Silver paints in medieval manuscripts: a first molecular survey into their degradation

The lack of comprehensive studies on silver paints in manuscripts is a handicap to new stabilization strategies and treatments. This work develops and tests a methodological approach for the study of the degradation of silver paints, at the molecular level, using as a case study a French Book of Hours dated from the 15th century from Mafra National Palace collection. In this paper, we show that the global equation commonly used by the cultural heritage community to describe the tarnishing of silver proves to be insufficient to understand the main factor causing its severe and extensive darkening in medieval illuminated manuscripts. This equation depicts the degradation of silver instantly with a sulphide containing species as H₂S. Recent research has solidly proved that the first intermediate of silver oxidation is Ag₂O. This theme will be presented in a forthcoming review. Within this framework, we selected four typologies of silver paints, all analysed by in situ techniques and four as microsamples; due to the very small size and fragility of the microsamples, new methods of sample preparation were developed and tested. The main research questions include: (1) understanding how silver-based colour are formed, disclosing its full stratigraphy; (2) define if the other compounds present, such as colour paints and the ground, may affect silver oxidation by stabilizing or accelerating it; (3) comprehend if it is possible to characterize the degradation products of silver paints with the methodology developed in this work, both in the bulk and at the interfaces between layers. Overall our results show that silver-based colours exhibit a complex stratigraphy that impacts its conservation condition. Further research at the molecular level is required on the correlation of the main components of the ground (binder and filler such as gypsum and calcium carbonate) and the paints that are applied as layers over or under the silver leaf. Our case studies have shown that both in situ analytical techniques and scanning electron microscopy with energy dispersive spectroscopy analysis in the microsamples, provide relevant information on the silver colour stratigraphy. However, the information obtained studying cross-sections is more accurate. We consider that the study of the degradation mechanism of silver in medieval illuminated manuscripts will require the combination of both approaches; in situ methodologies support the relevance of the data acquired on the microsamples. Microsamples studies will make possible the acquisition of data that is fundamental to understand the darkening of silver paints and this, in turn, is essential to develop informed and sustainable conservation policies.