Raman and ATR-FTIR spectroscopies applied to the conservation of archaeological Baltic amber

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2006; 37: 1221–1227Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/jrs.1586

Raman and ATR-FTIR spectroscopies applied to theconservation of archaeological Baltic amber

Yvonne Shashoua,1∗ Mai-Britt Lund Degn Berthelsen2 and Ole Faurskov Nielsen2

1 Department of Conservation, National Museum of Denmark, Brede, 2800, Kongens Lyngby, Denmark2 Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

Received 2 September 2005; Accepted 4 December 2005

A recent survey suggested that the National Museum of Denmark had more than 17 000 pieces of amber inits collections. It also concluded that 45% of the amber in the collections exhibit deterioration, manifestedby surface crazing, powdering and discolouration. Although the chemical composition of Baltic amberhas been investigated, and it is known to comprise an alkyd polymer containing 8% succinic acids andterpenoid components, degradation pathways have not been defined. It is important that the ambercollections are preserved for future study and enjoyment, and a preventive conservation strategy is underdevelopment. Such an approach involves controlling the environment in which the objects are stored toslow the rate of deterioration. Experience at the Natural History Museum, London, suggested that the rateof degradation was related to relative humidity (RH) of the storage environment. In this study, Raman andattenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopies were used to quantifyand compare the extent of deterioration of amber after thermal ageing at various levels of RH. ATR-FTIRspectra provided information about surfaces, because the electromagnetic radiation penetrated only theupper few microns of the amber, whereas Raman spectra reflected chemical changes at the millimetre level.

Raman spectra showed a loss in C C groups on ageing of the amber samples at all levels of RH, whileATR-FTIR spectra indicated an increase in the concentration of carbonyl for the same samples. Bandscorresponding to unsaturated carbon bonds (1646 cm−1) were strong in Raman spectra and weak in FTIRspectra, while the opposite was true for bands representing carbonyl groups (1730 cm−1). The findingswere attributed to reaction of oxygen with unsaturated carbon–carbon bonds in the terpenoid componentsof amber that formed acids containing carbonyl groups (dehydroabietic and 7-oxodehydroabietic acids).ATR-FTIR spectroscopy suggested that the rate of deterioration was slowed by high RH, although furtherinvestigations are required. Copyright 2006 John Wiley & Sons, Ltd.

KEYWORDS: Baltic amber; degradation; ATR-FTIR spectroscopy; preventive conservation

INTRODUCTION

A survey completed in the year 2000 suggested that theNational Museum of Denmark has more than 17 000 piecesof amber in its collections. The collections are among thelargest in Europe. Most of the amber is the Baltic form, and atleast 50 million years old. It is generally accepted that Balticamber originated from a vast ‘amber forest’ in Scandinavia atthe beginning of the Tertiary period. It was redistributed firstby the glaciation of northern Europe and then by post-glacialrivers. As a result, Baltic amber is now found on the east coastof England, eastern parts of the Netherlands, throughoutsouthern Scandinavia, northern Germany, Poland and in

ŁCorrespondence to: Yvonne Shashoua, Department ofConservation, National Museum of Denmark, Brede, 2800,Kongens Lyngby, Denmark. E-mail: [email protected]

Russia down to the Black Sea.1 After its original use, usuallyas jewellery or other decorative pieces, amber is likely tobe buried for many years in earth or under water in a bog(Fig. 1). As a consequence, most of the archaeological amberin the National Museum originates from one of these twoplaces.

Amber originates as a liquid resin produced by a conifertree, Pinus succinfera. Fossilization of amber (a process inwhich liquid resin is transformed to solid amber) occurs intwo clear stages. Partial polymerization occurs first; cross-linking and organization of terpeniods in the tree resintake place over thousands of years. The process follows afree-radical reaction mechanism in the presence of heat andpressure. Copal is the product of this stage, a hard material,although volatile terpene oils are still present. The oilsevaporate over a period of millions of years to form amber.

Copyright 2006 John Wiley & Sons, Ltd.

1222 Y. Shashoua, M.-B. L. Degn Berthelsen and O. F. Nielsen

Figure 1. Stone Age hunter’s jewellery from the collections ofthe National Museum of Denmark. The object is considered tobe in good condition.

