Detecting molecular changes in UV laser-ablated oil/diterpenoid resin coatings using micro-Raman spectroscopy and Laser Induced Fluorescence

Naturally occurring diterpenoid resins were extensively applied, mainly as oily mixtures, through the ages as protective and decorative coatings on paintings, metals and wood artifacts. When these coatings age, tend to generate tougher films than triterpenoid resins, completely insoluble, increasingly subject to darkening, and then very hard to remove using conventional methods. In this regards, laser sub-micrometric ablative techniques are being increasingly used in the cultural heritage field also for the treatment of synthetic and natural polymer coatings. Here, in the wake of the positive outcomes achieved on triterpenoid resin films, the present approach has been applied and extended to diterpenoid resin coatings, which have never been thoroughly studied yet. In detail, colophony, sandarac and Manila copal resin films, prepared as solvent and linseed oil formulations, were subject to light-ageing and then systematically irradiated at various exposure conditions using the 4th (266 nm) and 5th (213 nm) harmonics of a Q-Switched Nd:YAG laser. UV-Vis absorption spectroscopy was used as preliminary characterization of the films optical properties. The assessment of physico-chemical modifications induced by artificial light-ageing and ns UV laser irradiation were assessed non-invasively by mu-Raman spectroscopy, Laser Induced Fluorescence (LIF) and microscopic examination. The results underlined that, due to the presence of a polymer network, sandarac and copal resin coatings showed higher Fa, than those composed of colophony. Morphologically, all the coatings were subject to bubbling upon 266 nm irradiation, while damage-free at 213 nm. At molecular level, irradiation at 266 nm produced minor changes to nu(CH3)/nu(CH2), nu(C=0) and nu(C=C) modes, thus confirming an ablation mechanism mainly driven by photo-thermal bond-breaking through the ejection of gaseous by-products. Raman background fluctuations along with shifting and broadening of LIF maxima were supportive in the assessment of laser-induced surface modifications. Finally, the work performed indicates that the 213 nm wavelength is the most indicated for the treatment of aged solvent and oil diterpenoid-based coatings, as no side effects occurred. This outcome, corroborated by the previous results obtained in triterpenoid resin films, may have important implications in the field of cultural heritage conservation, as it extends the application range of solid-state Nd:YAG lasers to all types of protective and decorative terpenoid coatings.