Towards more stable natural resin varnishes for paintings

Patrick Dietemann, Towards more stable natural resin varnishes for paintings. The aging of triterpenoid resins and varnishes, Tese de Doutoramento, Swiss Federal Institute of Technology, Zurich, 2003

Resumo:

Natural triterpenoid resins, dammar and mastic, are widely used as varnishes on paintings. Unfortunately, these varnishes are not stable, but oxidize and yellow with time. The latter is of major importance since yellowing obscures a painting and can considerably change its appearance. As a consequence, varnishes have to be removed and replaced rather often, a harsh treatment that can damage the painting. It is therefore desirable to find ways to improve the performance of varnishes with respect to oxidation and, especially, yellowing. This would reduce the number of varnish replacements necessary, and valuable and unique works of art could be protected. This thesis studies the aging of triterpenoid resins with the aim to improve their stability and performance.

Conventionally, the aging of resin varnishes was described with a model that postulated very different aging pathways in light and darkness. Aging in light meant oxidation proceeding by radical chain reactions. The light was believed to be necessary for the initiation of these reactions, that would stop during dark storage due to recombination of the radicals. Aging in darkness, therefore, was considered to proceed by non-oxidative thermal reactions, such as aldol condensation. Yellowing was regarded as a typical consequence of these dark reactions, since yellowing is more pronounced in darkness.

This thesis, however, shows that commercial resins, usually considered “fresh”, are in an advanced stage of oxidation, and high amounts of radicals are abundant even in darkness. These results were obtained by two analytical methods newly introduced to the field of conservation science: electron paramagnetic resonance spectroscopy, and graphite-assisted laser desorption/ionization mass spectrometry. Consequently, several natural and artificial aging studies were performed to study the aging pathways in light and darkness in more detail. Mastic resin was harvested under protection from sunlight on the island of Chios, Greece. The idea was to suppress the formation of radicals in the fresh resin and obtain a more stable material for varnishes. The factors influencing the resins’ properties and aging behavior are discussed.

In contrast to the conventional view it was found that aging processes in light and darkness are actually very similar. Large amounts of radicals are abundant in triterpenoid resins under all storage conditions. Thus, oxidation proceeds by radical chain reactions also in darkness. Varnishes oxidize very quickly, most of the initial triterpenoids are oxidized after a few months, not years or decades. A kinetic study revealed that the radical concentrations change considerably within hours after change of illumination conditions. It is concluded that radicals are abundant not because of insufficient termination, but rather strong initiation reactions, also in darkness. This explains the rapid oxidation of varnishes and proves that aging reactions are very dynamic and vigorous. A consequence of the same aging processes taking place in light and darkness is the occurrence of the same aging products, as identified by GC-MS. No photoreaction products were found in all naturally aged samples, even when aged under harsh conditions (direct sunlight irradiation in a window). Thus, the influence of light on aging mainly consists in the enhanced formation of radicals. Aging then proceeds by the same pathways in both light and darkness, although rates may differ.

Oxidative radical reactions taking place in darkness give a straightforward explanation for yellowing. Allylic oxidation results in formation of unsaturated ketones from double bonds. In addition, formation of ketone and hydroxy functional groups enhance the possibilities for aldol condensation and enlargement of preexisting unsaturated systems by elimination of water. Thus, it is concluded that more compounds responsible for yellowing are formed in light than in darkness, but are bleached simultaneously. In darkness they accumulate, and yellowing seems more pronounced, although it is not a dark reaction.

Protection from any light during harvest was found to dramatically suppress radical formation and prevent polymerization of monoterpenes in fresh mastic. Dark harvested mastic contains no polymer, and yellowed significantly less during aging. However, radical formation and oxidation during aging could not be prevented, and radicals develop even in resins that are protected from any light during harvest, storage and aging. The reason of the reduced yellowing predisposition of dark harvested mastic is not completely clear at that point. It might be connected with the absence of mastic polymer, that is found to act as a natural radical stabilizer, reducing oxidation, but enhancing yellowing. Thus, the aging properties of a varnish can be influenced by the resin composition, which opens up new possibilities for improving the performance of natural resin varnishes.

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