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Polymer Degradation and Stability 96 (2011) 1996e2001
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Polymer Degradation and Stability
journal homepage: www.elsevier .com/locate/polydegstab
Photoageing of Baltic amber e Influence of daylight radiation behindwindow glass on surface colour and chemistry
Gianluca Pastorellia,b,*,1,2, Jane Richtera,1, Yvonne Shashouab,2
a School of Conservation, Royal Danish Academy of Fine Arts, 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 1 September 2010Received in revised form23 May 2011Accepted 23 August 2011Available online 30 August 2011
Keywords:Baltic amberPhotoageingPreventive conservationSpectrocolorimetryInfrared spectroscopyRaman spectroscopy
* Present address: The Bartlett School of Graduate SHeritage, University College London, Central House, 1don WC1H 0NN, UK. Tel.: þ44 (0)20 31 08 90 25.
E-mail address: [email protected] (G. Pastorell1 Tel.: þ45 33 74 47 00; fax: þ45 33 74 47 77. E-m2 Tel.: þ45 33 47 35 02; fax: þ45 33 47 33 27. E-m
0141-3910/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2011.08.013
a b s t r a c t
The aim of this study was to provide evidence about the interaction between Baltic amber and daylightbehind window glass, essential to understanding the mechanisms by which the material degrades inmuseum environments and to propose techniques for preventive conservation based on the control ofenvironmental parameters where amber objects are stored or displayed. To investigate the photo-degradation of Baltic amber, the methodology consisted of artificial ageing, in order to initiate degra-dation of model amber samples, and non-destructive analytical techniques, in order to identify andquantify changes in colour and chemical properties. Prism-shaped samples, obtained from a large amberpiece, were exposed to different microclimatic conditions, subjected to accelerated photoageing andanalysed by spectrocolorimetry, infrared spectroscopy and Raman spectroscopy. The experimentsprovided results about surface discolouration, oxidation of the molecular structure and breakdown ofunsaturated carbon-carbon bonds in various environmental conditions, confirming the degrading roleof daylight behind window glass. The conclusions of this study can be applied to the development oftechniques for preventive conservation of museum collections containing amber objects.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Amber is a natural polyterpene, which is derived from the fos-silisation of resins produced by different trees [1]. Because of itssensitivity to physico-chemical environmental factors, amber isextremely prone to progressive degradation and eventually tocomplete disintegration by atmospheric oxidation, accelerated byheat and light [2e4].
The role of light in the degradation process of amber is still thesubject of studies: ultraviolet (UV) radiation (10 nme380 nm) canpromote rapid chemical degradation (photodegradation) [2,3],while so far visible light (380 nme760 nm) is reported not to affectamber [3,5].
The material considered in this work, Baltic amber or Succinite,is one of the most common and investigated kind of amber due tothe abundance of its deposits.
tudies, Centre for Sustainable4 Upper Woburn Place, Lon-
i).ail: [email protected]: [email protected].
All rights reserved.
The aim of this research was to provide evidence about theinteraction between Baltic amber and daylight behind windowglass in different microclimatic conditions, essential for thecomprehension of degradation processes in museum environmentsand for devising methods to slow down the rate of degradation.
Museum collections containing Baltic amber objects are ofinterest to archaeologists and curators, because amber artefacts,such as jewellery, household goods and ceremonial items, reflectthe economic, social, religious and other cultural beliefs of thepeople who made and used them.
Because consolidants and adhesives on amber should be avoi-ded for conservation purposes [2,6], a more effective inhibitiveapproach is needed. Such a strategy, that is based on the control ofenvironmental parameters where amber objects are stored or dis-played, could be developed if the causes and modalities of amberdegradation were better understood.
Artificial photoageing was used to accelerate the degradation ofrepresentative Baltic amber samples in various microclimatesrelevant to use and display conditions. Samples were examined bythe use of non-destructive techniques, namely Commission Inter-nationale de l’Eclairage (CIE) L*a*b* spectrocolorimetry [7], used tomeasure changes in surface colour, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and Fourier
Table 1Microclimates, mimicked conditions and storage methods used for the acceleratedphotoageing of amber samples.
Microclimate Mimicked condition Storage methods
Sample exposed to chamberatmosphere (50% RH)
Object exposedto open air
Using open containers
Sample exposed to internalatmosphere (100% RH)
Object displayedin humid case
13 mL of deionisedwater placed insideclosed containers
Sample exposed to internalatmosphere (�20% RH)
Object displayedin dry case
5 g of silica gel placedinside closed containers
Sample exposed to internalacidic atmosphere(100% RH)
Object displayedin humid casein presence ofacidic pollutants
13 mL of 33% V glacialacetic acida aqueoussolution placed insideclosed containers
Sample exposed to internalalkaline atmosphere(100% RH)
Object displayedin humid casein presence ofalkaline pollutants
13 mL of 5% Vammoniumhydroxideb aqueoussolution placed insideclosed containers
a 100%, analysis grade from Merck.b 28.0e30.0% NH3 from SigmaeAldrich.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001 1997
transform (FT)-Raman spectroscopy, used to identify and quantifydegradation through changes in surface chemical properties.
