore aiways dry orexposed to thè RH proper of an indoor controlied environment, some other works, keptoutdoors, can be effectively covered, especially during thè night or in thè first hours of thèday. by a more or less thick layer of condensed water, or they are directly exposed to thè rain.

As a comparison and a baseline, we remind that average rates of atmospheric attack ofcopper and copper alloys in different environments are summarized by Shreir et al. in [13]and range from 0.3-1 um/y for rural sites to 0.7-2.6 um/y for marine sites and to 0.9-3.8 pm/yfor urban and industriai sites.

We indicate, in thè following, thè most relevant peculiar features of thè single examinedworks. and summarize thè results of thè investigation.

The Angel of thè City (Fig.5) represents one of thè largest sculptures of thè Collection,and is placed, in a dominant position, on thè white marble terrace facing thè Grand Canai,main "water entrance" to Palazzo Venier dei Leoni. The sculpture is therefore exposed to thèstrongly aggressive synergistic actions of marine aerosol, wind, direct sunlight and pouringrain.

The large surface of thè sculpture presents a wide range of different conservativesituations, corresponding to different sections of thè statue, more or less exposed to thèaggression of naturai agents and to thè offending action of thè visitors. Sections like thè horsemuzzle and tail or thè man's penis are thè object of continuous touching by thè numerous\isitors. and exhibit thè highest values of c.r., together with thè back of thè horse, moreheavily exposed to thè washing action of rain (Fig. 4).

After thè renewal of thè wax coating, thè statue is uniformly protected, showing valuesof corrosion rate never exceeding 0.35 um/y.

Amphara-Fruit; thè sculpture, characterized by a very smooth and uniform surface, isexposed in thè garden, in a very easily accessible position to thè visitors, whose continuoustouch and handling left visible signs, especially on thè "tip" of thè "Fruii", where thè naturaigreen patina, entirely covering thè rest of thè sculpture, was completely missing until aboutten years ago. After a decade of careful maintaining procedures, thè green patina is slowlyreforming, and is now gradually recovering thè exposed tip area (see Fig, 7).

As mentioned above, both protective layers (acrylic varnish and microcrystalline wax)-Aere recently renewed, allowing for thè determination of thè corrosion rate of thè

I 1:00

"unprotected" surface. Values of about 5-10 um/y were recorded in that condition,representing dangerously high corrosion velocities for a wet copper alloy structure(comparable to vaìues of bare copper structures in contaci with drinkable waters containingorganic mailer [14]), and indicaling a very delicale surface and Iherefore thè necessity ofcontinuous carenai contrai and maintenance.

The newly prolected surface shows corrosion rates about 30 times lower, varying fromabout 0.05 to 0.5 um/y.

The vaìues of c.r. measured on two years old protective coalings do noi appeardraslically decreased, evidencing a good resistance of thè Incralac-Soter couple in Iheseexposure condilions. The ralio between ihe average c.r. recorded on ihe "lip" and c.r.measured on Ihe remaining surface of thè sculpture ("back"), is about 2 in thè case ofunprotecled surface, and about 20 for Ihe Iwo years old proleclive coaling, removed on thè lipby thè slroking of visitors, Ihus indicaling that thè real protective action againsl corrosion isexerted, in thè present case, by Ihe attificial doublé coaling much more Ihan by Ihe naturaipalina, also missing on Ihe "lip".

The Ihickness of Ihe naturai patina ranges from 2 lo 22 nm. The proleclive layer isexiremeìy Ihin on Ihe back, where il is easier lo evenly lay off varnish and wax, and variesfrom aboul 1 to 20 firn on thè exposed areas (tip and tail), where il is necessary lo guaranlee amore efficienl proleclion and il is diffidili lo lay off a uniform coaling.

Walking Woman: Ihis sculplure shows a thin (about 10-30 um). dark brown patina,smooth and delicate, Ihal does noi seem lo guaranlee an efficienl proleclion. Values of c.r. of0.5 to 1 [im/y are recorded for thè newly protected surface. thè highest among Ihose ofdouble-coaled sculplures, increasing lo 1 lo 2 um/y after two years of exposure.

