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Polymer Degradation and Stability 98 (2013) 2317e2322

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Surface yellowing and fragmentation as warning signs ofdepolymerisation in Baltic amber

Gianluca Pastorelli a,b,*, Yvonne Shashoua b,2,*, Jane Richter a,1,*a School of Conservation, Royal Danish Academy of Fine Arts, Schools of Architecture, Design and Conservation, Esplanaden 34, 1263 Copenhagen K,DenmarkbResearch, Analysis and Consultancy Section of the Department of Conservation, National Museum of Denmark, IC Modewegsvej, Brede,2800 Kongens Lyngby, Denmark

a r t i c l e i n f o

Article history:Received 14 April 2013Received in revised form5 August 2013Accepted 16 August 2013Available online 26 August 2013

Keywords:Baltic amberThermal ageingDepolymerisationSpectrocolorimetryConfocal profilometryRaman spectroscopy

* Corresponding author.E-mail addresses: gianluca.pastorelli@gma

yvonne.shashoua@natmus.dk (Y. Shashoua), jr@kadk.1 Tel.: þ45 33 74 47 00; fax: þ45 33 74 47 77.2 Tel.: þ45 33 47 35 02; fax: þ45 33 47 33 27.

0141-3910/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2013.08.0

a b s t r a c t

It is well known that the surrounding environment affects the initiation and rate of degradation of Balticamber, causing changes in visual and mechanical properties. However, the mechanisms by which oxygen,the most important agent of degradation, causes those alterations are still unknown. Knowledge of suchmechanisms would allow developing more efficient preventive conservation measures for amber arte-facts in museums than those available today. In this study, an experiment using accelerated thermalageing was conducted on representative Baltic amber samples. Changes in colour, topography andchemical properties were assessed regularly using non-destructive techniques, to identify the degra-dation phenomena which play a significant role during the discolouration and fragmentation processesof amber surfaces. The breakdown of the amber polymer chains, caused by oxidative radical reactions,was determined by means of FT-Raman spectroscopy and involved the formation of olefinic bonds in theterminal position. This depolymerisation process was correlated to modifications in colour, particularlyyellowing, and in topographic features, analysed by spectrocolorimetry and confocal profilometryrespectively. Thus, surface yellowing and fragmentation of amber objects in museum collections may acttogether as indicators of progressive depolymerisation.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Museum environmental conditions contribute to the deterio-ration of amber, which is a natural polyterpene, making objectscrafted from this material vulnerable to damage through handlingand limiting their options for display and loan [1]. Understanding,monitoring and mitigation of degradation phenomena in amberartefacts are essential in order to develop suitable preventiveconservation strategies for museum collections, which aim to slowthe rate of natural ageing by controlling the surrounding environ-ment. The study of degradation processes in amber is a challenginganalytical problem, mainly because the mechanisms involved arecomplex and can result in various visible deterioration effects, e.g.,surface discolouration, crazing and powdering [2].

il.com (G. Pastorelli),dk (J. Richter).

All rights reserved.09

Baltic amber, or Succinite, is a copolymer of labdanoid diter-penes (Fig. 1), whose chemical characteristics are well known[3e5]. In common with many organic materials, it is knownthat oxygen accelerates the rate of decay [6e8], however, thedegradation mechanisms by which atmospheric oxidation causesalterations in visual properties (discolouration, especially yel-lowing [9]) and mechanical properties (fragmentation, eventu-ally leading to complete disintegration [10]) of Baltic ambersurfaces are poorly established. A likely cause of these deterio-ration effects is depolymerisation of the macromolecular struc-ture, where oxygen plays a significant role, rather than physicalbreakdown processes that only occur in an inert atmosphere[11,12]. Similar oxidation radical mechanisms are reported toenhance or lead to degradation in other natural terpenoid ma-terials [13e15].

