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Photoageing of Baltic amber – Influence of daylight radiation behind window glass on surface colour and chemistry
Polymer Degradation and Stability 96 (2011) 1996e2001
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Photoageing of Baltic amber e Influence of daylight radiation behind window glass on surface colour and chemistry Gianluca Pastorellia, b, *,1, 2, Jane Richtera,1, Yvonne Shashouab, 2 a b
School of Conservation, Royal Danish Academy of Fine Arts, Esplanaden 34, 1263 Copenhagen K, Denmark Research, 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
a b s t r a c t
Article history: Received 1 September 2010 Received in revised form 23 May 2011 Accepted 23 August 2011 Available online 30 August 2011
The aim of this study was to provide evidence about the interaction between Baltic amber and daylight behind window glass, essential to understanding the mechanisms by which the material degrades in museum environments and to propose techniques for preventive conservation based on the control of environmental parameters where amber objects are stored or displayed. To investigate the photodegradation of Baltic amber, the methodology consisted of artificial ageing, in order to initiate degradation of model amber samples, and non-destructive analytical techniques, in order to identify and quantify changes in colour and chemical properties. Prism-shaped samples, obtained from a large amber piece, were exposed to different microclimatic conditions, subjected to accelerated photoageing and analysed by spectrocolorimetry, infrared spectroscopy and Raman spectroscopy. The experiments provided results about surface discolouration, oxidation of the molecular structure and breakdown of unsaturated carbon-carbon bonds in various environmental conditions, confirming the degrading role of daylight behind window glass. The conclusions of this study can be applied to the development of techniques for preventive conservation of museum collections containing amber objects. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Baltic amber Photoageing Preventive conservation Spectrocolorimetry Infrared spectroscopy Raman spectroscopy
1. Introduction Amber is a natural polyterpene, which is derived from the fossilisation of resins produced by different trees [1]. Because of its sensitivity to physico-chemical environmental factors, amber is extremely prone to progressive degradation and eventually to complete disintegration by atmospheric oxidation, accelerated by heat and light [2e4]. The role of light in the degradation process of amber is still the subject of studies: ultraviolet (UV) radiation (10 nme380 nm) can promote rapid chemical degradation (photodegradation) [2,3], while so far visible light (380 nme760 nm) is reported not to affect amber [3,5]. The material considered in this work, Baltic amber or Succinite, is one of the most common and investigated kind of amber due to the abundance of its deposits.
* Present address: The Bartlett School of Graduate Studies, Centre for Sustainable Heritage, University College London, Central House, 14 Upper Woburn Place, London WC1H 0NN, UK. Tel.: þ44 (0)20 31 08 90 25. E-mail address: [email protected] (G. Pastorelli). 1 Tel.: þ45 33 74 47 00; fax: þ45 33 74 47 77. E-mail: [email protected]. 2 Tel.: þ45 33 47 35 02; fax: þ45 33 47 33 27. E-mail: [email protected]. 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.08.013
The aim of this research was to provide evidence about the interaction between Baltic amber and daylight behind window glass in different microclimatic conditions, essential for the comprehension of degradation processes in museum environments and for devising methods to slow down the rate of degradation. Museum collections containing Baltic amber objects are of interest to archaeologists and curators, because amber artefacts, such as jewellery, household goods and ceremonial items, reflect the economic, social, religious and other cultural beliefs of the people who made and used them. Because consolidants and adhesives on amber should be avoided for conservation purposes [2,6], a more effective inhibitive approach is needed. Such a strategy, that is based on the control of environmental parameters where amber objects are stored or displayed, could be developed if the causes and modalities of amber degradation were better understood. Artificial photoageing was used to accelerate the degradation of representative Baltic amber samples in various microclimates relevant to use and display conditions. Samples were examined by the use of non-destructive techniques, namely Commission Internationale de l’Eclairage (CIE) L*a*b* spectrocolorimetry [7], used to measure changes in surface colour, attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectroscopy and Fourier
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