In thè Streets ofAthens: Ihe work of Max Ernsl is characlerized by very low and quilehomogeneous vaìues of corrosion rale (0.05 lo 0.24 \im/y for a 2 years old protective coating).Since thè coating procedure is exactly thè same used for Amphora-Fruir, we are allowed topresume thal thè Ihickness of thè naturai patina, in Ihis case, musi be substantially higher Ihanin ihe previous example (ranging from aboul 40 lo 50 firn).

The surface which has just been trealed is alinosi completely insensitive lo Ihe progressof corrosion (c.r. < 0.01 ^irn/y).

The Horse, dated 1914, is me oldesl sculplure in Ihe garden. Il exhibiled originally abrighi polished surface. Corrosion rate vaìues lending lo zero are recorded for this sculpturealso in thè unprolecled condition. It seems therefore sensible lo assume Ihal polishingoperaiions of Ihe pasl contribuled lo thè "sealing" of thè metallic surface, making ilimpervious lo waler and pollutants, and allowing for Ihe very slow formation of an extremelycompaci and proleclive thin stable patina.

Three Standing Figures: thè sculpture appears well protecled by a compaci and Ihickpatina. Very low vaìues of corrosion rale (below Ihe deleclion limil) characlerize thè surface\vhichhasjustbeen irealed, increasing lo 0.1 lo 0.3 um/y after Iwo years of exposure.

Two Figures is kepi on a basement placed in thè middle of a wild flower-bed,surrounded by Irees and shrubs, in a particularly humid section of Ihe garden, hardly exposedto sunlight. The sculplure is characlerized by a non-homogeneous surface, made of perfecllybrighi and smoolh areas nexl lo scratched and "unfìnished" zones.

Reliable c.r. measurements could only be carried out on thè smooth areas, like thè breastor thè knees of thè Iwo fìgures, due lo inlrinsic limilalions of Ihe probe (a fìat, even surface ofal leasl 2 cm2 is necessary for valid delerminalions), and lo Ihe particular features of thiswork. for which thè Sculplor studied and wished Ihe special effecl crealed by residuaimolding sand fìlling spaces, scralches and folds of Ihe bronze structure.

The patina is exlremely Ihin, as well as Ihe proleclive coaling, consisling of a monolayerof Incralac, completely missing where thè surface is covered by molding sand.

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Corrosion velocities are, as expected, very high, of thè order of 10 to 20 um/y. The usedprotection is not particularly effective.

Maiastra (Fig. 8a) is a polished brass sculpture, dated 1912, presently kepi in thè formerdining room of Palazzo Vernier. Brancusi's works are thè only examined cases of indoorsexposed sculptures, and corrosion rate measurements were carried out for Maiastra on thèbare surface, on thè new coating and after 2 years of exposure. The surface of thè sculptureshows small but evident porosities, consequence of a non-perfect casting process.

Results show that thè corrosion velocity is homogeneous on thè whole surface, with noparticular relevance for thè areas where a higher porosity or a light oxidation are visible onthè naked eye. The absolute absence of a naturai patina is confìrmed, and a thickness of thèIncralac monolayer of about 1-5 um is estimated. This protective coating alone allows for a250 times slow-down of thè corrosion velocity (from 50 to 0.2 um/y); however its protectiveeffect decreases with time, and after two years of exposure to thè controlied indooratmosphere and to thè naturai/artifìci al light, a c.r. of about 2 um/y was measured.

We need to highlight that in thè present case thè absolute values of corrosion rate are ofminor importance and are defmitely over-estimated, since they simulate, as mentioned above,conditions of constantly wet surface, and this work is never actually in contaci with any traceof water. Therefore, thè relevance of c.r. data, for indoors exposed works, is only to be foundin thè need of comparative evaluations, as it is evidently thè case under examination.

Biro in Space: of thè sanie Author of Maiastra, is a much later work (1932-40), again ofpolished brass, exposed indoors (Fig. 8b). The surface reveals a more perfect castingtechnique and a slightly different alloy (thè colour of thè brass is more yellow in this case, ascompared to a more reddish alloy for Maiastra). Results show evidence of a higher resistanceof this work to thè progress of corrosion (11 um/y for unprotected surface. 0.55 um/y afterrwo years of exposure).

ConclusionsAt thè end of this preliminary phase of investigation, encouraging results lead us to

conclude that thè proposed non-destructive method for thè evaluation of corrosion velocitycan be successfully applied to thè study of thè state of conservation of metallic sculptures.

