## 1 Introduction  
Historic wall paintings are highly appreciated not only for their unquestionable aesthetic value, but also because they provide insights into the art, history, architecture, and customs of the time [1]. As they are frequently outdoor-exposed, their conservation is highly dependent on their surrounding environment, among other factors. They can suffer alteration, decay, and even destruction due to the action of--and fluctuations in--various different environmental agents (solar irradiation, humidity, soluble salts, etc.) [2, 3, 4, 5]. This often manifests itself in the detachment and consequent loss of the painted surface and the appearance of lacunae (meaning cavity or lack) [2, 3, 6]. When this happens, professional restorers are engaged to repair these lacunae in a process known as chromatic reintegration or inpainting [2, 3]. The most widely reported inpainting techniques involve water-based(pigments generally dispersed in gum Arabic) and varnish-based (pigments dispersed in natural and synthetic resins) procedures [2; 7]. However, they only appear to be effective in indoor settings where environmental conditions can be controlled [3; 8]. [PERSON] et al. [9] evaluated different inpainting techniques, both natural and synthetic, and reported positive results in terms of colour stability with ammonium caseinate after accelerated ageing. However, cracks were observed after temperature and relative humidity fluctuations. Therefore, it is paramount to find an inpainting technique that can withstand the significant environmental fluctuations to which many wall paintings are partially or completely exposed [8]. This requires the use of a weather-resistant binder, which must also ensure the natural breathability of the wall, i.e., by creating a layer that allows the exchange of moisture between the wall and the environment. Despite this, little research has been conducted into chromatic reintegration in terms of the stability of the restored lacunae against environmental factors, their long-term durability and behaviour, and their visual compatibility with the original painting [8]. In this context, the use of silica-based paints has become more popular in recent decades [10; 11; 12], although little is known about their compatibility, durability, and reversibility when used in chromatic reintegrations [8]. Silica was first used as a binder in paintings at the end of the 19 th century, in a technique known as'stereochrome painting' (from the Greek _stereos_, meaning solid, and _cromia_, meaning colour), also referred to as'mineral painting' [12; 13; 14]. Soluble silicates (sodium and potassium) were first used by [PERSON] (1825) and later by [PERSON] (1881), who improved the method for obtaining these paints and helped disseminate the'mineral painting' technique [11]. From then on, technological advances in the manufacture of silica-based paints led to binders of this kind being used in the construction and artistic field [10; 11; 12; 15]. They seem to be particularly suitable for outdoor settings due to their water-repellent properties and their high resistance to harsh weather conditions [5; 16]. Various silicates that are widely used in cultural heritage restoration, such as sodium (Na) and potassium (K) silicates, appear to be unsuitable as binders for outdoor-exposed paintings. Na-silicates do not provide a water-resistant coating [15] and K-silicates have shown poor resistance to freeze-thaw cycles, leading to a loss of adhesion of the paint layer [16]. In this situation, nano-silicates, with their apparent chemical inertness and durability, could be considered a viable, environmentally friendly alternative. Cultural heritage restoration work such as chromatic reintegration must always meet certain requirements. According to [17], these are recognisability (i.e., ensuring that the inpainted area is visually distinguishable from the original painting), reversibility (i.e., it can be removed without damaging the original paintwork) and compatibility.