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ava i l ab l e a t www.sc i enced i r ec t . com

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The effect of uric acid on outdoor copper and bronze

E. Bernardia,⁎, D.J. Bowdenb, P. Brimblecombeb, H. Kenneallyb, L. Morsellia

aDipartimento di Chimica Industriale e dei Materiali, Università di Bologna, Viale del Risorgimento 4, 40136 Bologna, ItalybSchool of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

A R T I C L E D A T A

⁎ Corresponding author. Tel./fax: +39 051 2093E-mail address: elena.bernardi@unibo.it (E.

0048-9697/$ – see front matter © 2009 Elsevidoi:10.1016/j.scitotenv.2008.12.014

A B S T R A C T

Article history:Received 8 August 2008Received in revised form26 November 2008Accepted 5 December 2008Available online 20 January 2009

Bird droppings are often quoted as a decay agent for outdoor goods, in particular buildingsand statues. Undoubtedly, they represent one of the major causes of aesthetic damage onoutdoor materials, but the real chemical damage they are able to induce, in particular onmetals, is not so well studied. This work focused on the short term role of uric acid, themainconstituent of bird urine, with respect to copper, which make such an importantcontribution to architectural elements of buildings and outdoor sculpture. Preliminaryresults of laboratory tests and analyses on real exposed samples showed that uric acidchemically affects copper and bronzes: the surface of the metal is modified and copperurates formed. Also natural patina, formed on statues and roof, react with uric acid, even if itseems to afford some protection toward bird droppings. In general, experimental resultsconfirm that the potential chemical damage by bird droppings is significant whenconsidering external cultural heritage such as statues, metal monuments and buildingswith historic copper roofs.

© 2009 Elsevier B.V. All rights reserved.

Keywords:UrateStatueBird droppingCorrosionRaman spectroscopy

1. Introduction

Outdoor heritage is exposed to physical, chemical andbiological threats, which degrade their aesthetic appeal andmaterial integrity. The effects of air pollution and weather arefrequently discussed (Strandberg, 1997) and although thereare general comments (BBCNews, 2003, 2004) about the effectsof excreta from birds and other animals (Fig. 1), relatively littleresearch has been done on this topic.

There are mixed opinions about the importance of birddroppings as a decay agent for buildings and statues. Bassi andChiantante (1976) and Channon (2004) investigated the role ofbird excrement as a cause of deterioration, but their studieswere limited to indirect impact on stone works. Additionally,Gómez-Heras et al. (2004) pointed to the role droppings play asa source of salts and these droppings can also block drainagechannels and pipes, leading to damp ingress and associatedstructural problems (Feare, 1986). Our research investigatedthe potential for bird droppings to affect copper, which make

863.Bernardi).

er B.V. All rights reserved

such an important contribution to architectural elements ofbuildings and outdoor sculpture.

This preliminary work involved a laboratory investiga-tion of the short-term effects of analytical grade uric acidand real bird droppings in contact with copper to assessvisual change, erosion and secondary compound formationi.e. urates. In addition, we considered some real cases, byanalysing bird dropping stains from outdoor copper andbronze statues.

2. General overview

2.1. Damage caused by bird droppings

The simple aesthetic and visual damage to cultural heritagefrom bird droppings is often evident, but the extent of thechemical damage to the underlying materials is not so clear.Some isolated observations suggest bird droppings can

.

Fig. 1 –Nude male bronze statue at Anglesey Abbey (UK) showing bird dropping soiling on raised arm and side of torso.

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influence corrosion. Electrochemical measurements on thebronze sculpture El Karma, exposed in the La Recoletacemetery of Buenos Aires since 1927, show that zones incontact with bird droppings reveal less protective products.In addition, high concentrations of phosphates in theyellowish green and light green areas of corrosion couldbe derived from bird droppings (Cicileo et al., 2004). Thiswas confirmed in a study of the patina formation onhistorical copper roofs in Krakow (Stoch et al., 2001).Graedel et al. (1987) observed that surfaces of outdoorsculptures and roofs of copper often have small black spotsafter long exposure. These spots are normally very hard andadherent and contain some phosphorus, which suggests,“pigeons may, of course, be involved at times” because theelement is not very abundant in rainwater (Graedel et al.,1987). Skennerton et al. (1997), analyzing exposed coppersurfaces contaminated with bird droppings, felt it unlikelythat the phosphate and nitrate compounds found in theguano influenced, in their case, the atmospheric corrosionof the copper. However, concerning the effect of PO4

3− frombird dropping on outdoor bronzes, a specific copper corro-sion compound (cornetite: Cu3PO4(OH)4) was identified byRobbiola and Hurtel (1991) on a Rodin's bronze.

