An overview of principal components analysis approaches in raman studies of cultural heritage materials

The present overview answers the need of assessing the current state of the art concerning the application of principal components analysis (PCA) to Raman spectroscopy investigations of cultural heritage and related materials. An increment of the employment of this multivariate statistic technique to Raman results in the mentioned field began between 15 and 10 years ago, after a very slow start at the turn of the millennium. A delay of about a decade was observed with respect to PCA applied to elemental quantitative data of archaeometric analyses, likely a consequence of the required spectral pre-treatment and to results of complex interpretation. Therefore, it is by now the time to summarize this evolution in a comprehensive, yet very specific way. In this overview, painting constituents were considered, both colouring materials and binders, in addition to natural and synthetic glasses, and biogenic and mineral gemmological materials. A marked unbalance between the studies pertaining to the different sections has been noticed, revealing a concentration of the work mainly on painting materials, including the study of ageing and alteration. The different aims of PCA application to Raman spectra, the various approaches and the achievable results, with the possible arising problems, were underlined, too. Special attention was given to the pre-treatment of the spectra, which was observed to be essential to overcome the influence of several issues concerning bands intensity, spectral noise, background, fluorescence and so on

Pigments Checker version 3.0, a handy set for conservation scientists: A free online Raman spectra database

Pigments Checker is a collection of swatches of historical and modern pigments that offers art professionals, conservation scientists, conservators and fine art photographers, a tool to evaluate and test their imaging and spectroscopic methodologies for pigment identification. "Pigments Checker Free Spectra Database" is an ongoing project to thoroughly characterize each pigment in the collection with a series of spectroscopic and imaging techniques and to make the data open access. This paper presents the free and downloadable database of Raman spectra, adding to the reflectance spectral database already published. The Raman analysis is in agreement with the information provided by the pigments' manufacturers since most of the pigments could be identified by their Raman spectra which were consistent with the expected content reported in literature

CHARACTERIZATION OF SILK-COTTON AND WOOL-COTTON BLENDS PATTERN BOOKS BY FIBER OPTICS REFLECTANCE SPECTROSCOPY. THE BOOMING MARKET OF FIRST SYNTHETIC TEXTILE DYES IN EARLY 20th CENTURY

A remarkable number of early 20th century azo synthetic textile dyes was characterized by Fiber Optics Reflectance Spectroscopy (FORS). Items are part of silk-cotton and wool-cotton blends pattern books of Leopold Cassella & Co., a leading firm at that time. Focusing on early dyes from this privileged point of view, meant for us providing our own scientific contribution to restore luster to a historical period that represented the keystone from upmarket natural dyes to synthetic ones on a large scale. A selection of spectra on silk-cotton blend with the same dye concentration was compared color by color. In the case of wool-cotton blend textiles, two concentrations were available for each dye. Therefore, spectra comparisons between the same dyes used in silk-cotton and wool-cotton blends were carried out, as well as between the same dyes in different concentration in the case of wool-cotton textiles, simulating possible fading conditions. In addition, almost each analyzed dye was linked to its molecular structure. Finally, the obtained set of data has been statistically treated. The complete dataset was subjected to an exploratory analysis using PCA that exhibited an organization of the samples based on brightness and colors. The generation and validation of a PLS-DA model confirmed the recognizability of the samples based on color and pointed out the excellent conservation conditions implemented. The method provides a non-invasive, fast and low-cost approach for the recognition of azo dyes, based on the combined application of FORS, colorimetric parameters and chemometrics and lays the foundation for a reference database

Raman spectroscopy potentiality in the study of geopolymers reaction degree

AbstractAlkali‐activated materials (AAMs) and "geopolymers" are inorganic polymeric materials obtained by mixing of solid aluminosilicate precursors with an alkaline solution (generally, KOH or NaOH and Na2SiO3 mixed in various ratios). This class of aluminosilicate materials has emerged as a greener alternative to traditional concrete, for large‐scale as well as for niche applications such as conservation and restoration of built heritage. In this work we apply Raman spectroscopy both to aluminosilicate precursors (metakaolin, pumice, volcanic ash, volcanic soils, clayey sediments, ceramic waste) and to the respective AAMs. In the field of vibrational spectroscopy, Raman is much less employed in the literature with respect to Fourier transform infrared (FTIR) to have insights into the alkali activation process from a molecular point of view. The aim of this paper is to investigate the potentiality of a Raman approach to the comparison of the employed raw materials with the respective AAMs. Raman analyses during the first hours of geopolymerization were also carried out on the clayey sediments and ceramic waste‐based products. The results, differentiated according to the employed precursors, exhibit spectra relative to crystalline and amorphous phases that can give an indication about the newly formed aluminosilicate gel

Raman studies on zoisite and tanzanite for gemmological applications

AbstractTanzanite is the blue to violet‐blue variety of the sorosilicate zoisite; its colour is due to vanadium substitution of aluminium in the octahedral sites and is proved to appear after natural or artificial heating to approximately 500°C. Its colour makes it a highly appreciated gemstone, for this reason often imitated or thermally treated with the aim of enhancing its hue. Tanzanite loose gemstones are easily identified by classical gemmology methods that are not always applicable to mounted jewels. At the same time, zoisite Raman spectra are rarely reported in literature and never with a systematic approach. In this work, zoisite samples and cut tanzanite gemstones were considered, with the aim of filling this gap by addressing different methodological aspects. Namely, the orientational effects were investigated to explain the spectral variability, the photoluminescence bands were differentiated from the Raman signals using different excitation wavelengths, and the effectiveness of portable instrumentation in the correct identification of tanzanite was evaluated. The results are encouraging, showing that zoisite is revealed, notwithstanding orientational effects, by both portable and laboratory Raman instruments with comparable performances, opening the way to an effective identification of mounted tanzanite gemstones. Furthermore, the employ of different excitation wavelengths allowed to distinguish the photoluminescence bands with both categories of devices, thus identifying rare earth elements (REEs) likely associated with the mineral's genesis

