The article is an investigation of over 160 bronzes from Peru and Bolivia. Quantitative analysis of many pieces for copper and tin, and qualitative analysis for other elements are published in an effort to establish whether tin was added purposely. Comparisons with bronzes from Argentina and Chile, early post-Conquest written sources, comparison of the composition of different kinds of objects, and the analysis of ancient ores, leads the author to the conclusion that the tin was added purposely.