The empirical formula of Baltic amber has been stated both asC10H16O (Ref. 2) and C79H10.5O10.5 (Ref. 3). Sulphur is presentin small quantities (0.26–0.34%) and is used to distinguishBaltic amber from other natural resins. Baltic amber is alsoknown mineralogically as succinite, owing to the presenceof up to 8% succinic acid, HOOCCH2CH2COOH (Ref. 4). Asthe product of esterification of a polyvalent alcohol with adibasic acid, it has been described as a natural alkyd resin,but with some of the properties of an ion-exchange polymerbecause it contains free carboxylic groups.5

The survey also concluded that 45% of amber in the col-lections exhibits deterioration, manifested by surface crazing,powdering and discolouration (Fig. 2). The environment inwhich amber objects were found greatly influenced their con-dition and appearance. Amber found in bogs was in bettercondition than those from dry archaeological sites. Only 4%of amber objects originating in a bog exhibited a high degreeof degradation in comparison with 40% of objects buried indry earth. Amber found in dry earth frequently had a thick,pale coloured surface crust, which cracked and subsequentlycrumbled. Bog-found amber was dark brown in colour, andeither exhibited little or no surface crust.

The Museum’s amber collection is of interest to archae-ologists and curators because amber jewellery reflects theeconomic, social, religious and other cultural beliefs of thepeople who made and wore it. For these reasons, it is

Figure 2. Amber beads from the Stone Age exhibiting typicalsurface crumbling, disintegration and discolouration. Theobjects are from the collections of the National Museum ofDenmark.

important that the collections are preserved for study andenjoyment by future generations. Preventive conservationtechniques involve controlling the environments in whichobjects are placed during storage, with the aim of slowingthose chemical reactions, which cause or accelerate deteri-oration. This may involve limiting the exposure of objectsto sources of energy, oxygen, heat and light. Experience ofthe curators at the Natural History Museum, London, sug-gests that the rate of degradation is related to the level ofrelative humidity (RH) during storage. The main objectiveof this study was to systematically examine the influence ofmoisture on the rate of deterioration of Baltic amber, whichwould help in the development of a conservation strategyfor the National Museum of Denmark’s collections.

Until recently, the applications for infrared and Ramanspectroscopy have focussed on differentiating betweenamber types and between genuine and artificial ambers.6 – 8 Inthe study described here, attenuated total reflection-Fouriertransform infrared (ATR-FTIR) spectroscopy and FT-Raman

Copyright 2006 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2006; 37: 1221–1227DOI: 10.1002/jrs

Studies for the conservation of archaeological Baltic amber 1223

spectroscopy were used to quantify and compare the rates ofdeterioration of experimental samples of Baltic amber agedat various levels of RH.

EXPERIMENTAL

Preparation of amber samplesFreshly mined Baltic amber from Kaliningrad was cut tosize (approximately 10 ð 10 ð 10 mm) and polished on twosides under water-cooling. Samples were subject to thermalageing at 100 °C for 2 months in the absence of light becauseprevious investigations by the authors suggested such aregime to represent natural ageing in a museum store forat least 30 years. Microclimates containing different levelsof moisture were created by adding water (high RH), 2 g ofpre-conditioned self-indicating silica gel (low RH) or nothing(ambient) to Pyrex glass flasks (100 ml) before adding ambersamples and placing the closed containers in a convectionoven at 100 °C (Fig. 3). Tinyview Plus loggers were usedto determine the mean RH in closed containers during theageing period. Values were downloaded via an interfacecable and plotted against time using Gemini Data Logger’ssoftware. Microclimates containing water had a mean RH of94.7%, those containing silica gel had a mean RH of 6.5% andclosed containers had a mean RH of 33.2% at 100 °C.

Examination by FT-Raman and ATR-FTIRspectroscopiesRaman spectra were recorded at the surfaces of ambersamples before and after thermal ageing. To minimize theeffects of fluorescence exhibited by amber exposed to visibleradiation, the spectra were obtained with excitation froma Nd : YAG laser at 1064 nm with intensity 350 mW andspectral resolution 4 cm�1 on a Bruker RFS 100 FT-Ramanspectrometer. Raman spectra of polished faces of sampleswere more intense than those from unpolished faces, owingto lower laser scattering and reflection by the former, so

only spectra obtained from polished faces were studiedquantitatively.

