ETD

• atomic oxygen cleaning
• Gas-Chromatography
• laser cleaning
• Mass Spectrometry
• oil paints

The surface cleaning is a crucial procedure for cultural heritage conservation. Usually, it involves the use of solvents (wet cleaning) or physical removal of dirt from the surface (dry cleaning). However, these methods have their limitations in terms of efficacy, safety and sustainability. Search for new approaches lead to the development of non-contact methods, which present significant advantages when working with solvent-sensitive and mechanically unstable heritage object surfaces.
In this regard recently, an alternative for cleaning the surface of works of art was developed based on the use of atomic oxygen in atmospheric pressure plasma (AO). It’s cleaning mechanism is based on the oxidation of the contaminants by atomic oxygen. This is a promising non-contact technique particularly useful for cleaning carbon-based contaminants, like soot, from porous, water-sensitive and fragile surfaces. The pioneering use of AO cleaning for cultural heritage conservation showed promise before. However, before applying it to artworks, it is necessary to systematically study the chemical effects of AO on the substrate materials to evaluate the cleaning mechanism and carry out a systematic comparison to other established techniques.
To this aim, this study systematically investigates the impact from AO cleaning and compares it to cleaning carried out with two types of lasers: Er:YAG, suitable for varnish thinning, and Nd:YAG, commonly used by conservators for soot removal.
Experiments were performed on model oil paint layers (naturally aged for two years), consisting of ultramarine blue pigment and two different types of binders: linseed oil, used as oil binder since antiquity, and safflower oil, frequently encountered in modern paint formulations. Treated and untreated samples of identical composition were analyzed using a combination of analytical techniques such as SEM, optical microscopy, glossimetry and colorimetry to compare the surface morphology of the layers, ATR-FTIR and EGA-MS to assess the molecular changes undergone by the binder, and SPME-GC-MS to monitor the evolution of VOCs released from paint layers over time.
Additionally, to more thoroughly investigate the mechanism of action of atomic oxygen on the oil curing process, a treatment with isotopically marked atomic oxygen (18O) was performed on unaged (fresh) samples. The samples consisted of the binder linseed oil with cadmium red (CR), lamp black (LB), lead white (LW) and ultramarine blue, as well as the binder safflower oil with the pigment ultramarine blue. SPME-GCxGC-MS analyses were then performed to determine any changes in the natural isotopic ratio between 18O and 16O of the VOCs released from the oil paint layers.