## 4 Discussion  
The difference in appearance and thickness of the black, white, and red paint, observed in the first investigation of the four masks, is associated with the distinct properties of the pigments and plant products mixed, or not, with organic binders, and applied directly to the wooden support, without any preparation layer. Scientific analysis, informed by a conversation with artist [PERSON] (see Appendix A), corroborated and complemented the information reported by scholars on the use of different painting techniques for each of the three colors, and provided a deeper insight into the materials' selection and application. According to [PERSON]'s descriptions, two different kinds of black paint are used on Bwa masks: what he called a 'thick black', obtained by boiling the seed pods from the _Acacia nilotica_ tree, a synonym of _Vachellia nilotica_ (L.) [PERSON]. [22], a plant native to North Africa; and a 'thin black', prepared by mixing charcoal with a binder, such as egg [1]. The exclusive detection in the samples investigated in this study of tannins, and of no other organic binding media or black particles, indicated the black paint could be ascribed to the 'thick black' category. Tannins are polyphenic secondary metabolites of higher plants, both gymnosperms and angiosperms. They are mainly present in bark, seeds, roots, and leaves, with the primary function of protecting the plant from microbial and fungal attacks [23]. Due to their property of binding and precipitating proteins, specifically collagen, they have been widely used since ancient times to prepare leather from animal skin [24; 25]. In the art and archeological fields, tannins have also been used as writing and drawing material (iron gall ink) and natural dyes, often combined with metallic mordants for better stability [26]. From a chemical standpoint, tannins can be generally categorized into either condensed or hydrolyzable types, the former made of flavonoids linked together, and the latter characterized by a sugar core with ellagic and gallic acids linked to it [23]. The identification by Py-GCMS of both gallic and ellagic acid in the black paint samples (Figure 3C), and the absence of catechins, suggested the tannins present in the black paint to be hydrolyzable tannins and, in particular, the sub-category containing both gallotannins and ellagitanins. While the chemical composition of the black paint and the seed pod extract prepared in our laboratory was comparable, the consistency and color of the two were significantly different, with the extract obtained after 8 h of boiling in water having a dark brown color instead of black. Information provided by [PERSON] (Appendix A), and collected by [PERSON] during his time in Burkina Faso [5], helped to account for this difference and provided greater insight into the paint preparation process described by [PERSON]. The _Acacia nilotica_ seed pods are collected and crushed in their entirety, water is added in a 2:1 ratio, and the mixture is boiled over fire for several hours, or until the liquid turns a dark brown color. The mixture is filtered, and the brown extract is recovered and boiled again for several hours until it is reduced to a thick black liquid. This prolonged boiling and concentration of the extract allows the tannin-based liquid to turn into a thick black material usable as painting material. This product can be kept for up to 2 years and it is heated up again before use. The fact that the tannin-based extract is further concentrated to reach a tar-like consistency explains why the black paint appears textured and not flat, as in the red and white painted areas. All red paint samples showed a consistent composition, characterized mainly by a mixture of an iron-oxide-based pigment and a protein-based binding medium. The leanly bound nature of the red paint explains its matte appearance. Iron-based stones and clays have been reported in the literature as a common source of red pigments in African sculptures and objects [27; 28; 29; 30].