Moreno et al.9 related the Raman intensity of the bandat 1646 cm�1 (assigned to the simple aliphatic unsaturationof the olefin group C C) to the intensity of the band ataround 1450 cm�1 (assigned to the deformation of CH2

bonds) to quantify the process of breakdown of C Cbonds. In that study, the breakdown of C C bonds wasfound to be dependent on the extent of maturation of theamber from the liquid resin stage. An earlier investigationsuggested that these two same bands could be used toprovide information about ageing and degradation of Balticamber.10 While the heights of the bands at 1646 cm�1 changedwith deterioration, the heights of those at 1448 cm�1 wereunaffected. In the present investigation, Raman spectraobtained for amber samples before and after thermal ageingshowed well-resolved bands at around 1646 cm�1 and1448 cm�1 (Fig. 4). Spectra were normalized in relation tothe signal at 1448 cm�1, so that the band acted as an internalstandard, and baselines were corrected so that they wereset to zero at around 1800 cm�1. Three spectra were runon polished faces of the amber samples in each microclimateafter periods of 1 and 2 months’ thermal ageing. The intensityof the bands at 1646 cm�1 was measured to give a ratio(I1646/I1448) for each spectrum and the mean calculated.

ATR-FTIR spectroscopy was also used to compare thelevels of degradation present at surfaces of the samplesbefore and after accelerated thermal ageing. Spectra werecollected over 30 scans at a resolution of 4 cm�1 between4000 cm�1 and 600 cm�1 (the lower limit of sensitivity forATR) using an ASI DurasamplIR single reflection accessorywith an angle of incidence of 45° and fitted with a diamondinternal reflection element in a Perkin–Elmer Spectrum 1000FTIR spectrometer. Initial trials showed that 30 scans were theleast required to obtain well-resolved spectra for the samples.The high refractive index of diamond compared with thatof amber (2.4 and 1.5, respectively) allowed absorbance data

Figure 3. Freshly mined amber thermally aged at 100 °C for 2 months in microclimates containing ambient RH (left), high RH (middle)or low RH (right).



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Figure 4. Raman spectra of amber showing well-resolved bands at 1646 cm�1 (attributed to C C stretching) and 1450 cm�1

(attributed to CH2 scissoring). The relative intensities of these bands were used to compare the deterioration of amber aged atvarious relative humidities. (A) Amber aged at low RH. (B) Amber aged at high RH.

to be collected from a depth approximately equal to that ofthe wavelength of the infrared radiation, a maximum depthof approximately 2 µm.11 Background spectra of the empty,clean accessory open to air were run at hourly intervals.

The quality of spectra was highly dependent on achievingclose contact between the DurasamplIR reflection element(the diamond crystal had an active area of 1 mm diameter)and the surface of interest. Identical pressure distributionwas achieved for all samples using a pressure device (3 mmin diameter) supplied with the accessory in combination withthe torque limiter, which allowed the press to be tightenedto the same, repeatable level. Although the technique isnon-destructive, some samples became brittle on ageingand were damaged while undergoing examination by ATR-FTIR. Analysis of the unpolished areas of the amber samplesresulted in weak and poorly resolved spectra due to makinginadequate contact with the diamond; only highly polishedamber produced spectra of sufficient quality for quantitativemeasurements.

Beer’s Law, which states that spectral absorbance isproportional to the concentrations of two components ina material, was applied to the spectra. Absorbance ofa component may be determined using the area, widthor height of its band. Since most of the bands wereoverlapped and few were symmetrical, maximum heightfrom the baseline was used to quantify absorbance in thisinvestigation. Calculations to quantify degradation of amberrequired two independent absorbance bands.