In any case, bird droppings can alter the chemicalcomposition of the precipitation by increasing the pH valueof rainwater and the concentration of NH4

+, K+, PO43−, and

sometimes NO3− (Asman et al., 1982). Bird excreta are also

found to be a significant source of soluble salts on stone, in

particular NO3− and K+ (Cardell et al., 2003; Topal and Sözmen,

2003; Gómez-Heras et al., 2004).In particular applications (i.e. polishing of semiconductor

devices, chemical mechanical planarization), uric acid isindicated as an effective corrosion inhibitor for copper(Koito et al., 2002; Nagendra Prasad and Ramanathan,2007). In these cases uric acid is used to prevent corrosionfrom specific aggressive agents, such as amino alcohols andH2O2, and in particular conditions: high temperature, alka-line and buffered pH, mechanical stress, etc.; the inhibitoryaction occurs when all the copper object is treated with asufficient amount of solubilised uric acid. This indicatesthat, in specific conditions, uric acid can react with copperand form quite stable products. For outdoor metals, acorrosion inhibition by uric acid from bird droppings is notlikely, at least for the irregular covering of the surfaces. Ifformed and stable in atmospheric conditions, these pro-ducts scattered on the surfaces could even be a cause ofvisual damage chemically induced.

Moreover, there is evidence that solid uric acid at ambienttemperatures corrodes copper, brass and bronze with a rateb0.5 mm/yr, roughly the same rate as glacial acetic acid underthe same conditions. In comparison, other acids with anatmospheric or biological source show corrosion ratesN1.3 mm/yr, as sulphuric, nitric and phosphoric acid, orbetween 0.5 and 1.3 mm/yr, as citric acid. Within the samerange as citric acid falls sodium chloride, the main sea salt inatmospheric depositions (Corrosion Survey Database, 2002).

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2.2. Bird droppings and uric acid

Birds, unlike mammals, do not excrete urea as major endproduct of nitrogenous metabolism. Their faeces are usuallycream coloured and consist of two fractions: a clear liquid anda white part, generally viscous and mucoid, composed mostlyof uric acid (see Table 1) and other urates (McNabb, 1974;Sturkie, 1986; Drees and Manu, 1996).

Uric acid is only very slightly soluble in water (64 mg L−1 at310 K) (Verschueren, 2001) so it does not contribute to theosmotic activity of avian urine, where pH varies from 4.70 to8.00: male urine is about 6.4 and the female urine is 5.3, moreacidic with more ammonia and phosphate. When no calciumis being deposited on the eggshell or an egg is laid the urine isalkaline (pH=7.6). The uric acid in droppings is composed ofminute (0.5–13 μm) birefringent spheres (Lonsdale and Sutor,1971) of two components: soluble salts and uric acid dihydrate.Wetting can break up the spheres, dissolving the minorconstituents and leaving the uric acid dihydrate as an orderedphase.

Uric acid undergoes biological degradation, both before andafter excretion. Intestinal microbes transform uric acid intoammonia, short chain fatty acids and carbon dioxide (Singer,2003; Braun and Campbell, 1989). External decompositiongives mainly ammonia, that can be subsequently nitrified(Lindeboom, 1984), and, if degradation is incomplete, urea andother intermediates are found (Carlile, 1984). These transfor-mations suggest that not only uric acid, but also thecompounds deriving from its biodegradation are potentialagents for damage to materials (Bassi and Chiantante, 1976).

Chemically, uric acid consists of white and odourlesscrystals (rhombic prisms or plates) decomposing withoutmelting. It exists in two forms: dihydrate and anhydrous.The nitrogen and oxygen atoms enable uric acid to coordinateas a bidentate ligand with many divalent transition metalsincluding copper, lead and zinc (Gandour et al., 1994;Koksharova, 2002; Moawad, 2002).

