Characterization and Radiocarbon Dating of Archaeological Resins

Chapter 7

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Characterization and Radiocarbon Dating of Archaeological Resins from Southeast Asia 1

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C . D . Lampert , I. C. Glover , C. P. Heron , B. Stern , R. Shoocongdej , and G . B. Thompson 1

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Department of Archaeological Sciences, University of Bradford, Bradford BD7 1DP, United Kingdom Institute of Archaeology, University College London, London WC1H 0PY, United Kingdom Department of Archaeology, Silpakorn University, Bangkok-10200, Thailand

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Archaeologists have identified evidence for the use of resins in association with some Southeast Asian ceramics. Resins are largely mixtures of terpenoids with similar molecular structure and numerous isomers. Well-established methods to investigate solvent soluble components of modern resins include gas chromatography and GC-mass spectrometry, sometimes combined with pyrolytic techniques to explore the high M W or polymeric insoluble fraction. This approach is equally applicable to the characterisation of archaeological resins. Although resins are a widespread natural resource in Southeast Asia, it is not clear which of the many resinous species were exploited in the past. To this end, a study of archaeological resins from Southeast Asia is underway with objectives of identification, examination of use, comparisons across geographical distance and time, and evaluating their merit for radiocarbon dating. Samples have been analysed using G C and G C - M S and preliminary results suggest that Dipterocarp resins were used as adhesives or sealants.

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© 2002 American Chemical Society

In Archaeological Chemistry; Jakes, Kathryn A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Background Natural resins are found throughout the world (Figure 1) and their many valuable physical and chemical properties have led to their exploitation for a broad variety of purposes, both in the past and the present. Amongst other things they are flammable, insoluble in water, adhesive, aromatic and can act as biocides. Traditionally, resins have been used as adhesives and varnishes, as components of pigment media, as sealants and waterproofing agents, as incense or in perfumery, as components of torches or firelighters and for a diversity of medicinal purposes. Gianno ( i ) provides a detailed evaluation of applications resins have found worldwide.

Figure 1. Some of the many resins found worldwide.

Evidence for the use of resins in association with ceramics has been identified at a number of archaeological sites within Southeast Asia. Resins may be found in close association with an inorganic substrate such as a ceramic vessel due to their deliberate application, for example in the form of a coating, varnish or adhesive.


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Alternatively, they may be deposited during use, possibly as one component of a combination of substances mixed or contained in a pot or from the resins themselves having been stored or traded in a vessel. Deliberate application, particularly where a resin is used as a coating, may be intended to alter the properties of a vessel, for example to modify its permeability (2) or heating effectiveness (5), although a resin coat might also be applied for purely aesthetic reasons. One aspect of this research can be summarised as the identification of resinous resources used at specific sites in prehistory, an examination of how they were used and a comparison of the use of resins across geographical distance and time. Studying resins found in association with archaeological ceramics has the potential to further our understanding of the acquisition and use of natural organic resources by prehistoric populations and may shed light on the potential movement of resins as a traded resource. It may add to our awareness of technologies exploited in prehistory or broaden our understanding of vessel function. The other major objective is to explore the potential of resins for radiocarbon dating. Initially, we must consider whether it is possible to obtain valid, reliable and repeatable radiocarbon dates from resins. Once it can be established whether the radiocarbon dates we obtain are dependable, the question of whether they can be used to directly date the ceramics with which they are associated can be addressed.

Resins in Southeast Asia The flora of Southeast Asia, a geographical region stretching across some 4000 miles from the borders of Myanmar to Papua New Guinea, is diverse and home to many species of plants. Many produce exudates, including resins, often in response to injury. A large number of plants in Southeast Asia could have been used as a source of resin by ancient peoples. O f these, plants from the Dipterocarpaceae and Pinaceae families are widespread and perhaps the most plausible sources for resins used in association with ceramics, although plants from the Sapotaceae, Burseraceae and other families should not be ruled out. A smaller number of resins produced by insects are also found in the region but this paper concentrates on plant resins. As might be expected over such a broad area, significant floristic variations exist. The main differences are between the flora present on the bulk of mainland Southeast Asia and that of the Malay Peninsula and the islands to the


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east. Altitude and climate, in particular levels of rainfall in a given area, largely dictate the local flora. Each region's flora is distinct, without a gradual merging between adjacent regions (4), and each of the forest types can offer numerous resinous species. Mainland Southeast Asia is seasonal and is comparatively dry. Deciduous dipterocarp forests are widespread (5), with conifer forests at higher altitudes (6). In comparison, areas to the south and east, collectively known as Malesia, are wetter with evergreen tropical rainforests, again rich in dipterocarp species. However, there is a drier, more seasonal band with a high proportion of conifers covering the central zone of the Philippines, Sulawesi, the Moluccas and into Java before tropical rainforest takes over again in Irian Jaya and the east (4).

