Analysis of Resins in Archaeological Contexts

The development of sensitive and specific analytical techniques has improved significantly the opportunities for identifying organic molecules. In particular, 

GC and GC-MS (see Chapter 2), are ideal for the separation and characterization of individual molecular species. Characterization generally relies on the principle of chemotaxonomy, where the presence of a specific compound or distribution of compounds in the ancient sample is matched with its presence in a contemporary authentic substance. The use of such 6molecular markers is not without its problems, since many compounds are widely distributed in a range of materials, and the composition of ancient samples may have been altered significantly during preparation, use and subsequent burial. Other spectroscopic techniques offer valuable complementary information. For example, infrared (IR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy have also been applied. 

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). 

Structure 7.10 Dehydroabietic acid Structure 7.11 7-Oxodehydroabietic acid 

When resin or resinous wood is heated strongly, significant changes in resin composition occur, leading to the formation of tar and pitch. Chemical changes 

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