Amber degrades by oxidation in the presence of heat, light andairborne pollutants [7e9,16,17]. At first, probably due to the for-mation of chromophores [18], oxidation is revealed by a thinyellowish external layer surrounding the paler core of an amberobject [19]. In the advanced stages of degradation, breakdown ofthe polymer chains causes a large reduction in degree of

Fig. 1. Baltic amber and its chemical composition. Structures of labdatrienoid diter-penes (a), commonly found as monomers for Baltic amber polymerisation, andresulting polylabdanoid (b) are shown at the bottom of image. Numbering of carbonatoms according to Villanueva-García et al. (2005) is used. Compounds include com-munic acid (R ¼ COOH), communol (R ¼ CH2OH) and succinate ester (R ¼ CH2O-succinyl, from the reaction of the communol hydroxyl group with succinic acid), andmay include biformene (R ¼ CH3).

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polymerisation (DP), resulting in the loss of structure in the ma-terial, which is manifested by brittleness, crazing and inter-connected cracks on the surface [1]. In the long term, chemicalbreakdown of polymer chains results in physical powdering. Theaim of this work was thus to obtain quantitative data on changesin colour, topography and chemistry of surfaces of representativeBaltic amber samples during artificial thermal oxidative ageing inthe absence of light. In this way, surface discolouration and frag-mentation could be correlated to the depolymerisation process,making it possible to identify the oxidation-based breakdown ofpolylabdanoid chains as a potential and significant cause ofphysical deterioration.

To assess changes in colour, evenness and chemical properties atsurfaces of amber samples due to the thermal treatment, analyseswere carried out before and after ageing using non-destructivetechniques, namely Commission Internationale de l’Éclairage(CIE) L*a*b* spectrocolorimetry, confocal profilometry and Fouriertransform (FT) Raman spectroscopy. Spectrocolorimetry and FT-Raman spectroscopy have previously been applied to identify andquantify degradation processes in Baltic amber [9]. Such methodsare non-destructive and non-invasive, as sampling is not requiredand the object under investigation is neither physically norchemically damaged during the process of analysis. Furthermore,analyses can be performed in situ, also thanks to the recentdevelopment of field-portable Raman spectrometers [20], hencemaking these techniques suitable for heritage materials studies.Confocal profilometry is already a valuable imaging method forsynthetic polymer materials in the context of museum exhibitionsand storage spaces [21], and could considerably contribute to the

present practice of structural monitoring of further organic mate-rials, such as natural polymers. Other non-destructive imagingtechniques either cannot provide detailed surface morphology in-formation or are typically invasive and often resource-demanding.For instance, the study of micro-topography features is usuallylimited by the low magnification offered by optical stereo-zoommicroscopy, while portable polynomial texture mapping (PTM)kits require close contact with the object. Moreover, environmentalscanning electron microscopy (ESEM) studies can be performed onsite, but the aid of various auxiliary devices and considerableexperience are necessary. By contrast, confocal profilometry can beused in a non-contact and non-destructive high resolution imaginganalysis mode and in situ without the need of a high level ofexpertise [22].

2. Experimental

2.1. Sample preparation

Amber powder was produced from a large piece of raw Succinitefrom Rav Fehrn ApS (Søborg, Denmark). Pellets were obtained frompressed powder and were used as test material because presenta-tion of amber in that format resulted in excellent repeatability andoptimal signal to noise ratio in FT-Raman spectroscopic analyses, bywhich the samples were inspected in surface chemistry. Materialsand procedures used for the sample preparation are described indetail elsewhere [7]. In total nine, 2 mm-thick amber pellets wereprepared.

The nine samples were divided into three groups: group Aincluded three unaged pellets, group B included three pellets agedfor 35 days and group C included three pellets aged for 70 days. Inparticular, each sample from B and C groups was placed in a glassPetri dish and subjected to accelerated thermal ageing at a tem-perature of 70 � 2 �C, in the absence of light, in a Memmert UL 50oven. Ageing periods of 35 and 70 days were already tested inearlier experiments and resulted in a suitable length of time toproduce chemical changes in amber samples at 70 �C [7,8].

2.2. CIE L*a*b* spectrocolorimetry and FT-Raman spectroscopy

Instrumentation and procedures to quantify levels of degrada-tion of the amber samples by CIE L*a*b* spectrocolorimetry and FT-Raman spectroscopy during the accelerated ageing were based onthe same materials and methods which were previously used bythe authors of this work in another study [9] and can be summar-ised as follows.

