1742cm−1 attributed to the CQO stretching is present on the oils
IR spectra as well as on the resins IR spectra but shifted to1710–
1720 cm−1 [7,16]. Finally, concerning the resins, a band at
3060 cm−1 assigned to the stretching vibration of the aromatic or
olefinic CH groups [6] is present on the shellacs spectra (Fig. 1d).
This characteristic band appears also on the diterpenoid resins
spectra (Fig. 1f) at 3080cm−1, but is absent from the triterpenoid
resins ones (Fig. 1e). This weak feature is thus a useful criterion to
distinguish between the di-, triterpenic resins and shellacs groups.
However, one major problem is that all these specific features may
decrease, be shifted or broadened with ageing or degradation
[41,42] or if these media are part of a mixture [32]. On Fig. 1 the
spectrum in(g) is a surface sampling of the archeological copal in
(f). For example, in (g), the band around 1650 cm−1 is broader and
decreases, and the CH stretching region (around 2900 cm−1) is less
structured than in (f). Using simple spectral features to differenti-
ate between the media could be therefore compromised when
some alteration is involved. This underlines the necessity of a
more detailed spectra exploitation.