transform (FT)-Raman spectroscopy, used to identify and quantify degradation through changes in surface chemical properties. 2. Material and methods A large piece of raw Baltic amber (approximately 15 cm 12 cm 6 cm, Fig. 1) from Ravfehrn (Søborg, Denmark) was selected for its visible homogeneity, uniform translucency and colour, together with the absence of sedimentary matrix on the external surface 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 to limit any additional degradation effects due to the formation of free radicals [8]. A section of the amber lump was smashed into a number of fragments using a wood hammer; the impact had to be intense enough to obtain the required number of fragments in one attempt. Prisms (approximately 18 mm 9 mm 5 mm, 1 g each) were obtained from a few amber fragments using a Buehler Isomet low speed electrical saw, cooled with a 3% aqueous solution of additive for cooling fluid without amine compounds from Struers (Ballerup, Denmark). Each sample was placed inside a wide neck 100 mL Bibby Sterilin 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 photodegradation of amber. For each microclimatic condition three samples were exposed. Details about the experimental storage are shown in Fig. 2. Three control samples (blind samples later on) were added in order to verify the absence of further ageing factors than light. The three blind prisms were placed on the bottom of an open Pyrex glass flask, which was wrapped in aluminium foil to block the exposure to light. Samples were subjected to accelerated photoageing in an Atlas Ci3000þ light chamber. The used light radiation was daylight behind window glass, with a wavelength in the range 325 nm to 760 nm, emitted by a xenon lamp at an irradiance of 40 W m2 (calculated on the region from 300 nm to 400 nm of the spectrum emitted by the lamp) and filtered through a coated infrared absorbing (CIRA)/soda lime filters combination applied around the xenon lamp to obtain the absorption of most of UV and infrared radiations (Fig. 3). The experimental set-up provided an illuminance of 88,000 lx at the samples surfaces. The ageing environment was kept at room temperature, to separate the degradation effects of heat from light; a temperature of 30 2 C was the lowest possible. Relative humidity was approximately 50%. Samples were aged for 17 days; ageing periods of 17 days were already tested in pilot experiments and resulted in a suitable length of time to
Fig. 1. Baltic amber lump selected for the production of samples.
1997
Table 1 Microclimates, mimicked conditions and storage methods used for the accelerated photoageing of amber samples. Microclimate
Mimicked condition
Storage methods
Sample exposed to chamber atmosphere (50% RH) Sample exposed to internal atmosphere (100% RH)
Object exposed to open air Object displayed in humid case
Using open containers
Sample exposed to internal atmosphere (20% RH) Sample exposed to internal acidic atmosphere (100% RH)
Object displayed in dry case Object displayed in humid case in presence of acidic pollutants Object displayed in humid case in presence of alkaline pollutants
Sample exposed to internal alkaline atmosphere (100% RH)
a b
13 mL of deionised water placed inside closed containers 5 g of silica gel placed inside closed containers 13 mL of 33% V glacial acetic acida aqueous solution placed inside closed containers 13 mL of 5% V ammonium hydroxideb aqueous solution placed inside closed containers
100%, analysis grade from Merck. 28.0e30.0% NH3 from SigmaeAldrich.
result in visual and chemical changes in the amber samples at the described conditions. All the experimental materials employed were selected for their high physico-chemical stability on photoageing. Samples were checked before and after ageing to measure changes in surface colour and chemistry. To ensure the analysis of the same surfaces of the prisms every time, one side of each prism was marked by a permanent pen, in order to indicate the surface to analyse. Prism surfaces were washed with deionised water before each analytical measurement. 2.1. CIE L*a*b* spectrocolorimetry Colour of samples was measured using a Minolta CM-2600d portable spectrophotometer. Data in the CIE L*a*b* colour space were acquired using a diffuse illumination/8 observation geometry and including the specular reflection component, through a measuring field of 3 mm in diameter. Standardized daylight at 6500 K (D65) containing 100% of UV radiation was used as illuminant, CIE 10 was used as observer.
Fig. 2. Experimental storage used for photoageing of amber samples exposed to different microclimates.
1998
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
Fig. 4. Infrared bands observed in ATR-FTIR, which were used to quantify the oxidation level of amber samples.
since the two bands of interest were not symmetrical, their maximum heights were determined on raw absorbance spectra, without manipulations or baseline corrections (Fig. 4). Because of the slight heterogeneity of the prism surfaces, data of three random points were measured on each prism and each absorbance value was calculated as average of the three measurements.
2.3. FT-Raman spectroscopy Fig. 3. CIRA/soda lime filters transmission curve for a xenon source.
Each prism was placed on a 301-A chart with white and black backgrounds from Sheen Instruments. Because of the slight transparency and heterogeneity of the prisms, values of three random points on each sample surface were measured against the black background, then the procedure was repeated on the white background and the final CIE L*a*b* result was calculated as average of the six measurements.
