Dating study of two rock crystal carvings by surface microtopography and by ion beam analyses of hydrogen
Two artefacts made of rock crystal (quartz) from the collection of the Musée du quai Branly in Paris, France, a skull approximately half of the size of a real cranium and a smaller anthropomorphic head, purportedly attributed to pre-Columbian Mesoamerican cultures, were studied to assess their authenticity. The surface of the artefacts were examined by means of optical microscopy and scanning electron microscopy (SEM) and were analyzed nondestructively by ERDA (Elastic Recoil Detection Analysis), an ion beam analytical method that can measure hydrogen concentration profiles in depth. Optical and SEM imaging of tool marks indicates that the skull has been cut from a rock crystal block using machine lapidary techniques unavailable to pre-Columbian artisans, whereas the anthropomorphic head has more likely been carved and polished with manual techniques comparable to ancient ones. Hydrogen depth profiles in the first micron below the surface of the artefacts have been measured by ERDA with a 3-MeV He beam in a controlled helium atmosphere. Recently the progressive penetration of water at the surface of a quartz sample exposed to the natural environment has been proposed as a dating method (labeled quartz hydration dating or QHD) applicable to archaeological artefacts made of this material. The shallower penetration of H clearly indicates that the rock crystal skull was manufactured more recently than the reference quartz sample cut in 1740. As for the anthropomorphic head, the deep penetration profiles indicate an older artefact. Thus the converging micro-topographical examinations and hydrogen profiles of the samples surfaces indicate that the skull is probably not a pre-Columbian artefact but has been carved in the 18th or 19th century. The anthropomorphic head, on the other hand, could have been carved in the pre-Columbian period. In addition, the ERDA method carried out with an external beam presented here provides a new and simple approach for the nondestructive authentication of quartz-based archaeological artefacts by QHD.