Abstract
Recently, historical and conservation studies have attached an increasing importance to investigating the materials used in historic documents. In particular, the identification of the animal species from which parchments are made is of high importance and is currently performed by either genetic or proteomic methods. Here, we introduce an innovative, non-invasive optical method for identifying animal species based on light-parchment interaction. The method relies on conservation of light energy through reflection, transmission and absorption from the sample, as well as on statistical processing of the collected optical data. Measurements are performed from ultraviolet (UV) to near-infrared (NIR) spectral ranges by a standard spectrophotometer and data are processed by Principal Component Analysis (PCA). PCA data from modern parchments, made of sheep, calf and goat skins, are used as a database for PCA analysis of historical parchments. Using only the first two principal components (PCs), the method confirmed visual diagnostics about parchment appearance and aging, and was able to recognise the origin species of historical parchment of among database clusters. Furthermore, taking into account the whole set of PCs, species identification was achieved, with all results matching perfectly their proteomic counterparts used for method assessment. The validated method compares favourably with genetic and proteomic methods used for the same purpose. In addition to animals’ proteomic and genetic signatures, a unique “optical fingerprint” of the parchments’ origin species is revealed here. This new method is non-invasive, straightforward to implement, potentially cheap and accessible to scholars and conservators, with minimal training. In the context of cultural heritage, the method could help solving questions related to parchment production and, more generally, medieval writing production.
Highlights
The knowledge of the animal origin of parchment folios is a question of great interest in codicology: a discipline that, traditionally, studies the materiality of manuscript books, i.e. codices[1]
In order to show the consistency of the method, we measured the following on a set of modern parchments (21 samples) and historical parchments (20 samples) – diffuse reflectance and transmittance (Rd, Td), hemispherical reflectance and transmittance (R, T) and absorbance (A) over a broad wavelength (λ) range, from UV (200 nm) to NIR (2350 nm), using a standard spectrophotometer equipped with an integrating sphere (Fig. 1b)
In order to prove that this method is able to recover the underlying physics of light-parchment interaction, spectral data collected from all modern and historical parchments were organized in a data matrix X of size M × N, where M is the number of observations and N is the number of variables
Summary
The knowledge of the animal origin of parchment folios is a question of great interest in codicology: a discipline that, traditionally, studies the materiality of manuscript books, i.e. codices[1]. The most frequently used animals are goat, calf and sheep[7] At macroscopic scale, both sides of the parchment are distinguishable (Fig. 1a), at least, in good quality parchments[8,9]; on the grain side, hair follicles are visible, whereas, on the flesh side, the surface looks more homogeneous. Optical methods are intrinsically non-invasive and require no preparation They are already employed to evaluate the stage of degradation of parchments[17,18] or to distinguish between modern, historical and artificially aged parchments[19]. Our non-invasive method can be made portable by using a fibre optic photospectrometer and standard optical accessories It can be used by trained people in libraries and museums. Those advantages are very attractive in cultural heritage science, where fragile and precious manuscripts need to be examined under stringent conditions, in order to preserve them
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