Existing evidence for the use of resins in Southeast Asia Ethnographic and archaeological evidence pointing to the association of resins and ceramics in Southeast Asia exists from both the mainland and insular regions. Resin-coated potsherds were excavated during the 1960s at Spirit Cave, in Northwestern Thailand, a Hoabinhian rock-shelter site and potsherds and ceramic spindle whorls bearing traces of a coating of a resinous substance were more recently recovered from burials at the Iron Age site of Noen U-Loke on the Khorat Plateau in Northeastern Thailand. A coating, thought to be a resin, has been observed on the interior of ceramic vessels from the Plain of Jars in Laos and a resinous deposit, possibly used as an adhesive, was discovered around the runs of lidded funerary urns at Hau X a , a late Sa Huynh coastal site in Central Vietnam. The use of resins on the mainland is not restricted to prehistory. Resins, thought to have been used as binders in brickwork and mortars, were used in the construction of some Cham temples in Vietnam. In addition, evidence exists for trade in certain resins. Ceramic transport vessels were found on 15 -17 century shipwrecks off the coast of Thailand with their cargo of resin still intact and, on these same ships, resins were also used to seal the lids onto large storage jars (7). th

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Examples of resin-coated ceramics from archaeological contexts in insular Southeast Asia include potsherds from a number of sites in Malaysia and Sumatra (8). Ethnographic examples of an organic post-firing treatment come from Moluku, Sulawesi and from Northwestern Luzon, where pots hot from firing are coated with a pine resin (Agathis philippinensis) and it has been shown that the location of the resin on the pot is dependent on the proposed use of the vessel (9). Similarly coated pots are manufactured in the Central


88 Moluccas (10) where Agathis alba resin is used and some potters in Papua New Guinea use a resin coat to seal water pots (//).


Resin chemistry The chemistry of resins is complex but they are related to plant essential oils and are largely terpenoid based. Terpenoids, sometimes also referred to as terpenes, are "usually cyclic, unsaturated hydrocarbons, with varying degrees of oxygenation in the substituent groups (alcohol, aldehyde, lactone, etc.) attached to the basic skeleton" (12). The cyclic carbon 'backbone' or 'skeleton' in terpenoids is of importance to studies of archaeological resins as, although functional groups may alter or be lost over time, the carbon ring-structure is generally preserved, and can be used as a biomarker to relate a degraded substance in archaeological material back to its equivalent compound in an authentic resin. The terpenoids can be defined as "a group of natural products whose structure may be divided into isoprene units" (75). Isoprene, or isopentenyl, is a C (five carbon atom) compound, which can be viewed as the terpenoid building block (Figure 2). Both the lower terpenoids (monoterpenoids and sesquiterpenoids) and the higher terpenoids (diterpenoids and triterpenoids) are built up from multiples of these isoprene units. When freshly exuded, resins tend to be liquid or semi-liquid, as they contain either one or the other of the two higher terpenoids in solution in a mixture of the more volatile lower terpenoids. The terms 'terpene' and 'terpenoid' appear in many publications as largely interchangeable, although 'terpene' is sometimes used to refer only to hydrocarbons based on the C isoprene unit whilst 'terpenoid' may encompass compounds which are derived from a number of C isoprene units but have a varying number of carbon atoms, for example the C steroids (14). 5

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Biosynthesis of resins In practice, isoprene, although a convenient model to help understand the structural chemistry of terpenoids, is not the precursor to the terpenoid family in nature. In fact, the initial biosynthetic building block for the terpenoid family is mevalonic acid (15). From this compound, via a long series of intermediate steps, are formed an extensive range of terpenoid compounds (16), including those found in resins. Many plants have biosynthetic pathways that can produce both mono- and sesquiterpenoids and these are often found together in plant tissues. However,



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Characterization and Radiocarbon Dating of Archaeological Resins

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