Terpenoid resins ageing

M. Regert, T. Deviese, A. S. Le Ho and A. Rougeulle, Reconstructing ancient Yemeni com mercial routes during the Middle Ages using structural characterization of terpenoid resins, Archaeometry, 50, 668 695 (2008). 

D. Scalarone, J. van der Horst, J. J. Boon and O. Chiantore, Direct temperature mass spectro metric detection of volatile terpenoids and natural terpenoid polymers in fresh and artificially aged resins, J. Mass Spectrom., 38, 607 617 (2003). 

Terpenoids are susceptible to a number of alterations mediated by oxidation and reduction reactions. For example, the most abundant molecule in aged Pinus samples is dehydroabietic acid [Structure 7.10], a monoaromatic diterpenoid based on the abietane skeleton which occurs in fresh (bleed) resins only as a minor component. This molecule forms during the oxidative dehydrogenation of abietic acid, which predominates in rosins. Further atmospheric oxidation (autoxidation) leads to 7-oxodehydroabietic acid [Structure 7.11]. This molecule has been identified in many aged coniferous resins such as those used to line transport vessels in the Roman period (Heron and Pollard, 1988 Beck et al., 1989), in thinly spread resins used in paint media (Mills and White, 1994 172-174) and as a component of resin recovered from Egyptian mummy wrappings (Proefke and Rinehart, 1992). 

The botanical species exploited in the past for the collection of resins vary according to the geographical area, and thus, the availability of adequate reference materials of a known botanical origin is crucial for resin identification. Pine resin and resin-derived pitch, mastic, frankincense, myrrh, benzoe, and birch bark tar are the most common terpenoid materials in archaeological artifacts from the Mediterranean area. Terpenic resin composition depends on aging and exposure to air and light, so the biomarkers used for their identification are... 

Once in the burial environment, biodeterioration may promote further alterations in the structure of the higher terpenoids. Whilst the particular chemical character of the burial environment will dictate specific changes, once again, alterations likely to be encountered will include oxidation, hydrogenation, etc. Looking then at the composition of freshly exuded and aged resins, whilst the more stable terpenoids may be present in both, the older material is likely to exhibit a higher proportion of oxidised terpenoids. 

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