cause yellowing remain absent as well. Stabilization has a dramatic effect on solubility (Figure 2). For example, after more than 3000 h of light aging using a UV filter, dammar films con taining 2% Tinuvin 292 and 3% Tinuvin 328 can still be removed with 100% cyclohexane. Unstabilized films aged us ing a UV filter for a similar period re quire a mixture of 29% acetone and 71% toluene, whereas those films (with and without additives) that were light-aged without using a filter show rapid dete rioration as judged by solubility prop erties. IR spectra and GPC data of unstabi lized films show major changes despite the use of a UV filter. GC/MS shows that little of the triterpenoid fraction survives in such films (26). The IR spectra of films of dammar (containing 2% Tinuvin 292 and 3% Tinuvin 328) light-aged using a UV filter show little change from the unaged product. GPC of these films affords chromatograms that are essentially identical to those of the unaged resins. GC/MS of the triter penoid fraction of the aged material re
veals that all original components sur vive, indicating that authentic stabili zation has been achieved. Use of dammar varnish with additives is a via ble alternative for museums that have eliminated the UV component from their light sources. Future directions
We are currently investigating several commercially available synthetic resins that are more stable than ketone resins. These include hydrocarbon resins, which are often oligomers obtained from dicyclopentadiene. The most sta ble hydrocarbon resins are fully hydrogenated and free of functional groups. They dissolve in aliphatic sol vents and, because of their cyclic na ture, have relatively high RIs. We also are studying several condensation products of urea and aliphatic alde hydes, one of which has shown excep tional stability. Although work is still in progress, it is already clear that many of the resins under investigation have stabilities that are far superior to the products
currently in use by conservators. When formulated with stabilizing additives such as HALS, films of these resins are extremely stable even in the presence of UV light. Several of the resins are being used in experimental varnishes by a number of conservators. Many conservators have already expressed enthusiasm for the products; some have claimed that they have been able to achieve the same appearance with them as with natural resins, although typically the desired effects can only be achieved with synthetic resins by ap plying a final spray coating. Natural resin varnishes apparently have unique rheological properties that allow for subtle manipulation during brushing. Obviously, the complex chemical composition of these resins and the presence of both a low-MW fraction and a polymeric fraction con trol these properties. However, it is not yet clear which factors are most impor tant, and more precise information about the optical effects of varnish should be obtained. Goniophotometry (i.e., the measurement of the amount of
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CIRCLE 3 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 2 1 , NOVEMBER 1, 1989 · 1239 A