2.2. ATR-FTIR spectroscopy ATR-FTIR spectra were collected using a Perkin Elmer Spectrum One FTIR spectrometer equipped with a deuterated triglycine sulphate (DTGS) detector and the program Perkin Elmer Spectrum version 6.2.0. The ATR accessory used was an ASI DurasamplIR single reflection tool with an angle of incidence of 45o, fitted with a diamond/ZnSe internal reflection element (active area of 1 mm in diameter) and supplied with a pressure device (3 mm in diameter). Absorbance spectra were acquired over the range of 4000 cm1 and 650 cm1, with 4 scans at a resolution of 4 cm1; background spectra were run at hourly intervals. Each prism was placed on the ATR accessory, subjected to a force of 70 N and analysed. The method developed to quantify levels of oxidation of the amber samples by ATR-FTIR during the accelerated ageing was based on a similar method previously used in other studies [9,10]. The BeereLambert law, which states that spectral absorbance is proportional to the concentration of absorbing species in a material, was applied to the spectra. Oxidation of the molecular structure was calculated on the base of relative absorbance values of the infrared band at 1735e1700 cm1 (assigned to C]O groups of esters and acids). Absorbance values at this band were calibrated against a band at 1450 20 cm1 (attributed to CeH bonds of >CH2 and eCH3 groups) which, during initial tests, did not show significant changes due to the ageing process. Absorbance of a chemical group may be determined using the area, width or height of its band. In this investigation,
FT-Raman analyses were performed with a Bruker RFS 100 spectrometer equipped with Nd:YAG laser at 1064 nm (near infrared), liquid nitrogen cooled Ge-diode detector and the program Bruker Optik Opus version 5.5. The power of the laser was set at 350 mW. Spectra were acquired over the range of 3500 cm1 and 10 cm1, with 500 scans at a resolution of 4 cm1. Each prism was placed on the sample holder of the instrument and analysed on one point of the surface; initial trials showed that one analyses on each sample was enough to give reliable results. The method developed to quantify levels of degradation of the amber samples by FT-Raman during the accelerated ageing was based on similar methods previously used by other authors [9,11]. The breakdown of C]C bonds in the molecular structure was calculated on the base of relative intensity values of the infrared band at 1650e1600 cm1 (assigned to C]C groups of olefins). Intensity values at this band were calibrated against the band at 1450 20 cm1 (attributed to CeH bonds of >CH2 and eCH3 groups) which, during initial tests, did not show relevant changes due to the ageing process. Maximum heights of the two bands of interest were determined on raw Raman spectra without manipulations or baseline corrections (Fig. 5).
Fig. 5. Raman bands observed in FT-Raman, which were used to quantify the degradation level of amber samples.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
Fig. 6. CIE L* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
1999
Fig. 8. CIE b* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
3. Results and discussion The data obtained from the techniques described in section 2 were analysed through both critical evaluation and simple statistical tests, e.g., t-test for paired data. 3.1. CIE L*a*b* spectrocolorimetry Data from colour measurements of amber samples were expressed using the CIE L*a*b* colour system. Figs. 6e8 present three plots of the changes in each colour component versus all ageing conditions. In most of the instances results showed discolouration after ageing. Lightening and yellowing were the general effects on prisms surfaces colour. Samples aged in alkaline atmosphere and blind samples did not show significant variations of the CIE L*a*b* parameters. The CIEDE2000 (DE00) colour-difference (i.e., index related to the difference between CIE L*a*b* parameters of an aged sample and an unaged one [12]) was calculated for each experimental condition (Table 2). The DE00 values lay in a range between 5 and
14, which signifies a considerable change of colour [13]. Only the colour-difference values associated to the blind samples lay out of this interval. Values of relative humidity less than or equal to 50% caused the highest changes. The change in colour may be explained by appreciating that oxidation of amber is usually manifested by a thin yellowish external layer surrounding a paler core [9]. However, it was not clear why low levels of relative humidity enhanced discolouration.
3.2. ATR-FTIR spectroscopy Oxidation levels were determined using ATR-FTIR spectroscopy by measuring the ratio between absorbance values of C]O (carbonyl group) and CeH infrared bands, which were observed at 1735e1700 cm1 and 1450 20 cm1 respectively. In Fig. 9, the changes in carbonyl group concentration, in all the ageing conditions, are illustrated. ATR-FTIR spectroscopy data showed an increase in intensity in the absorbance band attributed to carbonyl groups (C]O) in almost all the samples, indicating an intense photo-oxidative activity. The increase in concentration of carbonyl groups is likely related to the formation of carboxylic acids [9]. These observations might contrast with the results by Williams et al. [3], where exposure of non-Baltic amber specimens to visible light is reported to give no appreciable spectroscopic change, however the authors do not provide full details about the experimental method and it is not clear whether the UV radiation is totally excluded or not. Samples aged in alkaline atmosphere and blind samples showed no significant change.
