Aged resin

Mono- and sesquiterpenoids are of limited use for the identification and classification of aged resins. Due to their volatility, they are rarely found in ancient samples except when they have been conserved in very particular conditions [88,98], On the other hand, the di-and triterpenoids enable us to identify resins thereby identifying their botanical origin [2,99]. Figures 1.1 and 1.2 show the main diterpenoid and triterpenoid structures. 

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). 

Numerous studies carried out in the last decade on fresh, artificially and naturally aged resins and varnishes have demonstrated that by means of THM-GC/MS a number of di- and triterperpenoids can by identified and, among them, the marker compounds that can be unequivocally used for resin recognition in real pictorial samples. 

A comprehensive survey of the chemistry of natural resins relevant to art historical and archaeological contexts is given in Mills and White (1994 95-128) and is not repeated here. Rather, those aspects of structure and chemistry relating to molecular transformation and to the composition of aged resins encountered in archaeological contexts will be emphasized. 

Table I also compares the heat stabilities of 60- and 120-mil plaques at 140 °C. The relative order of stabilities has changed slightly. The stability of Resin A in the thicker sections has equalled or surpassed the stability of the long term heat aging Resin B. Resins E, C, and H have improved their position, displacing I, the most stable resin in the 25-mil sheets, which is now third best in the 60-mil plaques and only fourth best in the 120-mil plaques.

Chemical Analysis One of the primary objectives of this program was to carefully monitor microchemical changes that occur in the resin system during the aging processes therefore sensitive methods were required to characterize the starting materials, the fresh and the aged resin. 

Table V. Average activation energies for the production of chemiluminescence from aged resin.

Vaporization Gas Chromatography and Mass Spectrometry of Aged Resin The... 

A series of Vap GC analyses on approximately 50 mg of all the aged resin samples was conducted. Ethanal, propanal, isobutyraldehyde, propenal, butenal, methyl ethyl ketone (MEK), and methyl penetenal were positively identified by mass spectrometry. With the exception of MEK, all compounds identified (the major products) were aldehydes. Methyl ethyl ketone, the solvent used in MY720, remains even after cure. The aldehydes, however, are not impurities in either the MY720 or DOS and represent compounds which are characteristic of the resin alone. These compounds are either produced during the curing process or are formed from the thermal decompositon of a labile compound which is formed during cure. 

Table VI. Activation energies for the release of propenal from aged resin samples.

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. 

PAE resins require the application of heat for the chemical reactions to occur. Usually, the maximum level of wet-strength is not achieved on the paper machine. The paper requires ageing (resin curing) to fully impart the wet-strength to the fiber network. At higher dryer temperatures the wet-strength development of the paper is improved both "off-machine" and with ageing [23, 47]. 

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