2. Material and methods
A large piece of raw Baltic amber (approximately 15 cm� 12cm� 6 cm, Fig. 1) from Ravfehrn (Søborg, Denmark) was selectedfor its visible homogeneity, uniform translucency and colour,together with the absence of sedimentary matrix on the externalsurface as well as organic/inorganic inclusions.
Samples were prepared in form of right rectangular prisms(simply prisms later on); specific procedures were used in order tolimit any additional degradation effects due to the formation of freeradicals [8]. A section of the amber lump was smashed intoa number of fragments using awood hammer; the impact had to beintense enough to obtain the required number of fragments in oneattempt.
Prisms (approximately 18 mm� 9 mm� 5 mm, 1 g each) wereobtained from a few amber fragments using a Buehler Isomet lowspeed electrical saw, cooled with a 3% aqueous solution of additivefor cooling fluid without amine compounds from Struers (Ballerup,Denmark).
Each sample was placed inside a wide neck 100 mL Bibby Ster-ilin Pyrex glass flask with polypropylene cap and silicon gasket,baked out at 100 �C for four days before use to remove impurities.
Samples were exposed to five different microclimates (Table 1)in order to investigate the potential effects of relative humidity(RH) and presence of airborne pollutants on the photodegradationof amber. For each microclimatic condition three samples wereexposed. Details about the experimental storage are shown inFig. 2.
Three control samples (blind samples later on) were added inorder to verify the absence of further ageing factors than light. Thethree blind prisms were placed on the bottom of an open Pyrexglass flask, which was wrapped in aluminium foil to block theexposure to light.
Samples were subjected to accelerated photoageing in an AtlasCi3000þ light chamber. The used light radiation was daylightbehind window glass, with a wavelength in the range 325 nm to760 nm, emitted by a xenon lamp at an irradiance of 40 Wm�2
(calculated on the region from 300 nm to 400 nm of the spectrumemitted by the lamp) and filtered through a coated infraredabsorbing (CIRA)/soda lime filters combination applied around thexenon lamp to obtain the absorption of most of UV and infraredradiations (Fig. 3). The experimental set-up provided an illumi-nance of 88,000 lx at the samples surfaces. The ageing environmentwas kept at room temperature, to separate the degradation effectsof heat from light; a temperature of 30� 2 �C was the lowestpossible. Relative humidity was approximately 50%. Samples wereaged for 17 days; ageing periods of 17 days were already testedin pilot experiments and resulted in a suitable length of time to
Fig. 1. Baltic amber lump selected for the production of samples.
result in visual and chemical changes in the amber samples at thedescribed conditions. All the experimental materials employedwere selected for their high physico-chemical stability onphotoageing.
Samples were checked before and after ageing to measurechanges in surface colour and chemistry.
To ensure the analysis of the same surfaces of the prisms everytime, one side of each prism was marked by a permanent pen, inorder to indicate the surface to analyse. Prism surfaces werewashed with deionised water before each analytical measurement.
2.1. CIE L*a*b* spectrocolorimetry
Colour of samples was measured using a Minolta CM-2600dportable spectrophotometer. Data in the CIE L*a*b* colour spacewere acquired using a diffuse illumination/8� observation geom-etry and including the specular reflection component, througha measuring field of 3 mm in diameter. Standardized daylight at6500 K (D65) containing 100% of UV radiation was used as illumi-nant, CIE 10� was used as observer.
Fig. 2. Experimental storage used for photoageing of amber samples exposed todifferent microclimates.
Fig. 3. CIRA/soda lime filters transmission curve for a xenon source.
Fig. 4. Infrared bands observed in ATR-FTIR, which were used to quantify the oxidationlevel of amber samples.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e20011998
Each prism was placed on a 301-A chart with white and blackbackgrounds from Sheen Instruments. Because of the slight trans-parency and heterogeneity of the prisms, values of three randompoints on each sample surface were measured against the blackbackground, then the procedure was repeated on the white back-ground and the final CIE L*a*b* result was calculated as average ofthe six measurements.
Fig. 5. Raman bands observed in FT-Raman, which were used to quantify thedegradation level of amber samples.