3. Experimental

Laboratory tests were carried out in order to verify if and howintensively uric acid in bird droppings is able to damageoutdoor copper. Analyses were performed, both on laboratoryand on real outdoor copper and bronze samples, to identify theproducts formed from interaction with uric acid. In order tohave reference compounds for Raman analyses, the principalcompounds expected to form, that is urates of copper, leadand zinc, were synthesized from analytical grade reagents.

Table 1 – Distribution-ranges of nitrogen in the urine of differen

Species Uric acid Urea

Chicken (Farner and King, 1972; Sturkie, 1986) 58.0–84.0 2–10.4Duck (Farner and King, 1972; Sturkie, 1986) 52.0–78.0 1.5–4.2Penguin (Lindeboom, 1984) 81–84 –Pigeon (Harr, 2002) 2.5–12.6a –

Where no data is given this does not imply none was detected but rathera mg/dL.

3.1. Materials

Samples used in the corrosion tests were sheets of purecopper; the nominal compositions were verified by EDSspectroscopy. The samples typically had a light tarnish andwere not acid cleaned, to observe the effect of uric acid onnatural tarnished metals. Samples were washed with purewater and degreased by acetone.

Some heavy patined copper samples were obtained fromthe original copper roof of the Wellington Pier at GreatYarmouth — Norfolk Coast (UK). The roof, dated 1895–1905,was removed in 2003 during restoration work. The samplesshowed a green patina of atacamite with black areas contain-ing cuprite and carbon (Raman analyses). In addition, samplesof corrosion products from areas that appeared damaged bybird droppings were collected, using wooden spatulas, fromoutdoor copper and bronze statues (19th C.) at Anglesey Abbey(Cambridgeshire, UK). A sample from a lead statue was alsocollected. Althoughmany damage spots were observed on thestatues, most were only tarnish marks and lacked easilycollectable material on themetal surface. Thismade samplingvery difficult as virtually no material could be collecteddirectly, so swabs were taken. A few spots had white orcream coloured crystalline corrosion products on the metalsurface, where about 10 mg for each sample were collected.

3.2. Test procedures

Several tests were performed placing drops of differentcorrosive media, all containing uric acid, on the pure copperand on the copper roof samples. In particular, the corrosivemedia consisted in: aqueous pastes containing uric acid(0.01 g), uric acid and sodium nitrate (2:1 mass ratio), uricacid and potassium dihydrogen phosphate (2:1 mass ratio)and bird droppings as collected. All the chemicals were oflaboratory analytical grade. NaNO3 (BDH) and KH2PO4 (BDH)were added to uric acid (Fluka) to examine the influence ofnitrate and phosphate, naturally present in bird droppings, onthe uric acid action. Real bird excreta were considered so as tocompare the effect of pure uric acid with that contained innatural small wild bird droppings. The excreta were collectedby pliers on a clean surface in a private garden,where feedwasspread to attract birds, put in sterile plastic boxes, stored in adry place and used within two days. Bird droppings were usedas-collected to allow greater amounts of uric acid to be incontact with the surfaces. In fact, the outer coatings of birddroppings contain most of the uric acid, as shown by thepositive response of themurexide test (useful spot test for uricacid as it gives a characteristic purple red colouration).

t birds expressed as percentage of total urinary nitrogen

NH4+ Purines Creatine and creatinine Amino acid

6–23.0 9.6–20.0 1–8.0 1.7–103.2–30.3 0.5 /–2.5 /–2.76.4–7.2 – – 8.6–11– – 0.26–0.40a –

analysis was not reported in the study referenced.

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All the treated samples were maintained in a temperaturecontrolled humidity cabinet at 25 °C and 100% relativehumidity (RH) for 15 days. Because pure uric acid pastes tendto dry over 24 h even at RH close to 100%, samples treatedwithpure uric acid were kept wet by adding a drop of water, twice aday.

