1742cm−1 attributedtotheCQO stretchingispresentontheoils
IR spectraaswellasontheresinsIRspectrabutshiftedto1710–
1720cm−1 [7,16]. Finally,concerningtheresins,abandat
3060cm−1 assigned tothestretchingvibrationofthearomaticor
olefinic CHgroups [6] is presentontheshellacsspectra(Fig. 1d).
This characteristicbandappearsalsoonthediterpenoidresins
spectra(Fig. 1f) at3080cm−1, butisabsentfromthetriterpenoid
resinsones(Fig. 1e). Thisweakfeatureisthusausefulcriterionto
distinguish betweenthedi-,triterpenicresinsandshellacsgroups.
However,onemajorproblemisthatallthesespecific featuresmay
decrease,beshiftedorbroadenedwithageingordegradation
[41,42] orifthesemediaarepartofamixture [32]. On Fig. 1 the
spectrum in(g)isasurfacesamplingofthearcheologicalcopalin
(f). Forexample,in(g),thebandaround1650cm−1 is broaderand
decreases,andtheCHstretchingregion(around2900cm−1) isless
structured thanin(f).Usingsimplespectralfeaturestodifferenti-
ate betweenthemediacouldbethereforecompromisedwhen
some alterationisinvolved.Thisunderlinesthenecessityofa
more detailedspectraexploitation.