Because the DurasamplIR accessory was situated externalto the Perkin–Elmer Spectrum 1000 FTIR spectrometer, watervapour in the surrounding air could come into contact

with the samples, rendering the use of the broad bandsdue to OH groups unreliable to use quantitatively. Bandsat 1646 cm�1 (C C) and 1448 cm�1 (CH2) were identifiedin the ATR-FTIR absorbance spectra of non-aged amberat around the same positions as they were in the Ramanspectra. However, the band at 1646 cm�1 was weak andlargely overlapped, thereby hindering independent intensitymeasurements on raw spectra (Fig. 5). In an attempt toinvestigate whether manipulating the spectra would reduceor remove the overlap of bands around 1646 cm�1, spectrawere deconvoluted and normalized with respect to themost intense band in the spectrum, that due to carbonyl at1730 cm�1. In addition, a curve-fitting operation was appliedto the area of overlap (1550–1800 cm�1); however, the bandat 1646 cm�1 was still considered too weak and overlappedto measure accurately (Fig. 6).

Instead, the strong band around 1730 cm�1 attributed tothe carbonyl (C O) stretch was employed. The same bandwas visible but very weak in the Raman spectrum. Heightsof bands at 1730 cm�1 and 1448 cm�1 were determined onraw absorbance spectra without manipulations or baselinecorrections and the ratio (A1730/A1448) used to quantifydegradation. Three spectra were run on polished faces of theamber samples in each microclimate after 1 and 2 months’thermal ageing. The mean ratio (A1730/A1448) was calculated.

RESULTS AND DISCUSSION

High-quality, well-resolved FT-Raman spectra were obtainedfor all amber samples before and after thermal ageing.Measurements of the mean ratio of C C groups to CH2



4000,0 3000 2000 1500 1000 600,0

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1646cm-1

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Figure 5. ATR-FTIR spectra of non-aged amber (upper), amber aged at low RH (middle) and high RH (lower). The band at 1646 cm�1

was too weak and overlapped to be measured reliably. Instead, the strong peak due to carbonyl was ratioed against that due to C–H.

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Figure 6. ATR-FTIR spectra deconvoluted and normalized with respect to the most intense band in the spectrum, that due tocarbonyl at 1730 cm�1. In addition, a curve-fitting operation was applied to the area of overlap (1550–1800 cm�1); however, theband at 1646 cm�1 was still considered too weak and overlapped to measure accurately. (A) Non-aged amber. (B) Amber aged atlow RH. (C) Amber aged at high RH.

groups (I1646/I1448) for amber samples before and after 1 and2 month’s thermal ageing suggested that degradation hadtaken place because of thermal ageing (Fig. 7). A reductionin C C groups was measured after ageing in ambient, lowRH and high RH microclimates, which was the indicatorfor degradation in this study. With the exception of sam-ples aged in low RH, loss of C C groups continued afterthe first month of ageing. However, since (I1646/I1448) ratiosfor aged amber showed little variation with microclimate

(between 0.55 at ambient and 0.58 for low RH) compared toa ratio of 0.61 for the non-aged material, it was not possibleto define clearly a relationship between the rate or extent ofdegradation and the water content of ageing climate.

Because the band at around 1646 cm�1, attributed to C Cvibrations, was weak and largely overlapped, a strong bandat 1730 cm�1 attributed to a carbonyl (C O) stretch was usedinstead to quantify degradation of amber from ATR-FTIRspectra. The height of this band was measured on the raw



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Figure 7. Measurements of mean ratio of C C/CH2 groups from Raman spectra indicating that some degradation had occurred onthermal ageing.

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Figure 8. Measurements of mean ratio of C O/CH2 groups from ATR-FTIR spectra suggesting that the extent of oxidation afterageing in high RH was similar to that of non-aged amber and lower than that experienced by amber aged at low or ambient RH.

spectra and compared with another unaffected by amber’sdegradation reaction at 1448 cm�1 �CH2�. Measurementsof (A1730/A1448) for amber samples before and after 1 and2 month’s thermal ageing, suggested that an increase inC O groups had occurred on ageing (Fig. 8). Thermalageing in ambient and low RH microclimates resulted inthe greatest increase in the concentration of carbonyl groups(2.25), while ageing under high RH resulted in a similar,low concentration to that seen in non-aged amber (1.45)although the concentration began to increase after 2 month’sageing. The results from ATR-FTIR spectroscopy suggestthat water either has no influence on the rate of oxidationof amber or acts to slow the reaction. Under low RHconditions, the rate of degradation is significantly fasterthan at high RH.

