New insight in the making of purple codices

The purple codices are biblical texts written with gold or silver inks on parchment dyed or painted with purple colorants. These manuscripts were produced in Late Antique and early Medieval times [1], most probably to be used in a royal or imperial context if we consider the strong symbolic value of the purple colour in association with noble metals. The making of these highly precious manuscripts, though, still represents a mystery from the scientific point of view. Even if the use of purple dyes or paints to impart colour to parchment of precious codices is mentioned since Roman times, a clear description of the materials and of the procedures used can hardly be found in the ancient technical treatises. According to the high symbolic value of purple codices, it has been hypothesised on historical basis that parchment had been dyed with Tyrian purple, the famous dye extracted from molluscs. Indeed, few chemical analyses have been carried out on purple codices [2-8], mostly by means of non-invasive measurements, and in no case the evidence of the use of Tyrian purple was given. In order to increase the present knowledge on the making of these masterpieces of the ancient book production, a mixed approach involving both non-invasive and destructive measurements has been applied to the study of a small fragment of purple parchment detached from the Codex Brixianus, a purple codex datable to the 6th century A.D. kept in the Biblioteca Queriniana at Brescia (Lombardy, Northern Italy). The sample was taken from a portion of folio that was formerly torn off from the book. Non-invasive measurements included UV-visible Diffuse Reflectance Spectrophotometry with optic fibres (FORS), Spectrofluorimetry and X-ray Fluorescence Spectrometry (XRF). Results indicated that orchil, the dye extracted from Roccella, Dendrographa and Lecanora genera of lichens, and/or folium, the dye extracted from Chrozophora tinctoria, are present on the parchment rather than the expected Tyrian purple. Bromine was identified by means of XRF but this element cannot be considered as a marker for Tyrian purple since it has been evidenced that both lichens and Chrozophora tinctoria contain it in significant amounts and in a water-soluble form. The availability of a micro-sample of parchment allowed the use of more powerful techniques. Surface-Enhanced Raman Spectroscopy (SERS) was applied on a very small fragment (< 1 mm) upon hydrolysis with formic acid in order to desorb the dye from the parchment; orchil was identified. On a portion of similar size a MALDI-TOF-MS analysis was performed, again upon hydrolysis with formic acid. In this case the results seem to indicate that both orchil and folium could be present, so that further investigation by HPLC-DAD-MS will be needed as a confirmation. Finally, a cross-section of parchment was prepared in order to verify the penetration of the colourants used and to yield information useful to elucidate how the parchment had been coloured. It appears that the parchment of Codex Brixianus has been coloured by a method that penetrated the entire skin. This suggests a dyeing technique, though the thinness of the parchment (less than 100 µm) could also allow complete penetration by the colour when painted onto the surface. These results must be confirmed, of course, by analysis on other purple codices.