Table 2 DE00 values related to the different experimental conditions.
DE00
Microclimate
Fig. 7. CIE a* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
Sample exposed Sample exposed Sample exposed Sample exposed Sample exposed Blind sample
to to to to to
chamber atmosphere (50% RH) internal atmosphere (100% RH) internal atmosphere (20% RH) internal acidic atmosphere (100% RH) internal alkaline atmosphere (100% RH)
13.90 8.97 12.62 8.36 5.70 0.46
2000
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
4. Conclusions
Fig. 9. C]O group concentration changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
3.3. FT-Raman spectroscopy Degradation levels were determined using FT-Raman spectroscopy by measuring the ratio between intensity values of C]C and CeH infrared bands, which were observed at 1650e1600 cm1 and 1450 20 cm1 respectively. In Fig. 10, the changes in unsaturated carbonecarbon bond concentration, in all the ageing conditions, are illustrated. FT-Raman spectroscopy data showed a slight decrease in intensity in the band attributed to carbonecarbon double bonds in most of the samples, indicating a breakdown of CeC bonds caused by light. These observations agree with the results by different authors [9,11,14], where the loss in C]C groups is the most common effect of maturation or ageing of amber. Reaction of oxygen with C]C bonds in the terpenoid components of amber likely resulted in the formation of carboxylic acids, as it was detected by ATR-FTIR spectroscopy. On the other hand, samples exposed to highly acidic and alkaline atmospheres, as well as blind samples, did not show significant changes.
The results of this study confirmed the exposure of Baltic amber to daylight behind window glass as a significant degradation factor. Photoageing resulted in considerable degradation, as exposure to light hastened change of colour, formation of C]O groups and breakdown 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 light during exposure of amber to extreme values of relative humidity and airborne pollutants concentrations are necessary. Although it is not clear whether Baltic amber is also sensitive to visible light or only to the near UV area (325 nme380 nm) included in the experimental spectral power distribution, the conclusions of this study can be applied to the development of techniques for preventive conservation of museum collections containing Baltic amber items. Moderated exposure to light, either natural or artificial, is relevant to amber objects in use before they are collected by museums and to objects on display, but not to objects in storage or during transport which are usually covered and consequently not exposed to any kind of illumination. During exhibitions light must be accepted, but its intensity and incidence should be regulated to a compromise between the sensitivity of the material and what is required for the proper display of the objects. For example, showcases should not be placed close to windows, while artificial illumination should not directly point toward the objects. Continuous artificial illumination could also be avoided, for instance applying timed relays, and the use of camera flashes should be forbidden. The use of filtering films is only partially applicable in this context, since Baltic amber might be sensitive not only to UV radiation but also to visible light, as it was suggested by this research and could be confirmed by further investigations. When showrooms are closed to the public, showcases should be covered or else the environment should be kept in the dark, closing window shutters 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 the technical assistance. The School of Conservation of the Royal Danish Academy of Fine Arts and the Department of Chemistry of the University of Copenhagen for having provided all the materials and experimental equipment used for this research. The European Union’s Marie Curie Programme, for having offered the financial support which made this study possible. References
Fig. 10. C]C bond concentration changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
[1] Anderson KB, Crelling JC. Amber, resinite, and fossil resins. Washington: American Chemical Society; 1996. [2] Beck CW. Authentication and conservation of amber: conflict of interests. In: Science and technology in the service of conservation, preprints of the contributions to the IIC Washington congress; September 3e9, 1982. p. 104e7. [3] Williams RS, Waddington JB, Fenn J. Infrared spectroscopic analysis of central and south American amber exposed to air pollutants, biocides, light, and moisture. The Society for the Preservation of Natural History Collections collection forum 1990;2(6):65e75. [4] Zivancevic MP, Stojiljkovic D, Brzakovic M. Conservation of amber from the collection of the National Museum in Belgrade. The magic of amber e Archaeological Monographies 2006;18:400e19. [5] Thickett D, Cruickshank P, Ward C. The conservation of amber. Stud Conserv 1995;40:217e26. [6] Clydesdale M. A note on the conservation of deteriorated amber. Quarter News Magazine Scottish Soc Conserv Restor 1999;4(10):11. [7] Johnston-Feller R. Color science in the examination of museum objects: nondestructive procedures. Los Angeles: The Getty Conservation Institute Publications; 2001.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001 [8] Urbanski T. Degradation of amber and formation of free radicals by mechanical action. Proc R Soc London Ser A Math Phys Sci 1971;1562(325):377e81. [9] Shashoua Y, Degn Berthelsen ML, Nielsen OF. Raman and ATR-FTIR spectroscopies applied to the conservation of archaeological Baltic amber. J Raman Spectrosc 2005;37:1221e7. [10] Pastorelli G. A comparative study by infrared spectroscopy and optical oxygen sensing to identify and quantify oxidation of Baltic amber in different ageing conditions. J Cult Herit 2011;12:164e8.
