Academic journal article Heritage Science

Vibrational Spectroscopy for the Study of Chilean Cultural Heritage

Academic journal article Heritage Science

Vibrational Spectroscopy for the Study of Chilean Cultural Heritage

Article excerpt

Authors: Marcelo M Vallette Campos (corresponding author) (equal contributor) [1]; Tom?s Alvarado Aguayo (equal contributor) [1]

Background

Techniques that involve micro-sampling or non-destructive are important aspects to consider when dealing with cultural heritage and unique objects while pursuing characterization through analyses of ancient samples [1]?[4]. Raman spectroscopy is a powerful technique for the analysis of different kinds of materials, showing advantages related to the specificity, sensitivity, reproducibility, applicability, mobility and resolution (spatial and spectral) [5]; moreover, it is a non-invasive and non-destructive technique by itself. These advantages, coupled with recent developments in instrumentation and techniques, particularly the surface enhanced Raman spectroscopy (SERS), have made it possible to extend its use in archaeometry and conservation [5]?[11]. Raman spectroscopy allowed identifying pigments and dyes used in the preparation of manuscripts, paintings, ceramics and textiles [12]?[17]. Its meaningful disadvantage lies in the formation of fluorescence, which is an accompanying phenomenon in the measurements of diverse materials; the nanostructured metal surfaces in the SERS technique, and the adequate use of specific lasers reduce, or totally quench the fluorescence.

Infrared spectroscopy (FTIR) has been widely present in the study of cultural heritage and it has become a permanent part of the instrumental techniques used in routine inspection of objects and materials related to the conservation of cultural objects. It is one of the most appropriate techniques to identify organic compounds presents in most several materials (e. g. binding media, varnishes, adhesives, etc.). It does, however, present some disadvantages (i.e. water absorption) related to the experimental procedures. Classically, a considerable amount of sample was required to disperse in an ?IR transparent? material, such as KBr or NaCl, to form a pellet from where the information was obtained. This resulted, in most cases, in the sacrifice of the sample in order to obtain good quality results. Fortunately, instrumental advances have allowed obtaining the same information with fewer amounts of sample and practically without preparation. One of these advances is the use of attenuated total reflection (ATR-FTIR) sampling method, in which a small fragment of the sample is pressed against a crystal used as a medium to make the IR radiation to interact with the material. Despite this, in most of the cases the original form of the sample is modified somehow. FTIR besides being micro-destructive to the object is also destructive to the sample.

The present work deals with our last results concerning the spectroscopic studies, mainly vibrational Raman and infrared, of different materials in the Chilean cultural heritage. The interest is focused on archaeological samples [18], [19], Diaguita pottery [20], mural painting [12], painted beams [21] and historical silks [22].

Results and discussion

Studied objects, selected to be presented in this work, represent a wide sample of the materials found in the Chilean cultural heritage. The findings allow us to show the versatility of the vibrational tools used, as well as to help improving the contexts in which the objects are/were in each case. Also it gives us the opportunity to share knowledge and to give value to objects that not always are of the public domain. The specific insights on each object are presented in the following sections.

Archaeological pigments

Samples of yellow pigment blocks from the archaeological site Playa Miller 7 (PLM7) of the Formative Period (3700?1500 years B.P.) on the northern coast of Chile were studied using micro-Raman data [see Additional file 1] [18]. Samples from complete blocks, detached fragments or powder were chosen. Yellow blocks (Fig.?1a, b), from the archaeological site consist mainly of a finely ground pigment. …

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