Evidence concerning oxidation as a surface reaction in Baltic amber

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  • Spectrochimica Acta Part A 89 (2012) 268 269

    Contents lists available at SciVerse ScienceDirect

    Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

    j our na l ho me p age: www.elsev ier .co

    Evidence concerning oxidation as a surface react

    Gianluca Pastorelli a,b,, Jane Richtera,1, Yvonne Shashouab,2

    a School of Con markb Research, Ana Denm

    a r t i c l

    Article history:Received 26 JuReceived in reAccepted 2 Jan

    Keywords:Baltic amberOxidationArticial ageinCross-sectionMicro-ATR-FT

    videnf the

    collecd to apy, inng oxmal a

    1. Introdu

    The aim as a surface

    The role of oxygen in the degradation process of Baltic amberis still the subject of studies. A deep understanding of theamberoxygen interaction modalities is essential for the compre-hension of degradation processes and for the development of bothactive, i.e., based on the application of polymeric coatings, and pre-ventive, i.econservatio

    One of tamber is oxpenes, i.e., where unsgroups [3,4the low poof gases thprovide evi

    AttenuaFTIR) spect

    CorresponSustainable, HPlace, London

    E-mail add(G. Pastorelli),

    1 Tel.: +45 32 Tel.: +45 3

    y andperthe prr an

    repeatability and the optimal signal to noise ratio on ATR-FTIR spec-troscopic analyses. Successively, micro-ATR-FTIR spectroscopy wasapplied on cross-sections produced from both unaged and ther-mally aged amber pellets, in order to obtain information aboutthe development of oxidation in relation to the distance from the

    1386-1425/$ doi:10.1016/j.., based on the control of environmental parameters,n techniques for museum collections of amber objects.he major chemical degradation mechanisms in Balticidation of diterpenoid components (labdanoid diter-

    communic acid, communol and succinate ester [1,2]),aturated carboncarbon bonds are oxidized to acid]. Oxidation is thought to be a surface reaction due torosity of the material and the resulting slow diffusionrough it [5], but new experiments were necessary todence.ted total reectance-Fourier transform infrared (ATR-roscopy has previously been used on Baltic amber to

    ding author at: The Bartlett School of Graduate Studies Centre foreritage, University College London, Central House, 14 Upper WoburnWC1H 0NN, UK. Tel.: +44 0 20 31 08 90 25.resses: g.pastorelli@ucl.ac.uk, gianluca.pastorelli@gmail.com

    jr@kons.dk (J. Richter), yvonne.shashoua@natmus.dk (Y. Shashoua).3 74 47 00; fax: +45 33 74 47 77.3 47 35 02; fax: +45 33 47 33 27.

    external surface.

    2. Materials and methods

    A large piece of raw Baltic amber (approximately 15 cm 12 cm 6 cm) from Rav Fehrn ApS (Sborg, 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.

    In order to prepare the pellets, specic procedures were usedto limit any additional degradation effects due to the formation offree radicals [8] and to minimise the presence of pores, which mightinuence the diffusion of gases through the material. The authors ofthe present report describe these procedures in details in a previouswork [6]. In total nine 2 mm-thick amber pellets were prepared.

    Three pellets (group A) were kept unaged, each one of theremaining six (three in the group B and three in the group C) wasplaced in a glass Petri dish and subjected to accelerated thermalageing at a temperature of 70 2 C, in the absence of light, in aMemmert UL 50 oven, for 35 days (group B) or 70 days (group C).

    During the analysis, each pellet was cut along the diameter intwo equal parts, using a Buehler Isomet low speed electrical saw,

    see front matter 2012 Elsevier B.V. All rights reserved.saa.2012.01.003servation, Royal Danish Academy of Fine Arts, Esplanaden 34, 1263 Copenhagen K, Denlysis and Consultancy Section of the Department of Conservation, National Museum of

    e i n f o

    ne 2011vised form 24 October 2011uary 2012

    g

    IR

    a b s t r a c t

    The aim of this study was to provide eBaltic amber. A clear understanding oconservation techniques for museumpressed amber powder, were subjecteanalyzed by infrared micro-spectroscoThe experimental results showed strosamples subjected to long-term therthe surface.

    ction

    of this research was to provide evidence about oxidation phenomenon during degradation of Baltic amber.

    identifcal pro

    In tpowdem/locate /saa

    ion in Baltic amber

    ark, IC Modewegsvej, Brede, 2800, Kongens Lyngby, Denmark

    ce about oxidation as a surface reaction during degradation of amberoxygen interaction modalities is essential to developtions of amber objects. Pellet-shaped samples, obtained fromccelerated thermal ageing. Cross-sections of the pellets were

    order to identify and quantify changes in chemical properties.idation exclusively at the exterior part of cross-sections fromgeing, conrming that oxidation of Baltic amber starts from

    2012 Elsevier B.V. All rights reserved.

    quantify oxidation through changes in surface chemi-ies [57], but never to investigate bulk properties.esent study, pellets were produced from pressed amberd were used as test material because of the excellent

  • G. Pastorelli et al. / Spectrochimica Acta Part A 89 (2012) 268 269 269

    Fig. 1. ATR-FTThe infrared bsections are inthe decrease inin the aged am

    cooled without amine ccross-sectiolytical meas

    The ATRAutoImage reection eluride (MCTElmer Spec

    Absorbaand 650 cma measurinrun at hour

    Each croATR crystacross-sectiothe externabsorbancemeasureme

    The metamber sambased on thwork in oth

    3. Theory

    Micro-Aperformed ioxidation aments werestatistical te

    Fig. 2 prcentration v

    Shashoubecause of twork conrby micro-A

    The deteence betwewere aged frior part woccurs initithere was an

    O gellets for 35

    ratedt at t

    sequic amques

    amch wuriner obro-Ate an

    to cleae th

    grada

    wled

    authgaar

    Arts Scho

    thanents used for this research.

