Colour degradation in medieval manuscripts

The conservation of medieval manuscript illuminations: A chemical perspective

Illuminated manuscripts are the most abundant and well-preserved surviving medieval cultural artefacts. Created to contain sacred texts, their visual structure allows the reader to identify divisions and delight in their beautiful ornamentation and iconography. Western European manuscripts were written on parchment, which was the main writing support in the Middle Ages prior to the rise of paper production in the fourteenth–fifteenth centuries. Highly functional, durable animal skins were also used in medieval bookbinding (covers) and have been crucial in the preservation of the illuminations. These illuminations make wonderful use of form and colour. This chapter focuses on recent advances in the molecular characterization of these colours, used in medieval manuscripts produced by Western Europeans, and the challenges inherent in analysing materials that are intrinsically heterogeneous. We then discuss how molecular characterization may reveal conservation conditions and extend our analysis to parchment and iron gall inks. Finally, we address the challenges and possibilities for this flourishing field of research.

In short, we show in this chapter how analytical methods are used:

– To understand how medieval illumination colours were made and what palette defines them;

– To predict original colours and to understand why certain colours have remained in excellent condition, preserving both adherence and luminosity, while others have changed over the centuries;

– To assess deterioration (paints, parchment, writing inks) with the goal of determining what potential restoration measures could be taken, after careful consideration.

New Insights into Synthetic Copper Greens: The Search for Specific Signatures by Raman and Infrared Spectroscopy for Their Characterization in Medieval Artworks

A systematic investigation of medieval copper green pigments was carried out based on written sources: 21 manuscripts, dating from 50–70 to 1755 AD, were sourced and 77 recipes were selected, translating into 44 experiments. Reconstructions from medieval recipes were prepared and characterized through a multianalytical approach to disclose the original pigment formulation that is often described as verdigris. Based on the results obtained, we propose three main groups of copper green pigments, group 1, in which only Cu(CH3COO)2·H2O is formed; group 2, where this acetate is found together with copper oxalates; group 3, in which atacamite is present as the major green component or as a signature compound. The products formed are in perfect agreement with that predicted by the state-of-the-art research on the mechanisms of atmospheric corrosion of copper. This knowledge, together with our experience on craft recipes to prepare medieval paint materials, allowed us to recover a lost medieval recipe to produce a copper green pigment based mainly on atacamite, a basic copper chloride, which has been recently detected, by Raman and infrared spectroscopy, in artworks ranging from Catalonia and the Crown of Aragon panel painting to Islamic manuscripts.

Photochemistry of Artists' Dyes and Pigments: Towards Better Understanding and Prevention of Colour Change in Works of Art

The absorption of light gives a pigment its colour and its reason for being, but it also creates excited states, that is, new molecules with an energy excess that can be dissipated through degradation pathways. Photodegradation processes provoke long-term, cumulative and irreversible colour changes (fading, darkening, blanching) of which the prediction and prevention are challenging tasks. Of all the environmental risks that affect heritage materials, light exposure is the only one that cannot be controlled without any impact on the optimal display of the exhibit. Light-induced alterations are not only associated with the pigment itself but also with its interactions with support/binder and, in turn, are further complicated by the nature of the environmental conditions. In this Minireview we investigate how chemistry, encompassing multi-scale analytical investigations of works of art, computational modelling and physical and chemical studies contributes to improve our prediction of artwork appearance before degradation and to establish effective preventive conservation strategies.

Comparing the effectiveness of hyperspectral imaging and Raman spectroscopy: a case study on Armenian manuscripts

There is great practical and scholarly interest in the identification of pigments in works of art. This paper compares the effectiveness of the widely used Raman Spectroscopy (RS), with hyperspectral imaging (HSI), a reflectance imaging technique, to evaluate the reliability of HSI for the identification of pigments in historic works of art and to ascertain if there are any benefits from using HSI or a combination of both. We undertook a case study based on six Armenian illuminated manuscripts (eleventh-eighteenth centuries CE) in the Bodleian Library, University of Oxford. RS, and HSI (380-1000 nm) were both used to analyse the same 10 folios, with the data then used to test the accuracy and efficiency of HSI against the known results from RS using reflectance spectra reference databases compiled by us for the project. HSI over the wavelength range 380-1000 nm agreed with RS at best 93% of the time, and performance was enhanced using the SFF algorithm and by using a database with many similarities to the articles under analysis. HSI is significantly quicker at scanning large areas, and can be used alongside RS to identify and map large areas of pigment more efficiently than RS alone. HSI therefore has potential for improving the speed of pigment identification across manuscript folios and artwork but must be used in conjunction with a technique such as RS.

Organic dyes in illuminated manuscripts: a unique cultural and historic record

In this study, we successfully addressed the challenges posed by the identification of dyes in medieval illuminations. Brazilwood pigment lakes and orcein purple colours were unequivocally identified in illuminated manuscripts dated by art historians to be from the thirteenth to the fifteenth centuries and in the Fernão Vaz Dourado Atlas (sixteenth century). All three works were on a parchment support. This was possible by combining Raman microscopy and surface-enhanced Raman spectroscopy with microspectrofluorimetry. To the best of our knowledge, this is the first time that brazilein, the main chromophore in brazilwood lake pigments, has been unequivocally identified by surface-enhanced Raman spectroscopy in an illuminated work (the Dourado Atlas). Complementing this identification, through microspectrofluorimetry and micro-Fourier transform infrared spectroscopy, it was possible to propose a complete paint formulation by comparison with our database of references; the dark pink hues, in the three case studies, were produced by combining brazilwood pigment lakes and gypsum in a protein- and gum arabic-based tempera. Orcein purple, also known as orchil dye, has been previously identified in medieval manuscripts, dated from the sixth to the ninth centuries. Our findings in fourteenth-sixteenth century manuscripts confirm the hypothesis that this dye was lost during the High Middle Ages, to be later rediscovered.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.