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

Abstract

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 H 2 S. Recent research has solidly proved that the first intermediate of silver oxidation is Ag 2 O. 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.

Between past and future: Advanced studies of ancient colours to safeguard cultural heritage and new sustainable applications

“…The same holds for silver-based colours, profusely applied in the books of hours [56]. In both cases, we found black sulfide-based compounds as final products, such as PbS and Ag 2 S [46][47][48]. ii) Loss of cohesion and adhesion of the pictorial layer to the support, exemplified by bottle-green and lapis lazuli blue [38], Fig.…”

“…5. The former enables one to quantify the binders and additives, along with its conservation condition [38,[44][45][46][47][48]. Microspectrofluorimetry enables the acquisition of emission and excitation spectra in the same micro spot (in situ without any contact with the sample or work of art being analysed), which is decisive when using molecular fluorescence for dye identification.…”

Characterization of Pigments and Materials of the Original Body and Additions of the Book of Hours 50,1,1 from the Brazilian National Library

“…In this manuscript, silver, detected by XRF, was extensively applied in liquid form: in the representation of the metallic shine of weapons and armor, Figure 20(a), of light entering through windows, and in details of frames, Figure 20(b) -however, it is tarnished in most areas due to an oxidation process, probably forming silver sulfide (Ag 2 S) (Araújo et al, 2018).…”

Manuscripts in the Making: Art and Science

“…A direct link also exists between research by Araújo et al into the degradation of silver paint [11] and the investigation of a mid-fifteenth-century French Book of Hours (Cofre no 31 from the National Palace in Mafra, Portugal), reported as a case study by Carvalho et al [12]. Additional case studies include two articles published by separate research groups, one in France (Denoël et al [13]) and one in Austria (Frühmann et al [14]), discussing eighth-and ninth-century manuscripts from Carolingian scriptoria; the Emerson-White Book of Hours, whose late fifteenth-century Flemish illuminations were investigated by Mayer et al [15]; and a study of the materials used in other parts of manuscripts, such as the decoration of the sunk-panel binding of a Venetian 'Commissione Dogale' , reported by Rampazzo and Di Foggia [16].…”