The mobilization of copper was investigated by treatingsamples with individual deposits (5 mmdiameter) of pure uricacid and bird droppings: samples were extracted from thehumidity cabinet at progressive times of exposure. For eachsample, the deposit was carefully removed with a plasticspatula and the metal surface washed with pure water into aflask. The collected material was dissolved in about 10 ml ofmixed hydrochloric and nitric acid 1:1 warmed, cooled andmade up to 100 ml with pure water; Cu was analysed byInductively Coupled Plasma Spectrometry (ICP-Varian VISTA-PRO).

After polishing for 2 min in an ultrasonic bath containingpure water, the surface of affected areas on the coppercoupons was examined by spectrophotometry (DATACOLORSF 600× spectrophotometer; illuminant D65, 10° observer,beamof diffuse light of 6.6mm), Scanning ElectronMicroscopywith X-ray microprobe (Oxford Instruments INCA, Jeol JSM-5900LV) and Raman spectroscopy (Renishaw Raman SystemRM1000 laser wavelength 514.5 nm and diode 780.0 nm).

The corrosivity of uric acid was verified by stirring 5 g of cutbright copper wire pieces in a suspension of 0.20 g of uric acidin 5 ml of water left at room temperature (20 °C) for 15 days.The corrosion products formed were analysed by Ramanspectroscopy. Raman spectra were also recorded on thesamples collected from the statues at Anglesey Abbey.

3.3. Metal–urate synthesis

Copper, lead and zinc urates were synthesized along the linesof the procedure of Koksharova (2002) and Moawad (2002),using uric acid (Fluka) and the corresponding metal salt(CuSO4·5H2O–BDH, PbN2O6–Fluka, ZnN2O6–Aldrich). Thisinvolved 0.005 mol uric acid suspended in 100 ml of water(18MΩ) andmade alkaline to pH 9with sodiumhydroxide. Thiswas heated to about 90 °C whilemixedwith amagnetic stirrer,then a 50ml aliquot of 0.1 molar solution of themetal salt wasadded dropwise. The mixture was heated under stirring for5 min, cooled to room temperature, then heated again understirring at 80 °C for about 60 min. The precipitate was filtered,washed with hot water and air dried. As reported in literature(Koksharova, 2002; Moawad, 2002), copper urate shown a lightbrown/grey colour, while lead and zinc urates were white.

Fig. 2 –Colour variation of copper exposed to uric acid, as afunction of time. L⁎ (a) and ΔE⁎ (c) are the lightness coordinateand the total colour difference in the CIELAB colour space; C⁎ab(b) is the chroma in the CIELCH colour space. The continuousline in the ΔE⁎ graph indicates the perceptibility limit, equalto 3 units.

4. Results and discussion

4.1. Tests on copper sheets

The corrosion tests were carried out treating the metalcoupons with different corrosive media for fifteen days inthe humidity cabinet. These showed that laboratory grade uricacid corroded copper, leaving characteristic dark markings,usually in circular shapes reflecting the area of exposure to thesmall drops of uric paste.

Aesthetic alteration is evident considering the colourchanges measured in the CIELAB colour space. In this system,the difference between two colours is expressed by the ΔE⁎parameter:

DE4 = radq DL4ð Þ2 + Da4ð Þ2 + Db4ð Þ2� �

where L⁎, a⁎ and b⁎ are respectively the lightness, redness–greenness and yellowness–blueness axes. Information on thecolour purity can be obtained considering the polar systemCIELCH, where the C⁎ab parameter expresses the chroma (orcolour purity) and hab the hue (CIE, 1986; Hunt, 1998).

Copper sheets show a colour variation that change withtime andmore relevant in the first phase of the corrosion test.In particular, an initial significant darkening occurs, with aprogressive and simultaneous decrease in the red and yellowcomponents, then the colour parameters tend to stabilize.This is evident by the evolution of lightness and chroma that,during the first three days, drop, respectively, of 20% and 60%and afterwards remain nearly constant (Fig. 2a, b).

However, as the aesthetic effect depends on the directionand the magnitude of colour change, if we consider 3 units in

Fig. 3 –Raman spectrum of cuprite and copper urate recordedon a copper surface treated with uric acid after 15 days ofexposure.

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the CIELAB colour space as perceptibility limit (Berns, 2000;Volz, 2001), it can be observed that the total colour differenceΔE⁎ greatly exceeds the visual threshold at every time ofexposure (Fig. 2c). Therefore, uric acid induces on copper anevident aesthetic damage, especially due to the darkening,with a process taking place during the first days of exposure.

