Evidence for the degradation of an alloy pigment on an ancient Italian manuscript

An unusual metallic pigment was identified on an ancient Italian manuscript containing the Homilies on the Gospels of Gregory the Great, belonging to the Archive and Chapter Library of Vercelli (Italy) and datable to the IX century. Green uncial text and part of decoration on this manuscript were described as ‘being of golden fashion’ in a book written at the beginning of XX century. Investigations carried out with portable XRF and Raman instruments revealed that the green colouration is actually the result of a marked degradation process that has altered the original composition of the pigment, an alloy of copper, lead and zinc. Probably because of the conditions of storage of the manuscript, the pigment tarnished owing a chemical process that could resemble the infamous ‘bronze disease’: that is, the conversion of metallic copper into inorganic salts. Measurements were performed both in situ, i.e. directly on the manuscript, and on small fragments of ink collected from the page gutters.