Colour was measured with a Minolta CM-2600d portablespectrophotometer, using a standard illuminant D65/10� and adiffuse/8� geometry, and including the specular reflectioncomponent.

The FT-Raman spectra were collected using a Bruker RFS 100spectrometer equipped with Nd:YAG laser (at 1064 nmwith power350mW) and the program Bruker Optik Opus version 5.5. Chemicaldegradation levels were quantified by means of determining andcomparing the intensity of peaks on raw Raman spectra withoutmanipulations or baseline corrections. Concentrations of olefinicbonds were examined quantitatively, based on relative intensityvalues of C]C stretching at 1650e1600 cm�1, determinedbyheight,and calibrated against the unchanging band at 1450 � 20 cm�1

attributed to CeH bending of >CH2 and eCH3 groups.

2.3. Confocal profilometry

A mSurf Explorer confocal profilometer from NanoFocus AG(Oberhausen, Germany) was used to determine changes in

Fig. 2. Colour difference values observed in Baltic amber samples during the differentthermal ageing periods. Key to the symbols: white bar e after 35 days of ageing, filledbar e after 70 days of ageing. Error is negligible.

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topographic features of the amber pellets surfaces due to thethermal ageing. For this study the instrument was equipped with a512 pixels � 512 pixels 10ebit fast digital camera as image acqui-sition module and the program mSoft Control & Analysis (standardversion). Analyses were performed using a high efficiency LED at505 nm as light source, with an 800 XS optical module. The samplesunder investigation were placed directly on the analytical stagebelow the confocal microscope objective and examined withoutcontact with their surfaces or alteration of their condition. Data ofthree random points on the surface of each sample were acquiredwith a spot size at each location of 800 mm � 800 mm, a lateralresolution of 1.57 mm and a depth resolution of 0.1 mm. Surfacetopography values were calculated both on profiles and on areas.The arithmetic mean (Ra) and root mean square (Rq) profileroughness parameters (simply roughness parameters later) [23] arecommonly used for measuring the texture (i.e., combination offeatures referred to as peaks and valleys) along a line, while the

Fig. 3. Surface topography measurements of Baltic amber pellets: unaged (a), aged for 35 dacontrast in the false colour images shown at the bottom of image. Changes in roughness ande Rq, striped bar e Sa, filled bar e Sq. Error bars represent standard errors.

arithmetic mean (Sa) and root mean square (Sq) area roughnessparameters or area height parameters (simply height parameterslater) [24] represent an overall measure of the texture comprising asurface. Each roughness parameter and height parameter wascalculated as average of three values for each sample.

3. Results and discussion

Yellowing, changes in roughness and height parameters, andvariations in unsaturated carbonecarbon bonds concentrationwere measured on surfaces of Baltic amber samples during accel-erated thermal ageing.

3.1. CIE L*a*b* spectrocolorimetry

Data from colour inspection of amber samples were expressedusing the CIE L*a*b* colour system [25] and L*, a* and b* weremeasured for each sample. Several colour difference values,including the CIEDE2000 (D00) index for total colour variation[26,27], were calculated for each ageing period (Fig. 2). Duringthermal ageing of Baltic amber samples, significant surface yel-lowing (Db*) was observed. Differences in L* and a* parameterswere lower (in absolute value) in comparison with Db*. Addition-ally, the D00 values lay in a range between 9 and 14 units, whichsignifies a considerable change of colour, well above the 1.5 unitsselected as the threshold for perceptible changes by other authors[28]. Considering that the major contribution to the total colourdifference was given by an approximately proportional change inthe b* value, a variation corresponding to Db* � 2.5 may be areasonable estimate for the onset of perceptible yellowing occur-ring in Baltic amber (frequently a naturally yellow material) in realconditions.

ys (b) and aged for 70 days (c). The different roughness and height parameters lead toheight parameters are shown at the top. Key to the symbols: empty bar e Ra, dotted bar

Fig. 4. FT-Raman spectra of unaged (a), aged for 35 days (b) and aged for 70 days (c)Baltic amber samples. Two vertical lines are plotted at 1625 cm�1 and 1450 cm�1 tohighlight the infrared bands used to quantify degradation levels. Olefinic bonds con-centrations for each thermal ageing period are showed on the right. Error is beyondmeasurement limits.