2001
[11] Moreno YM, Christensen DH, Nielsen OF. A NIR-FT-Raman spectroscopic study of amber. Asian J Spectros 2000;4:49e56. [12] Sharma G, Wu W, Dalal EN. The CIEDE2000 color-difference formula: implementation notes, supplementary test data, and mathematical observations. Color Res Appl 2005;30:21e30. [13] Sharma G. Digital color imaging handbook. United States: CRC Press; 2003. [14] Clifford DJ, Hatcher PG. Structural transformations of polylabdanoid resinites during maturation. Org Geochem 1995;23(5):407e18.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Photoageing of Baltic amber e Influence of daylight radiation behind window glass on surface colour and chemistry Gianluca Pastorellia, b, *,1, 2, Jane Richtera,1, Yvonne Shashouab, 2 a b
School of Conservation, Royal Danish Academy of Fine Arts, Esplanaden 34, 1263 Copenhagen K, Denmark Research, 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
a b s t r a c t
Article history: Received 1 September 2010 Received in revised form 23 May 2011 Accepted 23 August 2011 Available online 30 August 2011
The aim of this study was to provide evidence about the interaction between Baltic amber and daylight behind window glass, essential to understanding the mechanisms by which the material degrades in museum environments and to propose techniques for preventive conservation based on the control of environmental parameters where amber objects are stored or displayed. To investigate the photodegradation of Baltic amber, the methodology consisted of artificial ageing, in order to initiate degradation of model amber samples, and non-destructive analytical techniques, in order to identify and quantify changes in colour and chemical properties. Prism-shaped samples, obtained from a large amber piece, were exposed to different microclimatic conditions, subjected to accelerated photoageing and analysed by spectrocolorimetry, infrared spectroscopy and Raman spectroscopy. The experiments provided results about surface discolouration, oxidation of the molecular structure and breakdown of unsaturated carbon-carbon bonds in various environmental conditions, confirming the degrading role of daylight behind window glass. The conclusions of this study can be applied to the development of techniques for preventive conservation of museum collections containing amber objects. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Baltic amber Photoageing Preventive conservation Spectrocolorimetry Infrared spectroscopy Raman spectroscopy
1. Introduction Amber is a natural polyterpene, which is derived from the fossilisation of resins produced by different trees [1]. Because of its sensitivity to physico-chemical environmental factors, amber is extremely prone to progressive degradation and eventually to complete disintegration by atmospheric oxidation, accelerated by heat and light [2e4]. The role of light in the degradation process of amber is still the subject of studies: ultraviolet (UV) radiation (10 nme380 nm) can promote rapid chemical degradation (photodegradation) [2,3], while so far visible light (380 nme760 nm) is reported not to affect amber [3,5]. The material considered in this work, Baltic amber or Succinite, is one of the most common and investigated kind of amber due to the abundance of its deposits.
