Macromolecular aspects of natural resins
Klaas Jan van den Berg and Jerre van der Horst

The ageing process of resins may result in the condensation of the low-molecular weight compounds to form larger molecules which could affect the solubility of varnishes. These high molecular weight fractions may account for the dramatic decrease of the triterpenoid fraction in triterpenoid varnishes (see project). In addition, resins added to drying oil in resin/oil paint or megilps may be incorporated into a polymeric structure. Before a more detailed study of polymers in aged natural resins, an analytical strategy was developed to isolate and identify polymer fractions in a selection of natural resins.
After extraction and precipitation, size exclusion chromatography (SEC) is performed to separate polymeric fractions. Various mass spectrometric and nuclear magnetic resonance studies are then performed to obtain more information on the structural elements, the linking and molecular weight distribution of the polymeric fractions.

Participants
Klaas Jan van den Berg, Jaap J. Boon, Gisela van der Doelen, Jerre van der Horst, Dr. Leslie Carlyle (CCI) and Dr. Joyce Townsend (Tate Gallery). Some pilot studies have been performed with LCMS techniques at the National Gallery of Art by Dr. Nobuko Shibayama.

PROGRESS REPORT 1995-1998
The chemical structure of the polymer fractions of the natural resins - mastic, Congo copal, sandarac, manila copal and kauri copal used by artists and restorers was studied. Isolation was carried out with a combination of precipitation reactions, extraction and SEC. The polymer fractions were then analysed with DTMS (Chemical ionisation), Py-TMAH-GCMS, FT-IR and NMR.
The structure of the mastic polymer, hitherto unknown, was identified and published in Tetrahedron Letters (Vol.39 (1998) p.2645-2648). The structure of the kauri copal polymer was confirmed to be a copolymer consisting of communic acid and communol. Congo copal contains a large polymer fraction, which consists of ozic acid and a monomer identified as ozol. Sandarac and Manila copal contain a copolymer consisting of communic acid with only low quantities of communol.
The polymers were found to be relatively unstable to air after isolation; the decrease in solubility (THF and DCM) of the fractions is interpreted as an indication that oxidative cross-linking was taking place. This process may well be worse after utilisation of the resins in or on a painting and could contribute to the loss of solubility of aged varnishes.
Results of studies on the formation of high molecular weight fractions in triterpenoid varnishes by Gisela van der Doelen are described in that project.

Future plans
Two publications about the work are in preparation. More studies on the stability of polymeric fractions of resins in e.g. copal varnishes are carried out in cooperation with Dr. Leslie Carlyle (CCI) and Dr. Joyce Townsend (Tate Gallery). In addition, incorporation of diterpenoid resins in oil networks will be studied in more detail (see also project #1).

Deliverables
Results of the studies will be presented in part in PhD theses of Gisela van der Doelen, Jorrit van den Berg, Georgiana Languri and Jaap van der Weerd. Some of the result will appear in the MOLART special report 'diterpenoids in paint media and paintings' by Klaas Jan van den Berg.

PROGRESS REPORT 1999
Many accounts in written sources point to the use of copal-oil media as varnish or paint medium. However, hardly any analytical evidence for the use of copal-oil media in paintings has been found to date. Studies on the ageing of copal-oil media were performed on varnish reconstructions of Leslie Carlyle with the aim to find any explanation and to develop analytical methods that could lead to the identification of copals in paintings.
GCMS studies showed that already in the process of manufacture the concentration of the diterpenes decrease relative to the oil components due to isomerisation, and probably polymerisation (Diels-Alder reactions etc.). This is explained by the extremely high temperatures that were applied (often >300 ºC) in the manufacturing process to melt the hard copal resin before adding the oil. In the drying and ageing process, the remaining characteristic diterpenes were found to disappear completely probably as a result of oxidative degradation and cross-linking. What remains is only the signature of a heat-bodied oil.
A possible solution to this problem could be to analyse the polymer fraction of the copals. Pyrolysis-GCMS is was applied using a novel two-step technique designed for selective analysis of remnants of polymer structures. The procedure involves a two-step pyrolysis-GCMS technique with on-line hydrolysis and derivatisation with TMAH, evaporation and analysis of the low MW material in the first step. In a second pyrolysis traces of polymer fractions in the residue are analysed specifically.
With this technique, it was found that some characteristic marker compounds of the copal polymers could still be found. As a major breakthrough traces of copals could be identified in two paintings from the Pre-Raphaelite artist J.E. Millais ('Vale of Rest' and 'Mariana in the Moated Grange; Tate Gallery) (see figure).
An article on this work (Recognition of copal resins in varnish and oil paint media) was published and the results presented at the ICOM-CC conference in Lyon.

Future plans
The new method will be applied in other cases were copal is a possible component of a varnish or paint medium as well as in other studies of high molecular weight fractions in oil and varnishes. We will participate in the project 'The analysis of nineteenth-century paint media: part II' at the Tate Gallery, London. Co-ordinator: Dr. J. Townsend. This project involves the analysis of copal-containing oil paints by Pre-Raphaelite painters.

Deliverables
Results of the studies were / will be presented in part in PhD theses of Gisela van der Doelen, Jorrit van den Berg, Georgiana Languri and Jaap van der Weerd. Some of the result will appear in the MOLART special report 'diterpenoids in paint media and paintings' by Klaas Jan van den Berg.

PROGRESS REPORT 2000
This project is finalised.