2.2. ATR-FTIR spectroscopy
ATR-FTIR spectra were collected using a Perkin Elmer SpectrumOne FTIR spectrometer equipped with a deuterated triglycinesulphate (DTGS) detector and the program Perkin Elmer Spectrumversion 6.2.0. The ATR accessory used was an ASI DurasamplIRsingle reflection tool with an angle of incidence of 45o, fitted witha diamond/ZnSe internal reflection element (active area of 1 mm indiameter) and supplied with a pressure device (3 mm in diameter).
Absorbance spectra were acquired over the range of 4000 cm�1
and 650 cm�1, with 4 scans at a resolution of 4 cm�1; backgroundspectra were run at hourly intervals.
Each prismwas placed on the ATR accessory, subjected to a forceof 70 N and analysed.
The method developed to quantify levels of oxidation of theamber samples by ATR-FTIR during the accelerated ageing wasbased on a similar method previously used in other studies [9,10].The BeereLambert law, which states that spectral absorbance isproportional to the concentration of absorbing species in a mate-rial, was applied to the spectra.
Oxidation of the molecular structure was calculated on the baseof relative absorbance values of the infrared band at 1735e1700cm�1 (assigned to C]O groups of esters and acids). Absorbancevalues at this band were calibrated against a band at 1450� 20cm�1 (attributed to CeH bonds of >CH2 and eCH3 groups) which,during initial tests, did not show significant changes due to theageing process. Absorbance of a chemical groupmay be determinedusing the area, width or height of its band. In this investigation,
since the two bands of interest were not symmetrical, theirmaximum heights were determined on raw absorbance spectra,without manipulations or baseline corrections (Fig. 4).
Because of the slight heterogeneity of the prism surfaces, data ofthree random points were measured on each prism and eachabsorbance value was calculated as average of the threemeasurements.
2.3. FT-Raman spectroscopy
FT-Raman analyses were performed with a Bruker RFS 100spectrometer equipped with Nd:YAG laser at 1064 nm (nearinfrared), liquid nitrogen cooled Ge-diode detector and theprogram Bruker Optik Opus version 5.5. The power of the laser wasset at 350 mW. Spectra were acquired over the range of 3500 cm�1
and 10 cm�1, with 500 scans at a resolution of 4 cm�1.Each prism was placed on the sample holder of the instrument
and analysed on one point of the surface; initial trials showed thatone analyses on each sample was enough to give reliable results.
The method developed to quantify levels of degradation of theamber samples by FT-Raman during the accelerated ageing wasbased on similar methods previously used by other authors [9,11].The breakdown of C]C bonds in the molecular structure wascalculated on the base of relative intensity values of the infraredband at 1650e1600 cm�1 (assigned to C]C groups of olefins).Intensity values at this band were calibrated against the band at1450� 20 cm�1 (attributed to CeH bonds of >CH2 and eCH3
groups) which, during initial tests, did not show relevant changesdue to the ageing process. Maximum heights of the two bands ofinterest were determined on raw Raman spectra without manip-ulations or baseline corrections (Fig. 5).
Fig. 6. CIE L* value changes in photoaged amber samples. Key to the symbols: emptybar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
Fig. 8. CIE b* value changes in photoaged amber samples. Key to the symbols: emptybar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001 1999
3. Results and discussion
The data obtained from the techniques described in section 2were analysed through both critical evaluation and simple statis-tical tests, e.g., t-test for paired data.
3.1. CIE L*a*b* spectrocolorimetry
Data from colour measurements of amber samples wereexpressed using the CIE L*a*b* colour system. Figs. 6e8 presentthree plots of the changes in each colour component versus allageing conditions. In most of the instances results showed dis-colouration after ageing. Lightening and yellowingwere the generaleffects on prisms surfaces colour. Samples aged in alkaline atmo-sphere and blind samples did not show significant variations of theCIE L*a*b* parameters.
The CIEDE2000 (DE00) colour-difference (i.e., index related tothe difference between CIE L*a*b* parameters of an aged sampleand an unaged one [12]) was calculated for each experimentalcondition (Table 2). The DE00 values lay in a range between 5 and
Fig. 7. CIE a* value changes in photoaged amber samples. Key to the symbols: emptybar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
14, which signifies a considerable change of colour [13]. Only thecolour-difference values associated to the blind samples lay out ofthis interval. Values of relative humidity less than or equal to 50%caused the highest changes.
The change in colour may be explained by appreciating thatoxidation of amber is usually manifested by a thin yellowishexternal layer surrounding a paler core [9]. However, it was notclear why low levels of relative humidity enhanced discolouration.