Blue coloured haüyne from Mt. Vulture (Italy) volcanic rocks: {SEM}-{EDS} and Raman investigation of natural and heated crystals

Haüyne mineral, belonging to sodalite group, characterizes two lithotypes of Mount Vulture area (Italy): phonolite of Toppo San Paolo and haüynophyre of Melfi. In the latter, haüyne phenocrysts can appear colourless-white, grey-to black or blue in colour. The blue colour of these crystals—sometimes in literature classified as lazurite—is known to be due to the presence of S 3 polysulphides radical anion chromophores. Since it has been previously observed that heating these rocks up to 750 C in oxidizing conditions leads haüyne crystals to acquire the blue colour, it was decided to investigate the reasons of this chromatic transformation. SEM-EDS and Raman spot microanalyses were performed on single crystals of the pristine rocks; furthermore, detailed X-ray and Raman maps were carried out on two white and two black haüyne crystals of haüynophyre before and after heating up to 750 C. This allowed to investigate the role of very fine inclusions in the chromatic change: the sulphur-, barium- and iron-based species included in haüyne, principally ‘pyrrhotite like’ crystals and baryte, with the increasing of temperature and under control of oxygen and sulphur fugacity, give rise to the formation of new phases, chiefly haematite and new baryte crystals and of S3 groups

A Possible Natural and Inexpensive Substitute for Lapis Lazuli in the Frederick II Era: The Finding of Haüyne in Blue Lead-Tin Glazed Pottery from Melfi Castle (Italy)

The blue color of glass and ceramic glazes produced in Apulia and Basilicata (Southern Italy) between the 13th and 14th centuries and connected to the Norman-Swabian Emperor Frederick II, has been, for a long time, under archaeometric investigation. On the one hand, it has usually been associated with lapis lazuli, due to the finding of the polysulphide blue chromophores typical of lazurite. Moreover, the observation that the mineral haüyne, which belongs to the sodalite group as well as lazurite, can be blue and/or can gain a blue color after heating, due to the same chromophores, has caused this automatic attribution to be questioned, and also considering that the mineral is characteristic of the rock haüynophyre of Melfi (Potenza, Southern Italy), a location of interest for glass and pottery findings. In this paper, for the first time, several haüyne crystals were found in the blue glaze of a ceramic dish found at Melfi Castle, leading to the hypothesis that, in this case, the local haüyne-bearing source could have been used as the coloring raw material. The discovery was possible thanks to SEM-EDS and Raman analyses that, respectively, highlighted the typical numerous presence of very fine sulphur-based inclusions in the crystals and the characteristic Raman signal of blue haüyne. This study was also focused on the composition of the crystals inclusions, aided by SEM-EDS and Raman maps, since the original very fine pyrrhotite was transformed into Cu and Pb phases (copper sulphates, copper sulphides, and lead oxide) due to reactions with cations that had mobilized from the glaze, while the migration of Si from the glass allowed the transformation of the rim of the haüyne, a silica-undersaturated mineral, into a corona of small euhedral and neomorphic Pb-rich feldspars, a silica-saturated phase

Physical-Chemical characterization of Sea-silk and its crafting phases

The sedentary mollusc Pinna nobilis L. produces fine but strong filaments of fibre beard called “byssus”. This fibre has the task of fixing itself to the sea grass and withstand the flow. Byssus was the basic raw material used to make Sea-silk, but this Mediterranean species is protected since 1992 [1] and production of its iridescent golden textile is therefore impossible. In the history of textile, Sea-silk constitutes a very small part, it is proven that the use of Sea-silk dates back to at least the Roman age while, today, Sardinia and Apulia (Taranto), are the production centres of Sea-silk and keep their importance. We studied samples of sea-silk kept at Commodity Science Museum of Bari University, where one valve, some pearls and all the different phases of crafting of this textile are represented, starting from the raw byssal threads up to the woven textile. The aim of this research is to characterize the threads from a morphological and chemical point of view considering above all the evolution and changing of the material with the different working phases. Due to the samples uniqueness, the smallest amount possible is taken from the byssal threads (about 20 mg). We used different techniques like optical microscopy (OM) and Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS) aimed to the observation of the morphological changing of the fibers examined in their different handmade production steps and ICP-MS for the determination of metal composition in some fibers after their mineralization and in deionized water, observing the metal lost characteristic of the desalination process. The results obtained show how the metal concentrations analysed, decrease after the different crafting steps, and the fiber of the Sea-silk change its microscopic aspect until to obtain the golden “soul of the sea”. [1] http://www.muschelseide.ch/it/biologie/byssus/faseranalyse.htm

“Indagini archeometriche su vasi della collezione Intesa Sanpaolo” Atti del Convegno. Giornata di Studi. SAVOIR-FAIRE ANTICHI E MODERNI Pittori e officine ceramiche nell’Apulia di V e IV secolo a.C.

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