Preliminary results from the characterisation of folium

The term folium has been attributed since long times to the purple dye extracted from Chrozophora tinctoria (L.) A. Juss., the plant native to coastal Mediterranean countries known as turnsole. In the past there has been confusion concerning its real nature and in many instances similar names were given to lichen dyes and to dyes obtained from Chrozophora species. Literary citations are several [1-3] and the first recipes are dating to 11th century A.D.; it is possible that turnsole was already in use in Roman times and that it corresponded to the Heliotropum tricoccum cited by Pliny the Elder [4]. According to these sources, three hues could be obtained extracting the dye from Chrozophora tinctoria: a red at acid pH, a blue at alkaline pH and a purple at neutral pH. The scientific knowledge on the composition of folium is at present relatively unsatisfying. Most of the available information can be circumscribed to the pioneering work by Guineau [5] who tentatively identified the dye in some 9th-11th century manuscripts by means of UV-visible diffuse reflectance spectrophotometry. In particular, the molecules responsible for purple/blue colour are unknown; it has been hypothesised a similarity with the molecules of orchil but without concrete scientific evidence. In our study we have carried out an historical reconstruction of folium starting from fruits of Chrozophora tinctoria in order to try to elucidate its chemical composition and to increase the possibility of its identification on artworks. The different parts of the fruit, i.e. the internal seed and outer and inner cuticles, have been treated separately in order to obtain aqueous extracts; depending on the ripeness of fruits it is possible to extract a purple and a blue dye, but not a red one. Different techniques have been applied to extracts and to painted/dyed samples: SERS and FT-Raman spectroscopy, FT-IR, FORS, spectrofluorimetry and MALDI-TOF-MS. Preliminary results are presented, together with some possible identifications on early medieval manuscripts.