3.2. ATR-FTIR spectroscopy
Oxidation levels were determined using ATR-FTIR spectroscopyby measuring the ratio between absorbance values of C]O(carbonyl group) and CeH infrared bands, which were observed at1735e1700 cm�1 and 1450� 20 cm�1 respectively. In Fig. 9, thechanges in carbonyl group concentration, in all the ageing condi-tions, are illustrated. ATR-FTIR spectroscopy data showed anincrease in intensity in the absorbance band attributed to carbonylgroups (C]O) in almost all the samples, indicating an intensephoto-oxidative activity. The increase in concentration of carbonylgroups is likely related to the formation of carboxylic acids [9].These observations might contrast with the results by Williamset al. [3], where exposure of non-Baltic amber specimens to visiblelight is reported to give no appreciable spectroscopic change,however the authors do not provide full details about the experi-mental method and it is not clear whether the UV radiation istotally excluded or not.
Samples aged in alkaline atmosphere and blind samples showedno significant change.
Table 2DE00 values related to the different experimental conditions.
Microclimate DE00
Sample exposed to chamber atmosphere (50% RH) 13.90Sample exposed to internal atmosphere (100% RH) 8.97Sample exposed to internal atmosphere (�20% RH) 12.62Sample exposed to internal acidic atmosphere (100% RH) 8.36Sample exposed to internal alkaline atmosphere (100% RH) 5.70Blind sample 0.46
Fig. 9. C]O group concentration changes in photoaged amber samples. Key to thesymbols: empty bar e before ageing, dotted bar e after ageing. Error bars representstandard errors.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e20012000
3.3. FT-Raman spectroscopy
Degradation levels were determined using FT-Raman spectros-copy by measuring the ratio between intensity values of C]C andCeH infrared bands, which were observed at 1650e1600 cm�1 and1450� 20 cm�1 respectively. In Fig. 10, the changes in unsaturatedcarbonecarbon bond concentration, in all the ageing conditions,are illustrated. FT-Raman spectroscopy data showed a slightdecrease in intensity in the band attributed to carbonecarbondouble bonds in most of the samples, indicating a breakdown ofCeC bonds caused by light. These observations agree with theresults by different authors [9,11,14], where the loss in C]C groupsis the most common effect of maturation or ageing of amber.Reaction of oxygen with C]C bonds in the terpenoid componentsof amber likely resulted in the formation of carboxylic acids, as itwas detected by ATR-FTIR spectroscopy. On the other hand,samples exposed to highly acidic and alkaline atmospheres, as wellas blind samples, did not show significant changes.
Fig. 10. C]C bond concentration changes in photoaged amber samples. Key to thesymbols: empty bar e before ageing, dotted bar e after ageing. Error bars representstandard errors.
4. Conclusions
The results of this study confirmed the exposure of Baltic amberto daylight behind window glass as a significant degradation factor.Photoageing resulted in considerable degradation, as exposure tolight hastened change of colour, formation of C]O groups andbreakdown of C]C bonds present at the surface of the samples,suggesting oxidation to be the major pathway. Nevertheless,additional and more precise analyses into changes induced by lightduring exposure of amber to extreme values of relative humidityand airborne pollutants concentrations are necessary.
Although it is not clear whether Baltic amber is also sensitive tovisible light or only to the near UV area (325 nme380 nm) includedin the experimental spectral power distribution, the conclusions ofthis study can be applied to the development of techniques forpreventive conservation of museum collections containing Balticamber items. Moderated exposure to light, either natural or artifi-cial, is relevant to amber objects in use before they are collected bymuseums and to objects on display, but not to objects in storage orduring transport which are usually covered and consequently notexposed to any kind of illumination. During exhibitions light mustbe accepted, but its intensity and incidence should be regulated toa compromise between the sensitivity of the material and what isrequired for the proper display of the objects. For example, show-cases should not be placed close to windows, while artificialillumination should not directly point toward the objects. Contin-uous artificial illumination could also be avoided, for instanceapplying timed relays, and the use of camera flashes should beforbidden. The use of filtering films is only partially applicable inthis context, since Baltic amber might be sensitive not only to UVradiation but also to visible light, as it was suggested by thisresearch and could be confirmed by further investigations. Whenshowrooms are closed to the public, showcases should be coveredor else the environment should be kept in the dark, closing windowshutters and turning off artificial illumination.
Acknowledgements
The authors are grateful to: Bent Eshøj (School of Conservation e
Royal Danish Academy of Fine Arts) and Ole Faurskov Nielsen(Department of Chemistry e University of Copenhagen) for thetechnical assistance. The School of Conservation of the Royal DanishAcademy of Fine Arts and the Department of Chemistry of theUniversity of Copenhagen for having provided all the materials andexperimental equipment used for this research. The EuropeanUnion’sMarie Curie Programme, for having offered the financial supportwhich made this study possible.
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