On the identification of folium by SERS: from crude extracts to illuminated codices

The ancient purple dye known as folium is still a mystery for both scientists and art historians. Today it is commonly assumed that folium was produced from the fruits of Chrozophora tinctoria (L.) A. Juss., a plant belonging to the Euphorbiaceae family, and efforts have been mainly devoted to highlight the analytical features of the dyes extracted from this plant, whereas detection in ancient manuscripts has been mainly based on poorly selective, non-invasive analytical techniques. As a consequence, the possibility that the actual source for the dye could have been so far misunderstood cannot be excluded. Surface Enhanced Raman Spectroscopy (SERS), a highly selective and sensitive analytical technique, has been used here to characterize both extracts from Chrozophora tinctoria and a micro-sample taken from a medieval manuscript. The behavior of the dyes as probes for SERS has been investigated, in order to set up an accurate and selective procedure for the identification of the dye in ancient artworks. By unambiguously detecting the dye by SERS in the micro-sample of the medieval manuscript, we also demonstrated that the purple dye mentioned in ancient treatises is definitely linked with the aqueous extract from purple fruits of Chrozophora tinctoria.