Identification of copper carboxylates as degradation residues on an ancient manuscript

Copper carboxylates were identified as degradation residues on the surface of a metallic pigment used in the illumination and in the text of a 9th century Italian manuscript containing the Homilies on the Gospels of Gregory the Great, belonging to the Archive and Chapter Library of Vercelli (Italy). These compounds are responsible of the greenish aspect that several text lines and decorated initials have developed over time from their original golden texture, starting frommore than a century as could be guessed from the early 20th century descriptions of themanuscript by art historians. Further, Raman investigations carried out on a particle recovered from the gutters between folios allowed (1) the identification with good accuracy of the nature of these compounds, confirmed by scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, (2) proposing a hypothesis on their origin as due to the interaction of copper with carboxylic acids and (3)making suggestions on the proper restoration intervention; moreover, evidence was found for similar compounds in three 9th–10th century Italian manuscripts coming from Bobbio abbey. Besides, the presence of orpiment in ink composition was verified and compared with citations in somemedieval texts.