Although a detailed degradation mechanism for Balticamber has not been published, it has been proposed thatoxidation is the most likely reaction. Reaction of oxygen inair with the carbon–carbon double bonds in the isopreneunits of the terpenoid components of amber would likelyresult in the formation of dehydroabietic acid (DHA) and7-oxodehydroabietic acid (7-oxoDHA) containing carbonylgroups.12 In this study, findings from both Raman andFTIR spectroscopies indicated that oxidation was the maindegradation process during the thermal ageing of amber inthe absence of light. Raman spectroscopy measured a loss inC C groups on ageing, while FTIR spectroscopy identifiedan increase in the concentration of carbonyl for the samesamples.



Loss of C C groups as determined by Raman spec-troscopy was low in all microclimates, whereas FTIR spec-troscopy showed a comparatively greater increase in theC O concentration. This may be explained by appreciat-ing that oxidation of amber is a surface reaction. Whenfreshly mined, amber has low porosity, and diffusion ofgases through it is slow. As a result, oxygen reacts first withthe surfaces; this is manifested by a thin yellow/brown layersurrounding a paler amber core. ATR-FTIR spectroscopyallows absorbance data to be collected from a maximumdepth of approximately 2 µm, so it would be expected tocome into contact only with degraded amber. However,since it has been previously determined that a Nd : YAGlaser with power 273.5 mW would penetrate amber to adepth of 2–3 mm, it seems likely that Raman spectroscopyhas analysed areas of amber samples that are less degradedthan at the surfaces that have been analysed by ATR-FTIRspectroscopy.13

CONCLUSIONS

Both Raman and ATR-FTIR spectroscopies are effective,reliable and largely non-destructive techniques for thedetection and quantification of the rate of degradationof Baltic amber. Results obtained from the two analyticaltechniques for amber aged in microclimates with variousrelative humidities suggested oxidation to be the majordegradation pathway. In addition, ATR-FTIR spectroscopyindicated that water either has no influence on the rate ofoxidation of amber or acts to slow the reaction.

Reaction of oxygen with the unsaturated carbon–carbonbonds in the isoprene units of terpenoid components ofamber would likely result in the formation of acids containingcarbonyl groups (DHA and 7-oxoDHAs). In this study,Raman spectra suggested a loss in C C groups on ageingof the amber samples aged in all microclimates, while ATR-FTIR spectra indicated an increase in the concentration ofcarbonyl for the same samples. Bands corresponding to theunsaturated carbon bonds (1646 cm�1) were strong in Ramanspectra and weak in FTIR spectra, while the opposite was truefor bands representing the carbonyl groups (1730 cm�1). Asa result, Raman and FTIR spectra were found to complementeach other.

Because Raman spectroscopy examined amber at a depthof 3–4 mm, while ATR-FTIR spectroscopy analysed surfacesdown to maximum of 2–3 µm, the extent of degradationshown by the same location by the two techniques was

different. Oxidation of amber occurs initially only at surfacessince oxygen is slow to diffuse through the dense material.It is likely that the extent of oxidation reduces with distancefrom the surfaces so that Raman spectroscopy detected lessdegradation than FTIR. In further studies, it is proposed touse a Raman confocal microscope to control the distancefrom the surfaces at which spectra are collected. In this wayboth Raman and ATR-FTIR spectra of identical areas couldbe obtained.

The main objective of this study was to examine theinfluence of moisture on the rate of deterioration of Balticamber. Results from ATR-FTIR spectroscopy suggestedthat the rate of deterioration was slow in a high RHenvironment. Further research with longer thermal ageingperiods is necessary to investigate that result. Because FTIRand FT-Raman spectroscopies were non-destructive, sampleswere subsequently available for investigation by additionaltechniques.

AcknowledgementsThe authors are grateful to Annemarie Juul and Jens Glastrup ofthe Department of Conservation, National Museum of Denmark, forhelp with photography and information about the degradation ofBaltic amber.

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Raman and ATR-FTIR spectroscopies applied to the conservation of archaeological Baltic amber