Challenges in the analysis of purple codices

Manuscripts with purple coloured pages exhibit a strong symbolic value in late antique and medieval book culture. Usually biblical texts were written with gold or silver inks on parchment dyed or painted with purple colorants. The symbolic value of the purple colour and the use of noble metals for inks indicate the representative character of the books and their use in a royal or imperial context. The use of purple dyes or paints to impart colour to parchment of precious codices is mentioned since Late Antiquity [1]. In the ancient literature, though, recipes explicitly describing the practice of colouring parchment are rare. As a consequence, our knowledge concerning the colourants effectively used and the way of using them is far from complete. One reason is due to the fact that in the ancient bibliographic sources there is sometimes ambiguity among the term “purple”, which recalls the use of Tyrian purple dye, and the actual compounds used to dye in purple. Moreover, ancient treatises were frequently compilations of more ancient sources and mistakes, misunderstandings and personal interpretations were always possible. The lack of bibliographic information adds to the scarceness of diagnostic information. Being valuable but also fragile artworks, purple codices have been rarely analysed [2-8] and not often the results allowed identifying a definite colourant or a mixture. At present, indeed, we have no single evidence of the use of Tyrian purple in codices, while all analyses carried out on purple codices hypothesised the presence of less valuable alternatives, such as orchil from Roccella, Dendrographa, and Lecanora genera of lichens, folium from Chrozophora tinctoria or anthraquinonic dyes such as kermes or madder, these last possibly in double dyeing with indigo. Anyway, since most of these analyses were performed in situ with portable techniques, we have no clear information even in cases where alternative dyes were supposed to be present on parchment, since the non-invasive identification of orchil and folium is difficult. Very little spectroscopic information is available from the scientific literature (in very few instances these dyes were identified on artworks). From these considerations, it appears clear that more studies are needed to verify data from bibliographic sources. In this study the suitability of different analytical techniques in the characterisation of purple parchments is discussed. Particular concern is given to non-invasive vs. invasive and destructive approaches; advantages and limits of each approach are pointed out. The following techniques are taken into consideration: • Non-invasive approach o Optical microscopy o UV-visible Diffuse Reflectance Spectrophotometry with optic fibres (FORS); o Spectrofluorimetry; o X-ray Fluorescence Spectrometry (XRF); • Non-destructive but invasive approach o Fourier Transform – Infrared Spectrophotometry in Attenuated Total Reflection mode (FT-IR-ATR); o Fourier Transform – Raman Spectroscopy; o Surface Enhanced Raman Spectroscopy (SERS) • Invasive and destructive approach o High Performance Liquid Chromatography coupled with Mass Spectrometry (HPLC-MS); o Nuclear Magnetic Resonance (NMR). The above cited techniques have been applied to the analysis of purple parchments of some codices. The manuscripts involved are datable from 6th to 8th century: they are known as Vienna Genesis (Vienna, Österreichische Nationalbibliothek), Krönungevangeliar (Vienna, Kunsthistorisches Museum Vienna, Treasury), Evangeliario di Sarezzano (Tortona, Archivio Diocesano) and Codex Brixianus (Brescia, Biblioteca Queriniana). In situ measurements were performed on all manuscripts with portable instrumentations, using therefore only the non-invasive approach. In the case of Codex Brixianus, a small sample of purple parchment was taken and this allowed performing also invasive and destructive analyses with all the techniques cited. In this way, the results obtained from the Codex Brixianus could be extended to the other manuscripts analysed only in situ.