The chemical action of uric acid on copper and copperbased alloy is evident from Raman analyses. After fifteen daysof exposure, besides cuprite, copper urates were detected onthe copper surfaces exposed to the uric acid pastes (Fig. 3,Table 2).

The presence of cuprite could be ascribed to the action ofuric acid but it could be partly due also to the naturaltarnishing of the used samples.

Copper corrosion is enhanced by the presence of salts(sodium nitrate and potassium dihydrogen phosphate) mixedinwith the uric acid. This could be due to the action of the saltsthemselves, but it is also likely that the salts, particularlynitrate, serve to maintain the moisture content of the uricpaste and enhance its corrosive action. The pastes includingsalts remained wet throughout the high humidity experi-ments (15 days). This hypothesis above is supported by Raman

Table 2 – Raman results from the analysis of new and historica

Material Corrosive mediu

Copper sheet Uric acid paste (kept wet)Copper sheet Uric acid and sodium nitrite pasteCopper sheet Uric acid and potassium dihydrogeCopper sheet Bird droppingsCopper wire Uric acid saturated solutionDark areas, brown crystalsWhite crystals

Copper roofa As receivedGreen areaBlack area

Copper roofa Uric acid pasteGreen areaBlack area

Copper roofa Bird droppings

n.d. — non detected do to high fluorescence of the sample.a Weathered copper roof from Wellington pier.

analyses that, on the samples treated with uric acid andsodium nitrate paste, revealed, in addition to the presence ofcopper nitrate, amore significant amount of copper urate thanin the case of the uric acid treatment (Table 2). Samples treatedwith uric acid and potassium dihydrogen phosphate pastewere too fluorescent to give good results from Ramanspectroscopy.

In any case the corrosive potential of uric acid was clearfrom experiments with suspension copper wire and uric acid.After a day the white suspension turned yellowish-brown andtwo weeks later part of the suspended powder had a greenishcolour. Typically, on the copper wires, there were white crustsconsisting in unreacted uric acid with some brown crystals ofcopper urate (Table 2). Copper urate was also found on thedarker underlying metal.

Bird excreta grew mould after a few days in the humiditycabinet and beyond a week the white coating, rich in uric acid,completely disappeared, suggesting a biological degradationtaken place (Carlile, 1984; Lindeboom, 1984). However, birddroppings left a brown patina that adhered to the coppersurface and themetal under this patinawas darker. In general,the marks were similar to those left by pure uric acid, exceptfor some green crystals grown after a week, likely due to thecorrosive activity of salts or metabolic products (Carlile, 1984;Lindeboom, 1984). Because of the presence of fluorescentnatural matter, it was not possible to identify any products byRaman spectroscopy.

Fig. 4 displays the results of the amount of coppermobilized over time from the copper samples by individualspot deposits of uric acid kept moist and bird droppings (5 mmdiameter). Only data collected in the first week of treatmentare compared, to avoid subsequent contributions from biolo-gical degradation of bird droppings. Generally, there is anincrease in the amount of copper dissolved over time, inparticular in the case of pure uric acid: from about 350 μg cm−2

after one day to about 1100 μg cm−2 after 8 days. The analyticalgrade uric acid mobilize an amount of copper nearly two orderof magnitude higher than bird droppings do, possibly becauseit is not bound with other organic materials and allows bettercontact with the metal for reaction than the uric acid

l metal sheet after 15 days treatment

m Compounds detected

Cuprite, copper urateCuprite, copper nitrate and copper urate

n phosphate paste n.d.n.d.

Copper urateUric acid

Atacamite — (Cu2Cl(OH)3)Cuprite and carbon

Atacamite and copper urateUric acid and copper uraten.d.

Fig. 4 –Copper mobilization (μg cm−2) in logarithmic scale asa function of time after exposure to uric acid. Solid squares:copper foil and laboratory grade uric acid kept moist. Opensquares: copper foil and bird droppings. 3 repetitions for eachpoint; percent errors between 10% and 15% for pure uric acidand bird droppings treatment, respectively.

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encapsulated in the droppings. The uric acidwould also have ahigher effective concentration as the pure compound.