* Present address: The Bartlett School of Graduate Studies, Centre for Sustainable Heritage, University College London, Central House, 14 Upper Woburn Place, London WC1H 0NN, UK. Tel.: þ44 (0)20 31 08 90 25. E-mail address: [email protected] (G. Pastorelli). 1 Tel.: þ45 33 74 47 00; fax: þ45 33 74 47 77. E-mail: [email protected]. 2 Tel.: þ45 33 47 35 02; fax: þ45 33 47 33 27. E-mail: [email protected]. 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.08.013
The aim of this research was to provide evidence about the interaction between Baltic amber and daylight behind window glass in different microclimatic conditions, essential for the comprehension of degradation processes in museum environments and for devising methods to slow down the rate of degradation. Museum collections containing Baltic amber objects are of interest to archaeologists and curators, because amber artefacts, such as jewellery, household goods and ceremonial items, reflect the economic, social, religious and other cultural beliefs of the people who made and used them. Because consolidants and adhesives on amber should be avoided for conservation purposes [2,6], a more effective inhibitive approach is needed. Such a strategy, that is based on the control of environmental parameters where amber objects are stored or displayed, could be developed if the causes and modalities of amber degradation were better understood. Artificial photoageing was used to accelerate the degradation of representative Baltic amber samples in various microclimates relevant to use and display conditions. Samples were examined by the use of non-destructive techniques, namely Commission Internationale de l’Eclairage (CIE) L*a*b* spectrocolorimetry [7], used to measure changes in surface colour, attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectroscopy and Fourier
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
transform (FT)-Raman spectroscopy, used to identify and quantify degradation through changes in surface chemical properties. 2. Material and methods A large piece of raw Baltic amber (approximately 15 cm 12 cm 6 cm, Fig. 1) from Ravfehrn (Søborg, Denmark) was selected for its visible homogeneity, uniform translucency and colour, together with the absence of sedimentary matrix on the external surface 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 to limit any additional degradation effects due to the formation of free radicals [8]. A section of the amber lump was smashed into a number of fragments using a wood hammer; the impact had to be intense enough to obtain the required number of fragments in one attempt. Prisms (approximately 18 mm 9 mm 5 mm, 1 g each) were obtained from a few amber fragments using a Buehler Isomet low speed electrical saw, cooled with a 3% aqueous solution of additive for cooling fluid without amine compounds from Struers (Ballerup, Denmark). Each sample was placed inside a wide neck 100 mL Bibby Sterilin 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 photodegradation of amber. For each microclimatic condition three samples were exposed. Details about the experimental storage are shown in Fig. 2. Three control samples (blind samples later on) were added in order to verify the absence of further ageing factors than light. The three blind prisms were placed on the bottom of an open Pyrex glass flask, which was wrapped in aluminium foil to block the exposure to light. Samples were subjected to accelerated photoageing in an Atlas Ci3000þ light chamber. The used light radiation was daylight behind window glass, with a wavelength in the range 325 nm to 760 nm, emitted by a xenon lamp at an irradiance of 40 W m2 (calculated on the region from 300 nm to 400 nm of the spectrum emitted by the lamp) and filtered through a coated infrared absorbing (CIRA)/soda lime filters combination applied around the xenon lamp to obtain the absorption of most of UV and infrared radiations (Fig. 3). The experimental set-up provided an illuminance of 88,000 lx at the samples surfaces. The ageing environment was kept at room temperature, to separate the degradation effects of heat from light; a temperature of 30 2 C was the lowest possible. Relative humidity was approximately 50%. Samples were aged for 17 days; ageing periods of 17 days were already tested in pilot experiments and resulted in a suitable length of time to
Fig. 1. Baltic amber lump selected for the production of samples.
1997
Table 1 Microclimates, mimicked conditions and storage methods used for the accelerated photoageing of amber samples. Microclimate
Mimicked condition
Storage methods
Sample exposed to chamber atmosphere (50% RH) Sample exposed to internal atmosphere (100% RH)
Object exposed to open air Object displayed in humid case
Using open containers
Sample exposed to internal atmosphere (20% RH) Sample exposed to internal acidic atmosphere (100% RH)
Object displayed in dry case Object displayed in humid case in presence of acidic pollutants Object displayed in humid case in presence of alkaline pollutants
Sample exposed to internal alkaline atmosphere (100% RH)
a b
13 mL of deionised water placed inside closed containers 5 g of silica gel placed inside closed containers 13 mL of 33% V glacial acetic acida aqueous solution placed inside closed containers 13 mL of 5% V ammonium hydroxideb aqueous solution placed inside closed containers
100%, analysis grade from Merck. 28.0e30.0% NH3 from SigmaeAldrich.
result in visual and chemical changes in the amber samples at the described conditions. All the experimental materials employed were selected for their high physico-chemical stability on photoageing. Samples were checked before and after ageing to measure changes in surface colour and chemistry. To ensure the analysis of the same surfaces of the prisms every time, one side of each prism was marked by a permanent pen, in order to indicate the surface to analyse. Prism surfaces were washed with deionised water before each analytical measurement. 2.1. CIE L*a*b* spectrocolorimetry Colour of samples was measured using a Minolta CM-2600d portable spectrophotometer. Data in the CIE L*a*b* colour space were acquired using a diffuse illumination/8 observation geometry and including the specular reflection component, through a measuring field of 3 mm in diameter. Standardized daylight at 6500 K (D65) containing 100% of UV radiation was used as illuminant, CIE 10 was used as observer.
Fig. 2. Experimental storage used for photoageing of amber samples exposed to different microclimates.