In particular, measurements carried out on different copper alloys, in differentenvironments, characterized by various surface conditions, gave positive and consistentresponses, precisely identifying correct experimental prnr.eHiirp« and evid<?neins tk^ intrincictechnical limitations of thè proposed method.

The investigation of protected surfaces, covered by insulating and sealing artifìcialcoatings, revealed thè possibility of employing thè results of polarization resistancemeasurements for comparative evaluations of thè condition of a metallic manufact in thècourse of its conservative iter, also in thè case of indoor exposed works, whose acumi rate ofcorrosive attack is much lower than thè one identified by Rp results.

On thè long terni, knowledge and familiarity with thè behaviour of thè single works, interms of resistance to thè progress of corrosion, will allow for a precise evaluation of thèalarm threshold for thè renewal of thè protective coating, or for minor conservativeprocedures, thus estimating thè minimal time intervals for preservative operations.

The comparison with a detailed and comprehensive data-base of corrosion ratemeasurements will also be recommendable in order to avoide too frequent maintenanceprocedures, inevitably causing a graduai modification of thè surface texture of a work, whosereading is a fundamental aspect of thè global aesthetical evaluation.

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AcknowledgementsWe acknowledge thè Peggy Guggenheim Foundation of Venice, in thè person of thè

Director, Prof. Philip Rylands, for having allowed and encouraged thè present work.

References1- C. Brandi "Teoria del Restauro", Einaudi, Torino, Italy, 1977, p.72- P.Edwards "Piano pratico per la generale custodia delle pubbliche pitture", G.Basile Ed.,

Istituto Centrale del Restauro, Roma, Italy, 1994.3- G. Urbani "Piano Pilota per la Manutenzione Programmata dei Beni Culturali in

Umbria", Istituto Centrale del Restauro, Roma, Italy, 1975.4- G.Urbani "La scienza e Parte della conservazione dei Beni Culturali", Ricerche di Storia

dell'Arte, n.16, 1982.5- G.Urbani, in "Chimica e Restauro: La scienza per la conservazione", Proc. of thè

Congress, Venezia, 1984.6- S.Angelucci, in "Arte Contemporanea, Conservazione e Restauro", S.Angelucci Ed.,

Nardini Editore, Firenze, Italy, 1994, p.101.7- C.Bartuli, B.Colombo, M.Marabelli, in Atti del 3° Congresso Nazionale AIMAT, De

Prede Editore, Napoli, Italy, 1996, p.1003.8- C.Bartuli, R.Cigna, B.Colombo, M.Marabelli, Materiali e Strutture, VI, 3, 1996, 127-1369- M.Marabelli, C.Bartuli, in Proceedings of thè Congress "Metallrestaurierung, Meta!

Restoration", Munich, 23-25 Oct. 1997, in press.10- Ministero Lavori Pubblici - Servizio Idrografico, Ufficio idrografico del Magistrato alle

Acque, Annali idrografici, Parte I.11- U. Fassina, Inquinamento, 16, 1, 1974, p.3412- M.Stern, E.D.Weisert, Proc. Am. Soc. Testing Materials, 59, 1959, 1280-91.13- (several contributors) Corrosion, L.L. Shreir, R.A. Jarman, G.T. Burstein Eds,

Butterworth-Heinemann Ltd, Oxford, U.K., vol.l, third edition, 1994, 4:47.14- L.L. Shreir, R.A. Jarman, G.T. Burstein, Corrosion, vol.l, p.4:47.15- J.P. Rehring and M. Edwards, Corrosion, 52, 4, 1996,307-317.

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CURRENTGENERATOR:GALVANOSTATICCIRCUIT

VOLTMETER:HIGH IMPEDANCEELECTROMETRICCIRCUIT

C ounterel ectrode

Reference electrode

Working electrode:bronze surface

Fig. 2: Flow chart of polarization resistance measurement

counterei ectrode ,_

stainless steelcounterei ectrode

bronze reference electrode

resin

/ wet lining-

r .,

A-A

stainless steel counterei ectrode

insulating resin layer

bronze reference electrode

Fig. 3: Schematic representation of thè probe designed for in situ measurements

1 1355

Marino Marini, The Angel of thè City, 1948Corrosion rate (|im/y) Thickness (/un)

