OIL OVERPAINTINGS REMOVAL USING ATMOSPHERIC PRESSURE PLASMA

Plasma cleaning is a contactless method with soft chemical action and nanometer precision; this makes it suitable for the highly demanding field of conservation of cultural heritage artifacts. Oil overpaintings that obscure the original murals are a common problem in the conservation of Eastern Orthodox churches. The removal of the often multiple layers of overpaint is a challenging task. The traditional cleaning methods, which combine chemical and mechanical treatment, are time consuming and may damage the underlying pictorial layer. In this paper, we study the benefits of combining plasma pretreatment with the traditional methods. We conducted experiments on removing oil overpaint using plasma, as part of the European project Plasma And Nano for New Age "soft" conservation (PANNA). We performed preliminary tests on laboratory samples as follows. First, we prepared wall painting samples, which we then coated with oil paint. Second, we treated the surface with compressed air plasma using a commercial plasma torch. Finally, the samples were analysed using Fourier transform infrared spectroscopy (FTIR). The results show that the oxidative plasma degrades the linseed binder present in the oil paint, facilitating its removal. We performed additional tests on authentic fragments from over-painted murals. The fragments were collected from the St. George church in Golyamo Belovo, Bulgaria. The original egg tempera decoration, dating back from 1852, was completely covered with oil paint in 1921. No isolation layer was used between the two. We first treated the fragments' surfaces with plasma (see Figure 1) and then completely removed the oil paint using organic solvent and a scalpel. As a next step, we conducted an in situ cleaning in the St. George church. We treated three selected areas, in the altar, following the methodology used for cleaning the fragments; see Figure 2. The cleaning effect was compared with that obtained with only traditional methods. Overall, our experiments are encouraging and demonstrate that plasma pretreatment facilitates the removal of oil overpaintings by decreasing the treatment time and the risk of damage to the pictorial layer.