Abstract
This work presents the first proof of concept for the use of molecular fluorescence signatures in medieval colours based on lac dye and brazilwood lake pigments. These two important medieval dyes were tested as markers using their UV–Visible emission and excitation spectra. These medieval paints had been previously fully characterized through a multi-analytical approach. In this work, molecular fluorescence spectra were acquired in manuscripts dating from 12th to 15th c., which were produced in monastic scriptoria or workshops. First, the spectral distribution and relative intensity of the emission and excitation spectra were discussed in detail by comparison with reference compounds, including reproductions of paints based on medieval technical texts. It was possible to group the spectra according to recipe specificities. Then, statistical methods (principal component analysis and hierarchical cluster analysis) were applied to the same fluorescence spectra and the generated clusters were compared with the previous ones. Principal component analysis was initially employed to eliminate redundancy in fluorescence data, so minimizing bias on the hierarchical cluster analysis results. Except for some misplaced spectra, the placement of samples per group was confirmed. The outliers resulted from either a poor signal to noise ratio or occurred because certain paints were unique, such as the colour produced by mixing lac dye and brazilwood, which was found in manuscripts from the Alcobaça monastic scriptorium. Previously, by using infrared or Raman spectroscopies, only lac dye could be detected. Notably, these paints compare well with a recipe that was reproduced from the text by Jean Le Begue, in which both dyes were required.
Highlights
Colour, a fundamental attribute of our heritage, is fading in precious artworks
It should be noted that most of the molecular fluorescence spectra obtained in medieval manuscripts belong to one of these two main groups, lac dye (Lac) 1 and Lac 3, or have some characteristics of one or both as will be discussed below, Fig. 4
Based on our knowledge of the reconstruction of medieval paints, we anticipated that lac dye formulations would be more complex than brazilwood lake pigments
Summary
A fundamental attribute of our heritage, is fading in precious artworks. in the last decade, progress has been made in understanding the complex mechanisms of degradation of historical dyes anchored in the Nabais et al Herit Sci (2021) 9:32and excitation data acquired in medieval illuminations for lac dye and brazilwood chromophores, Figs. 1, 2, 3. What motivates us to test a reference database based on original medieval colours, and why lac dye and brazilwood paints [25, 26]? Our database combines spectral information acquired in manuscripts preserved in Portuguese collections, having a Portuguese, French, or Flemish provenance, and dating from the 12th c. Molecular fluorescence spectra were acquired both in situ and in micro-samples (invisible to the naked eye, see in the Experimental Section for the average dimensions) We tested both sets and, we had a greater number of spectra acquired in situ, we chose to present in this work the data acquired in microsamples, as it is possible to guarantee a greater reproducibility in this set of samples
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