Tin-rich surfaces found on bronze antiquities may be the result of man-made or natural processes. The difficulties encountered in distinguishing between these processes are discussed and illustrated with reference to the surface structures obtained on experimentally-tinned copper and bronze. Heat treatments were carried out on the experimental material to allow observation of the growth of the intermetallic compound layers on the surface, and of changes that occur at various significant temperatures. The experimental reduction of cassiterite directly onto the surface of copper and bronze is illustrated for the first time. Examples of tin-rich surfaces found on various antiquities are illustrated and are compared with the experimental material for interpretation. Scanning electron microscopy and microanalysis were the primary tools used in this study.