4.2. Tests on patinated copper roof and outdoor statues

Considering the short-term effects of the tested corrosivemedia on the naturally patinated copper from the roof, it ispossible to notice that bird droppings and all the uric acidpastes produced light brown coloured marks. Heavy patinasseem to protect the underlying metal from corrosion, at leastduring the first two weeks. The spots created by uric acidmodified the patina, but did not penetrate completely throughto themetal surface. In particular, in the treated areas, besidesatacamite and cuprite, Raman spectroscopy revealed theformation of copper urates (Table 2). This slight modificationoccurring in the copper patina is also evident by comparingthe results of SEM-EDS analyses from the as-received roof withthose from the patina underneath the uric acid and birddropping treatments (Table 3). Moreover, in the samplestreated with bird droppings, their contribution of biologicalmaterials is evident (see amount of phosphorus). Furtherstudies could investigate the long-term evolution of the patina

Table 3 – SEM-EDS analysis on samples from Wellington Piergrade uric acid and bird droppings in RH cabinet

Corrosive medium Element Historicex

Mean

Uric acid (laboratory grade) Copper 85.2Chloride 2.8Oxygen 10.0

Bird dropping Carbon 18.0Calcium 2.5Chloride 2.4Copper 50.4Oxygen 29.7Phosphorus 1.4

a Analysed after the careful removal of the corrosive medium and washi

exposed to these corrosive media and possible subsequentreactions involving the formed urates.

Samples from Anglesey Abbey's statues were analysed todetermine the effect of exposure to bird droppings under realconditions. The murexide test never gave a positive responsefrom the corrosion products on outdoor statues, Ramananalyses allowed to confirm that uric acid from bird droppingcan react with the original metal. Fluorescence prevent usefulresults from many corrosion samples and the paper tissuesample swabs were burned under the laser, such that theresulting carbon emission masked any useful results. How-ever, two bronzes (Silenus and young Bacchus and VersaillesDiana) gave good Raman spectra, revealing the presence ofcopper urates.

5. Conclusions

Laboratory tests show that uric acid is able to leave cleartarnish marks on copper, and copper urates are found oncopper coupons exposed to uric acid or bird droppings. Thus itwould appear that urates form during this corrosion process.The corrosive action of uric acid is marked, especially when itremain wet. Natural patina seems to protect copper fromattack by uric acid, at least in the short-term, although thesurface of the patina is still marked and its corrosion products(i.e. atacamite) react with uric acid to give copper urate. Underhumid conditions, uric acid in bird droppings is rapidlybiodegraded, suggesting that its corrosive action is temporary.However further studies need to investigate the role of itsmetabolites and the evolution of the corrosion productsformed. Analyses on outdoor bronze statues affected by birddropping stains confirmed that, under real conditions, uricacid from bird droppings is able to form metal urate.

In conclusion, the results of this work suggest that uricacid in bird droppings could cause appreciable damage tocopper used in buildings and monuments. The role of waterin enhancing corrosion by bird droppings seems of parti-cular significance in management, suggesting that cleaningmight be particularly appropriate if conditions are likely toremain wet.

historic copper roof before and after exposure to analytical

copper roof beforeposure (wt.%)

Historic copper roof afterexposurea (wt.%)

SD n Mean SD n

– 2 63.2 0.4 20.11 3 3.0 0.8 3– 2 27.2 1.3 22.8 3 8.0 1.0 31.7 8 3.5 1.6 70.9 7 2.7 0.7 95.1 6 59.3 9.3 63.5 7 31.5 6.1 50.6 6 2.6 1.1 7

ng of the affected areas with pure water.

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Acknowledgements

Heidi Kenneally was supported by the Nuffield ScienceBursaries scheme and we appreciated the help of Jayne Fosterand Helen Sayer in administering this through SETPOINTNorfolk Education Business Exchange Ltd. Elena Bernardi wassupported by the “Programma Marco Polo” Servizio EuropeoAlmaUE — Alma Mater Studiorum Università di Bologna. Wewould like to thank Francesca Ospitali of the University ofBologna for the Raman analyses and Chris Calnan of TheNational Trust for assisting us in sampling deposits on statuesin Anglesey Abbey garden Cambridgeshire.

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