## 4 Discussion
[PERSON] confirmed that the red pigment is obtained by grinding laterite, an iron-oxide-rich clay that can be locally found as both rock and soil. In the masks studied at AIC, no enamel paint was identified, even in the most recent examples. [PERSON] himself mentioned that modern paints are used only if requested (Appendix A). With regard to the binding medium, complementary analytical techniques with different levels of specificity were used for its identification. The application of untargeted bottom-up proteomics was necessary to achieve a high degree of protein specificity, which is valuable when investigating artworks that have, until now, received little scientific attention. Proteomics suggested the use of whole eggs from the helmeted guinea fowl (_Numida meleagris_) and wild turkey (_Melegaris gallopavo_), which were found together in the red sample from the Fish mask. The helmeted guinea fowl is native to Burkina Faso (Appendix A) and genomic studies showed that it was domesticated in West Africa around 1300-5500 years ago [31]. The turkey is native to central Mexico, but during the Spanish colonization, it was imported to Europe and then distributed in the 17 th century across other continents, including Africa [32]. [PERSON] verified that, besides guinea fowls, eggs from other birds including turkey are also used for painting. The presence of whole egg proteins from both the helmeted guinea fowl and turkey on one of the masks might be evidence for two different painting campaigns, or could alternatively indicate the use of a mixture of eggs from the two birds. Investigation of the white paints showed a greater variety of materials in the four masks. Gypsum, most likely from schoolroom chalk, as mentioned by [PERSON] [1], was used to paint white areas of the Hawk (1970.103) and the two Plank masks (2008.190 and 2000.313). Kaolinite was also identified in one of the two samples taken from each Plank mask, suggesting the use of kaolin clay, which is widely available and used as a pigment in different regions of Africa [29,30,33,34]. The two inorganic compounds were not identified as a mixture, which might indicate that they represent different painting campaigns. In the white paint of the Fish mask (1958.116), FTIR, Py-GCMS, and SEM analysis instead showed the presence of uric acid and/or urates, consistent with the use of reptile or bird excrement. Both uric acid and its salts are found in the excrement of these animals, varying in relative quantity according to the animal diet and other factors [20]. The application of such material as a pigment on the surface of African sculptures and other objects, and for other purposes, has been reported in previous studies. [PERSON] recorded the use of snake excrement among the Tiv people in Nigeria, mixed with herbs, cooked and powdered, and used to bless, for example, hunters before a hunt [35]. [PERSON] described the use of a mixture of snake or lizard excrement, rice, and limestone, as a white pigment among the Dogon in Mali [36]. More recently, urate salts have been scientifically identified by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in micro-samples from the accumulative surface of a wooden statuette from the Dogon [37]. This analytical technique was also used by the same authors to characterize the white pigment applied on a Dogon rock painting; ToF-SIMS results in this case showed the white pigment to be composed of uric acid [21]. In our research, mass spectrometry-based proteomics was applied to provide a more certain determination of the animal of origin. Proteomics allowed us to confidently exclude a bird's origin, and data suggested the excrement to come from a lizard species, based on the detection of peptides specific to _Pogona vitticeps_, a lizard native of Australia. The absence of a more geographically appropriate match is not surprising since only a few proteins have been sequenced up to now for indigenous, or lately introduced, reptiles of Burkina Faso [38], but is nevertheless convincing evidence of the use of excrement from lizards.