Red dyes in seven belts (trariwe) belonging to the Mapuche culture, dated as beginning of the XXth century and stored in Museo Regional de la Araucanía, Chile, were studied by using ultraviolet-visible, liquid chromatography coupled to mass spectrometry, Raman and Surface Enhanced Raman Scattering spectroscopies. Red dyes were extracted and analysed; the spectral analysis allowed identifying that the principal red dye in five samples is highly consistent with a commercial synthetic aniline, the azopigment PR57. Another monoazopigment, an orange benzimidazolone, was identified in two samples. A molecular model for the PR57/Ag surface interaction supports the idea that the dye mainly exposes to the metal the carboxylate and sulphonate groups; the red azopigment is oriented rather tilted with the two aromatic rings being almost coplanar to the surface. The electrostatic interactions are the main factor of the PR57/Ag layer interaction.