Conservation of ethnographic artefacts: Selective laser ablation of deposits from doum palm fibers

This work approaches the challenging cleaning problem of fragile ancient ethnographic artefact crafted using lignocellulosic fibers, which undergo different and concomitant degrading reactions (oxidation, hydrolysis, depolymerization) over time. Here, the fundamental wavelength and second harmonic of Q-Switched Nd:YAG laser were comparatively tested for the removal of deposits from woven-fibers `angarêb, which is exhibited at the Africa Hall of the National Geographic Society Museum (Cairo, Egypt). After a careful fiber identification, laser-induced effects were assessed on `angarêb fibers and fresh, naturally and artificially aged doum palm (Hyphanae Thebaica) reference samples by means of stratigraphic examination, UV-induced Vis fluorescence emission, Raman spectroscopy, ESEM-EDX analysis and optical microscopy. Irradiation at 532 nm affected the color appearance and structural integrity of the fibers. Bond-breaking/depolymerization and bleaching occured at this wavelength, due to the significant absorption of lignin/phenolic-carbohydrate and lignin-quinonoid complexes. In contrast, laser irradiation at 1064 nm did not induce any detectable discoloration or structural alteration, either in the short- or long-term diagnostic assessments. The results achieved highlight the possibility of using the latter wavelength for recovering the original surface of soiled ethnographic artefacts made of fragile vegetable fibers, which are otherwise untreatable.