1998
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
Fig. 4. Infrared bands observed in ATR-FTIR, which were used to quantify the oxidation level of amber samples.
since the two bands of interest were not symmetrical, their maximum heights were determined on raw absorbance spectra, without manipulations or baseline corrections (Fig. 4). Because of the slight heterogeneity of the prism surfaces, data of three random points were measured on each prism and each absorbance value was calculated as average of the three measurements.
2.3. FT-Raman spectroscopy Fig. 3. CIRA/soda lime filters transmission curve for a xenon source.
Each prism was placed on a 301-A chart with white and black backgrounds from Sheen Instruments. Because of the slight transparency and heterogeneity of the prisms, values of three random points on each sample surface were measured against the black background, then the procedure was repeated on the white background and the final CIE L*a*b* result was calculated as average of the six measurements.
2.2. ATR-FTIR spectroscopy ATR-FTIR spectra were collected using a Perkin Elmer Spectrum One FTIR spectrometer equipped with a deuterated triglycine sulphate (DTGS) detector and the program Perkin Elmer Spectrum version 6.2.0. The ATR accessory used was an ASI DurasamplIR single reflection tool with an angle of incidence of 45o, fitted with a diamond/ZnSe internal reflection element (active area of 1 mm in diameter) and supplied with a pressure device (3 mm in diameter). Absorbance spectra were acquired over the range of 4000 cm1 and 650 cm1, with 4 scans at a resolution of 4 cm1; background spectra were run at hourly intervals. Each prism was placed on the ATR accessory, subjected to a force of 70 N and analysed. The method developed to quantify levels of oxidation of the amber samples by ATR-FTIR during the accelerated ageing was based on a similar method previously used in other studies [9,10]. The BeereLambert law, which states that spectral absorbance is proportional to the concentration of absorbing species in a material, was applied to the spectra. Oxidation of the molecular structure was calculated on the base of relative absorbance values of the infrared band at 1735e1700 cm1 (assigned to C]O groups of esters and acids). Absorbance values at this band were calibrated against a band at 1450 20 cm1 (attributed to CeH bonds of >CH2 and eCH3 groups) which, during initial tests, did not show significant changes due to the ageing process. Absorbance of a chemical group may be determined using the area, width or height of its band. In this investigation,
FT-Raman analyses were performed with a Bruker RFS 100 spectrometer equipped with Nd:YAG laser at 1064 nm (near infrared), liquid nitrogen cooled Ge-diode detector and the program Bruker Optik Opus version 5.5. The power of the laser was set at 350 mW. Spectra were acquired over the range of 3500 cm1 and 10 cm1, with 500 scans at a resolution of 4 cm1. Each prism was placed on the sample holder of the instrument and analysed on one point of the surface; initial trials showed that one analyses on each sample was enough to give reliable results. The method developed to quantify levels of degradation of the amber samples by FT-Raman during the accelerated ageing was based on similar methods previously used by other authors [9,11]. The breakdown of C]C bonds in the molecular structure was calculated on the base of relative intensity values of the infrared band at 1650e1600 cm1 (assigned to C]C groups of olefins). Intensity values at this band were calibrated against the band at 1450 20 cm1 (attributed to CeH bonds of >CH2 and eCH3 groups) which, during initial tests, did not show relevant changes due to the ageing process. Maximum heights of the two bands of interest were determined on raw Raman spectra without manipulations or baseline corrections (Fig. 5).
Fig. 5. Raman bands observed in FT-Raman, which were used to quantify the degradation level of amber samples.
G. Pastorelli et al. / Polymer Degradation and Stability 96 (2011) 1996e2001
Fig. 6. CIE L* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
1999
Fig. 8. CIE b* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
3. Results and discussion The data obtained from the techniques described in section 2 were analysed through both critical evaluation and simple statistical tests, e.g., t-test for paired data. 3.1. CIE L*a*b* spectrocolorimetry Data from colour measurements of amber samples were expressed using the CIE L*a*b* colour system. Figs. 6e8 present three plots of the changes in each colour component versus all ageing conditions. In most of the instances results showed discolouration after ageing. Lightening and yellowing were the general effects on prisms surfaces colour. Samples aged in alkaline atmosphere and blind samples did not show significant variations of the CIE L*a*b* parameters. The CIEDE2000 (DE00) colour-difference (i.e., index related to the difference between CIE L*a*b* parameters of an aged sample and an unaged one [12]) was calculated for each experimental condition (Table 2). The DE00 values lay in a range between 5 and
14, which signifies a considerable change of colour [13]. Only the colour-difference values associated to the blind samples lay out of this interval. Values of relative humidity less than or equal to 50% caused the highest changes. The change in colour may be explained by appreciating that oxidation of amber is usually manifested by a thin yellowish external layer surrounding a paler core [9]. However, it was not clear why low levels of relative humidity enhanced discolouration.