Fig. 5. ATR-FTIR spectra of unaged (a), thermally aged (at 70 � 2 �C, in the absence oflight, after 35 days; b) and photoaged (with daylight behind window glass radiation,with an illuminance of 88000 lx at the samples surfaces, at a temperature of 30 � 2 �C,after 17 days; c) Baltic amber samples. Several spectral features due to oxidation, e.g.,the increase in intensity of the band at 1715e1710 cm�1 (attributed to C]O stretchingin carbonyl groups of acids), the negative slope of the shoulder at 1250e1175 cm�1

(attributed to CeO stretching in eCOeOe groups of succinate) and the decrease inintensity of the band at 887 cm�1 (attributed to CeH out-of-plane bending in ]CH2

groups of terminal olefins), are evident in the aged amber spectra, with a greaterextent after photoageing.

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3.2. Confocal profilometry

Non-contact confocal profilometry analyses were performed onthe surfaces of amber pellets, to obtain information about alter-ations in topographic features due to the thermal ageing (Fig. 3). Asubstantial increase in all the standard parameters used to evaluatetopography was measured. This progressive reduction in surfacehomogeneity, attributed to variations in profile roughness (Ra andRq parameters) and area height (Sa and Sq parameters), wasevident in the three-dimensional images rendered by the analyticalsoftware and could clearly be identified as surface fragmentation.After 35 days of ageing the measured levels of roughness andheight parameters showed a uniform increase of approximately0.2e0.5 mm. In samples that were aged for 70 days, a greater in-crease in height parameters was observed. During thermal ageing asimilar degradation pattern is shown by oxidation measurementsat amber surface [8], suggesting a possible correlation betweendevelopment of peaks and valleys expressed as height parameters,and surface oxidation.

3.3. FT-Raman spectroscopy

Chemical degradation levels were determined using FT-Ramanspectroscopy by calculating the ratio between intensity values ofC]C stretching and CeH bending infrared bands, which wereobserved at 1650e1600 cm�1 and 1450 � 20 cm�1 respectively.Spectroscopic data showed a consistent increase in intensity in theband associated with unsaturated carbonecarbon bonds present inthe molecular structure of the amber samples, indicating the pro-gressive formation of new C]C bonds caused by thermal ageing(Fig. 4). The infrared band at 1650e1600 cm�1, which was used toevaluate the concentration of C]C bonds, is assigned to olefinic

bonds in the terminal position [16]. Therefore, it is probable thatthe thermal ageing caused a breakdown process, due to oxidativereactions, of the amber polymeric structure with formation of ter-minal unsaturated carbonecarbon bonds in the position 14e15(Fig. 1). This depolymerisation of the material was manifested byyellowing and increase in unevenness of the sample surfaces.

The observed formation of olefinic bonds contrasts with theresults by Shashoua et al. [19] and Pastorelli et al. [9], where the lossin C]C groups in the terpenoid components of Baltic amber is oneof the effects of thermal ageing at 100 �C and photoageingrespectively after ageing periods shorter than 35 days. This may beexplained by appreciating that different levels and types of energyemployed in accelerated ageing experiments can induce differenteffects on the same material. As an example, Fig. 5 presents a stackplot of attenuated total reflectance-FT infrared (ATR-FTIR) spectraof unaged, thermally aged and photoaged Baltic amber samplesproduced in previous studies [7e9]. Samples which have beenphotoaged with “daylight behind window glass” radiation, with anilluminance of 88000 lx at the samples surfaces, at a temperature of30 � 2 �C, for 17 days show greater degradation, noticeable byseveral spectral features [16], than samples which have been agedthermally at 70 � 2 �C, in the absence of light, for 35 days.