muzzlehe adback

buttocks(a) tail

man's abdomenrigh t leg

loft legpenis

muzzlehcadback

buttocks(b) tail

man's abdomenrighi Icg

left legpenis

0.230.14 [0.050.080.03

0.13 [0.07 [0.03

0.35 [Z

2.78

52I 57

j 6289

1 4863

\65

\

• &1.02 • | 59

2.99 | • 560.11 [

0.49 ;0.08

0.35 [̂0.18

. . . i i 1-22 ,

\

L 1 M\ **

| 5*

70^25

4 3 2 1 O 50 100 150 200

(a) new protective coating (b) 2 years old prolective coating

Fig. 4: Corrosion rates (um/y) and thickness of thè insulating layers (^ini) for The Angelofthe City

Fig. 5: The Angel of thè City, bronze sculpture of Marino Marini (1948), exposedoutdnors, facing thè Grand Canai.

1356

Jean Arp, Amphora-Fruit, 1946 (?)

Conosion tale Qim/y) Jìuclaitss (fan)bick

npluì

back

tìPtali

«ci "Puil

0.05

U..VI

0.46 I

14 12 10 8 6 4 2 (I !0 20 36 40 56

(a) no coating (b) new prouciive coaiing (e) 2 years old proiettile coaling

Alberto Giacometti, Walking Women, 1932

Corrosion rale (|im/y) Ttnckness (tua)

breasi, shouldcrs

(a) abdomen

back

hieasi, - l i . u I , l < i • -

(b) . 'In . . i i. n

2.S 2 1.S 1 0.5 O 26 40 60 M

(a) QCW proiettive coating (b) 2 years oid protective coating

(foni io reo

t from head

back

(toni torso

v. from head

back

Max Ernsl, In ihe streets of Athens, 1960aie (iim/y) TJuctaav (pio)

1 0.75 0.5 0.25 O 20 40 60

(a) 1 1 . M proiettive coaling (b) 2 years old proieclìve coating

Raymond Duchamp-Villon, The Morse, 1914

Coirosion iate(|im/y) Thicltness(iun)

i n i | > , frunl ba

d'I

1 0.75 0.5 0.25 O 20 46 60

(a) no coaling (l>) 2 vcars old proleclive coaiing |i

Fìg.6 (continued)

Henry Moorc, Three Standing Figures, 1953

Corrosion rate (iim/v) Thifinrss (firn)

Ut figure

Jnd figu re

3cd figure

base

Ut fìgur*

2nd figure

3rd figure

base

0.5 0.25 O 30 60 90 120

(a) new protcclivc coaiing (b) 2 \tars old proiettive coatmg

Constant in Brancusi, Maìasira, 1912

Curiosino rate (pm/y) Thiciaitss(fn)

Luciano Minguzzì, Two Figures, 1950-52

Corrosion rate (lim/y) Tbiclmest (fan)

righi leg, small fig.

(a) brusì, small fig,

breasl, largc fig.

righi leg, - i n II MI;

(b) breasi, small fìg.

20 10 O 20 40 60

(a) new proiettive coaiing (b) 2 \ears old prole e live con ting v j

Constanlin Brancusi, Bird in Space, 1932-40

Corrosion rale (inn/yj Tbidatess (pia)

(a) n-hole surface

(b) whole surface

60 SO 40 30 20 10 O 2 1 6 8 IO

) no coaling (b) ne«- proiective coating (e) 2 years old proteciive coaiing

14 12 10 8 6 4 2 0 2 4 6 8 IO

(a) no eoaltng (b) 2 years old protettive coaling vjjj

Fig. 6: f'orrosion rales (fim/y) and thickness of thè insulating lavers (fim) for Amphora-Fruit (i), Walking Woman (ii), /« lite Streets oflihens (iii), T/J^ Wor-st" (iv), r/trf-f? Standing Figures (v), TVo Figures (vi), Maiastra (vii) and ft'rrf m jpace (vili).

I35S

Fig. 7: Detail of thè "tip" of Amphora-Fruit, Jean Arp (1946?), where thè naturai patinais gradually regenerating.

a) b)

Fìg. 8: Maiastra (1912) (a) and Bird in Space (1932-40) (b), polished brass sculptures byConstantin Brancusi.