###### Abstract
painting and was initially detected through UVF photography, which showed a blueish UV fluorescence on the whole artwork surface. As already mentioned in [PERSON] et al. [1] and discussed in detail in Section 3.1, the varnish was later confirmed as a ketone resin through Py-GC/MS analysis. _Spitz-Rund_ underwent several conservation treatments over the years due to the sensitivity of the constituent materials and to compensate for the effects of traveling around on the occasion of international exhibitions. Indeed, in the period between the 1930s and 1970s, the painting was glued on an auxiliary wooden support to make its transport easier. Several delaminations of the paint layers and a dense craquelure, probably caused by thermo-hygrometric variations, were previously treated with consolidation and filling processes. It is likely that the application of the ketone resin varnish occurred after the formation of these cracks. The craquelure is dense and extends through all layers down to the paper support. Additionally, there is a pronounced color deformation phenomenon, which resulted in significant alteration of the surface. The ketone-based varnish that was applied further hid the irregular surface texture and sipped into some of the paint cracks (Figure 2). The first patent for this class of resins dates back to 1926, but the ketone-based products spread in the field of conservation treatments only in the fifties and sixties of the 20 th century in order to replace natural resins [3]. Therefore, the application of this varnish on _Spitz-Rund_ was most likely accomplished from the 1950s onwards. The film layer showed several alteration phenomena like yellowing, bleaching and hardening. In addition to the degradation of the ketone varnish, a substantial amount of dust accumulated over time. The considerable adhesion between the synthetic varnish and the paint layers is another of the aspects investigated in this research. This led to the conclusion that the need to remove the non-original varnish depended both on aesthetic and conservation choices. The removal of organic film layers from pictorial surfaces is one of the most complex phases of a conservation treatment, as the original paint layers are stressed by the chemical, physical and mechanical forces exerted by the cleaning systems, hardly foreseeable or assessable without the aid of suitable scientific instruments. The water sensitivity phenomena of oil paints of the twentieth-century concern conservators as surface cleaning treatment can be complicated or prevented by it. There are many examples of not-varnished, water-sensitive oil paintings from the 20 th century reported in the literature, including works by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [4; 5; 6; 7; 8; 9]. In the case of _Spitz Rund_, in addition to the issues normally related to the traditional cleaning systems, we had to take into account the fragility and great variability in the thickness of the brushstrokes, both of the original and non-original paint layers. Therefore, given that the goal was to prove the efficacy of  
Figure 2: (**a**) Detail of _Spitz-Rund_ surface; (**b**) Schematic representation of the paint surface morphology: cardboard support (1); preparatory layer (2); pink background (3); ketone-based varnish (4). several cleaning treatments for the removal of a ketone-based varnish, reducing as much as possible the effects on the original paint, it was essential to provide an initial test phase on mock-ups that reproduced the original stratigraphy. In order to set up cleaning tests, the solubility parameters of ketone resin were deduced from previous works in the literature that investigated its properties and response to treatments after accelerated aging [3; 10; 11; 12]. Several tests have shown that ketone polymers are initially more stable than natural resins but become more difficult to dissolve over time exposure. Moreover, their films become weak and brittle over time.