3.2. ATR-FTIR spectroscopy Oxidation levels were determined using ATR-FTIR spectroscopy by measuring the ratio between absorbance values of C]O (carbonyl group) and CeH infrared bands, which were observed at 1735e1700 cm1 and 1450 20 cm1 respectively. In Fig. 9, the changes in carbonyl group concentration, in all the ageing conditions, are illustrated. ATR-FTIR spectroscopy data showed an increase in intensity in the absorbance band attributed to carbonyl groups (C]O) in almost all the samples, indicating an intense photo-oxidative activity. The increase in concentration of carbonyl groups is likely related to the formation of carboxylic acids [9]. These observations might contrast with the results by Williams et al. [3], where exposure of non-Baltic amber specimens to visible light is reported to give no appreciable spectroscopic change, however the authors do not provide full details about the experimental method and it is not clear whether the UV radiation is totally excluded or not. Samples aged in alkaline atmosphere and blind samples showed no significant change.
Table 2 DE00 values related to the different experimental conditions.
DE00
Microclimate
Fig. 7. CIE a* value changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
Sample exposed Sample exposed Sample exposed Sample exposed Sample exposed Blind sample
to to to to to
chamber atmosphere (50% RH) internal atmosphere (100% RH) internal atmosphere (20% RH) internal acidic atmosphere (100% RH) internal alkaline atmosphere (100% RH)
13.90 8.97 12.62 8.36 5.70 0.46
2000
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4. Conclusions
Fig. 9. C]O group concentration changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
3.3. FT-Raman spectroscopy Degradation levels were determined using FT-Raman spectroscopy by measuring the ratio between intensity values of C]C and CeH infrared bands, which were observed at 1650e1600 cm1 and 1450 20 cm1 respectively. In Fig. 10, the changes in unsaturated carbonecarbon bond concentration, in all the ageing conditions, are illustrated. FT-Raman spectroscopy data showed a slight decrease in intensity in the band attributed to carbonecarbon double bonds in most of the samples, indicating a breakdown of CeC bonds caused by light. These observations agree with the results by different authors [9,11,14], where the loss in C]C groups is the most common effect of maturation or ageing of amber. Reaction of oxygen with C]C bonds in the terpenoid components of amber likely resulted in the formation of carboxylic acids, as it was detected by ATR-FTIR spectroscopy. On the other hand, samples exposed to highly acidic and alkaline atmospheres, as well as blind samples, did not show significant changes.
The results of this study confirmed the exposure of Baltic amber to daylight behind window glass as a significant degradation factor. Photoageing resulted in considerable degradation, as exposure to light hastened change of colour, formation of C]O groups and breakdown 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 light during exposure of amber to extreme values of relative humidity and airborne pollutants concentrations are necessary. Although it is not clear whether Baltic amber is also sensitive to visible light or only to the near UV area (325 nme380 nm) included in the experimental spectral power distribution, the conclusions of this study can be applied to the development of techniques for preventive conservation of museum collections containing Baltic amber items. Moderated exposure to light, either natural or artificial, is relevant to amber objects in use before they are collected by museums and to objects on display, but not to objects in storage or during transport which are usually covered and consequently not exposed to any kind of illumination. During exhibitions light must be accepted, but its intensity and incidence should be regulated to a compromise between the sensitivity of the material and what is required for the proper display of the objects. For example, showcases should not be placed close to windows, while artificial illumination should not directly point toward the objects. Continuous artificial illumination could also be avoided, for instance applying timed relays, and the use of camera flashes should be forbidden. The use of filtering films is only partially applicable in this context, since Baltic amber might be sensitive not only to UV radiation but also to visible light, as it was suggested by this research and could be confirmed by further investigations. When showrooms are closed to the public, showcases should be covered or else the environment should be kept in the dark, closing window shutters 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 the technical assistance. The School of Conservation of the Royal Danish Academy of Fine Arts and the Department of Chemistry of the University of Copenhagen for having provided all the materials and experimental equipment used for this research. The European Union’s Marie Curie Programme, for having offered the financial support which made this study possible. References
Fig. 10. C]C bond concentration changes in photoaged amber samples. Key to the symbols: empty bar e before ageing, dotted bar e after ageing. Error bars represent standard errors.
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