Additionally, it is well known that while ATR-FTIR spectroscopyexamines surfaces at a depth of a few micrometres, depending onthe internal reflectance element used, infrared radiation in Ramanspectroscopy penetrates at a deeper level, which is on a millimetrescale in the case of museum translucent amber [19]. Therefore, thedetection of C]C groups by FT-Raman strongly depends on thedepth to which degradation persists. As illustrated in Fig. 6, ac-cording to the results of both ATR-FTIR and FT-Raman spectros-copies it is reasonable to propose that degradation of amber startsas depolymerisation of the polylabdanoid chains on the surface,with formation of unsaturated carbonecarbon bonds, followed byoxidation, where the C]C bonds are oxidised to acid groups [8]. Asthe surface of amber physically disintegrates, oxygen from air candiffuse increasingly deeper into the material and degradationprogresses. The proposed mechanism is a new important findingfor Baltic amber and it would be of interest to investigate the

Fig. 6. Correlation between the results obtained by spectroscopic analyses on Baltic amber samples and different types of artificial ageing. Using FT-Raman spectroscopy, an increasein concentration of C]C bonds due to depolymerisation is revealed at a deep level below the surface of an amber sample after moderately accelerated ageing, e.g., thermal ageing at70 �C for 35 days. In contrast, a decrease in concentration of C]C bonds due to oxidation is detected at the same depth after highly accelerated ageing, e.g., thermal ageing at 100 �Cfor 30 days or photoageing for 17 days. In both cases, progression of oxidation is confirmed by ATR-FTIR spectroscopy at a shallower depth on a micrometre-scale. Real verticalproportions are modified for better readability of the image.

G. Pastorelli et al. / Polymer Degradation and Stability 98 (2013) 2317e2322 2321

consequences of surface degradation in real conditions, where heatand light act together, in the future. One can speculate whetherother chemical processes involving the formation of C]C bonds,such as isomerisation and aromatisation of cyclic regions in theterpenoid components of amber [29e31], took place. During thisstudy no evidence to suggest different degradation pathways couldbe produced using the described methods. Hence, additional andmore precise investigations into changes induced in the molecularstructure of labdanoid diterpenes during thermal oxidative ageingare necessary. Nuclear magnetic resonance (NMR) spectroscopywould be a suitable analytical technique for further studies [32].

4. Conclusions

The results of this study indicated that depolymerisation is asignificant degradation mechanism in Baltic amber, since thethermally aged samples showed a progressive increase in theconcentration of terminal unsaturated carbonecarbon bonds pre-sent at the surface. A degradation pathway that comprises depo-lymerisation and oxidation of the amber macromolecular structurewas proposed. Yellowing and increase in roughness of the samplesurfaces were shown to be correlated with the depolymerisationprocess. For that reason, discolouration and fragmentation ofamber items in museum collections should be considered warningsigns of ongoing polymer chain breakdown and suggest preventiveconservation actions, such as exclusion of oxygen in the environ-ment where amber objects are stored or displayed. For this pur-pose, the use of anoxic cases should be established as arecommended practice.

It is clear that the application of non-destructive colorimetric,profilometric and spectroscopic techniques has a significant po-tential for monitoring of degradation of Baltic amber during naturalageing. In particular, confocal profilometry could be successfullyused for the characterisation of physical features as a consequenceof chemical degradation, while, similarly tomeasurements on otherpolymer materials, e.g., dental resins [33], FT-Raman spectroscopycould pave the way towards quantitative determination of amberDP.

Acknowledgements

The authors are grateful to:Heike Schmidt (NanoFocus AG, Oberhausen) and Ole Faurskov

Nielsen (Department of Chemistry e University of Copenhagen) forthe technical assistance.

The School of Conservation of the Royal Danish Academy of FineArts, Schools of Architecture, Design and Conservation, togetherwith NanoFocus AG and the Department of Chemistry of the Uni-versity of Copenhagen for having provided all the materials andexperimental equipment used for this research.

The European Union’s Marie Curie Programme, for havingoffered the financial support which made this study possible.

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