    European Unions Marie Curie Programme is thanked for theIR spectra of unaged (top) and thermally aged (bottom) Baltic amber.ands used to quantify oxidation levels of the amber pellets cross-dicated. The negative slope of the shoulder at 12501175 cm1 and

    intensity of the band at 888 cm1 due to oxidation are also evidentber spectrum.

    a water solution (3%) of additive for cooling uid with-ompounds from Struers A/S (Ballerup, Denmark). Pelletns were washed with deionized water before each ana-urement in order to remove any additive residues.-FTIR spectra were collected using a Perkin ElmerFTIR microscope equipped with an ATR diamond/ZnSelement, a liquid nitrogen cooled mercury cadmium tel-) photoconductive detector and the programme Perkintrum version 6.2.0 + AutoImage.nce spectra were acquired over the range of 4000 cm11, with 50 scans at a resolution of 8 cm1 and throughg eld of 100 m 100 m; background spectra werely intervals.ss-section was placed under the microscope objective-l and analyzed. Spectra related to 15 points on then (three random points at ve different levels betweenal surface and the core) were collected and the

    value of each level was calculated as average of threents.hod developed to quantify levels of oxidation of theples by ATR-FTIR during the accelerated ageing wase same method previously used by the authors of thiser studies [6,7] (Fig. 1).

    TR-FTIR analysis of amber pellets cross-sections wasn order to obtain information about the development of

    Fig. 2. Camber pB (aged errors.

    unsatupresenacids.

    Conof Balttechniings onapproacases dof amb

    Micto locastudiesobtainenhancthe de

    Ackno

    TheRoed of Fine

    TheArts isequipm

    The

    long the section. The data obtained from the measure-

    analyzed through both critical evaluation and simplests, e.g., t-test for paired data.

    esents a plot of the changes in the carbonyl group con-ersus distance from the surface.a et al. propose oxidation of amber as a surface reactionhe low permeability to oxygen of the material [5]. Thismed that oxygen reacts rst with the surface, as showedTR-FTIR spectroscopy results.cted levels of oxidation showed no signicant differ-en unaged and 35 days-aged samples. In samples thator 70 days, the strong increase of oxidation at the exte-as clearly visible, suggesting that oxidation of amberally only at the surface. Although it is not clear why

    induction period for oxidation, reaction of oxygen with

    nancial su

    References

    [1] J.S. Mills, R[2] K.B. Ander

    Resins, Am[3] C.W. Beck,

    96109.[4] C.W. Beck,

    the Contriington, 19

    [5] Y. Shashou1221122

    [6] G. Pastorel[7] G. Pastorel[8] T. Urbansk[9] M. Clydesdroup concentration changes at different levels in unaged and agedcross-sections. Key to the symbols: group A (unaged), group

    days), group C (aged for 70 days). Error bars represent standard

    carboncarbon bonds in the terpenoid componentshe surface likely resulted in the formation of carboxylic

    ently, the ideal conditions to obtain a good preservationber objects could include both active and preventive

    to limit surface oxidation. Because polymeric coat-ber should be avoided [4,9], a more effective inhibitiveould be appreciated, for instance using oxygen free

    g storage, transport and display of museum collectionsjects.TR-FTIR spectroscopy proved itself as a valuable methodd quantify oxidation of Baltic amber, allowing furtherbetter characterise the observed induction period, torer details on the oxidation prole and, in general, toe understanding of chemical mechanisms involved intion of museum amber objects.

    gements

    ors are grateful to Kim Pilkjr Simonsen and Metted (School of Conservation of the Royal Danish Academy) for technical assistance.ol of Conservation of the Royal Danish Academy of Fineked for providing all the materials and experimentalpport which made this study possible.

    . White, L. Gough, Chem. Geol. 47 (1984) 1439.son, in: K.B. Anderson, J.C. Crelling (Eds.), Amber, Resinite, and Fossilerican Chemical Society, Washington, 1996, pp. 105129.

    E. Wilbur, S. Meret, D. Kossove, K. Kermani, Archaeometry 8 (1965)

    Science and Technology in the Service of Conservation, Preprints ofbutions to the IIC Washington Congress, 39 September 1982, Wash-82, pp. 104107.a, M.L. Degn Berthelsen, O.F. Nielsen, J. Raman Spectrosc. 37 (2005)

    7.li, J. Cult. Herit. 12 (2010) 164168.li, J. Richter, Y. Shashoua, Polym. Degrad. Stabil. 96 (2011) 19962001.i, Proc. R. Soc. Lond. A 325 (1971) 377381.ale, Quart. News Mag. Scott. Soc. Conserv. Restor. 10 (1999) 11.

    Evidence concerning oxidation as a surface reaction in Baltic amber1 Introduction2 Materials and methods3 TheoryAcknowledgementsReferences