Markers, Reactions, and Interactions during the Aging of Pinus Resin Assessed by Raman Spectroscopy.

The resin extracted from the species of the Pinus genus (Pinaceae family) is a widely used material. Primarily, resins are made up of two types of diterpenoids: abietanes and pimaranes. Their composition changes with aging, affecting their chemical and physical properties; however, the chemical changes that occur during aging are not yet fully known. Understanding the evolution of pimaranes and abietanes and the chemical composition of the aged resins is essential to make the most of this substance and of its derivatives. A systematic study of the aging of Pinus resin with Raman complemented with infrared (IR) spectroscopy was carried out. This study provided new information about the interactions among the constituting molecules in resins aged over many years. In particular the formation of intermolecular hydrogen bonds in aged samples was detected for the first time, and the formation of acid anhydrides from the reaction between pimaranes was demonstrated. Furthermore, Raman and IR spectra band assignments are proposed, and the specific markers of the main compounds of the resin are tagged. This will facilitate the qualitative analysis of resin compounds.

Ageing of resin from Pinus species assessed by infrared spectroscopy.

Resins obtained from Pinus genus species have been widely used in very different fields throughout history. As soon as the resins are secreted, molecular changes start altering their chemical, mechanical and optical properties. The ageing processes are complex, and the chemical and structural changes associated with resin degradation are not yet fully known. Many questions still remain open, for instance changes happening in pimaranes, one of the two diterpenoid constituents of the resin. A systematic study of the ageing process of Pinus resins is done through Fourier transform infrared spectroscopy (FTIR) using chemical standards and complementing the obtained results with gas chromatography coupled to mass spectrometry (GC/MS) analysis when necessary. Moreover, long-term degradation processes are also investigated through the analysis of a selection of dated historical resins. This study overcomes the limitations of GC/MS and brings new information about the reactions and interactions between molecules during Pinus resin ageing processes. It also provides information about which bonds are affected and unaffected, and these can be used as specific markers of the degradation and of the resins themselves. Graphical Abstract Changes in the IR spectral features due to the Pinus resin ageing processes.

Optimal Sample Preparation for the Analysis of Micrometric Heterogeneous Samples.

Precise microanalytical techniques are essential in many fields such as cultural heritage materials, showing complex layered microstructures containing a wide range of materials of diverse nature and hardness. Noninvasive sample manipulation and preparation is required to avoid, as much as possible, sample contamination, which may strongly limit the materials identification. The method proposed consists in the application of thin gold or carbon protecting layers before embedding the samples in synthetic resin for microtoming. The validity and optimal procedure is checked for those materials most often found on the surface of paintings: varnishes (natural resins and wax). An artwork sample is similarly prepared and analyzed by optical microscopy (OM), scanning electron microscopy (SEM/EDS), micro-infrared spectroscopy (µFTIR/µSR-FTIR), and X-ray diffraction (µSR-XRD) with synchrotron light.

SR-XRD and SR-FTIR study of the alteration of silver foils in medieval paintings.

Altarpieces and polychrome carved wood from the fifteenth century AD usually exhibit golden and silvery areas by the application of a very thin foil of metal. The metal foils were normally protected from the atmosphere by a varnish or resin which maybe either preserved or absent. Moreover, they were glued to the background surface by adhesive substances (egg yolk, drying oil or animal glue). The high proportion of the glueing substances often renders the development of reaction compounds. With time, silver alters blacken or simply disappear completely. In this paper, we study the alterations to metal foils from a selection of fifteenth century artworks showing different glueing agents, organic coatings and several degrees of conservation of the organic coatings and metal leafs. The submillimetric layered structure and the high variability and low amount of most of the compounds present in the different layers, as well as their differing nature (organic and inorganic) make the use of micron-sensitive high-resolution techniques essential for their study. In particular, the high resolution, high brilliance and small footprint renders synchrotron radiation most adequate for their study. SR-XRD was performed to identify the reaction compounds formed in the different layers; µFTIR was used at to identify the silver protecting organic coatings, the metal foil glueing layers and the corresponding reaction compounds. The results obtained suggest that atmospheric corrosion is the dominant mechanism, and therefore that the degree of corrosion of the metal foils is mainly related to the conservation state of the protecting coatings.

Identification of reaction compounds in micrometric layers from gothic paintings using combined SR-XRD and SR-FTIR.

Synchrotron radiation X-ray diffraction (micro-SR-XRD) and Fourier transform infrared spectroscopy (micro-SR-FTIR) are used in the non-destructive identification of reaction and aging compounds from micrometric ancient painting layers. The combination of the micrometer size and non-destructive nature of the techniques together with the high resolution and brilliance of the synchrotron radiation has proved to be a procedure most advantageous for the study of reaction, aging and degradation processes. Copper, lead and calcium carboxylates and oxalates are determined in the chromatic, preparation and alteration layers from 15th century egg tempera and oil paintings. Their nature and crystallinity have been assessed. Some hypothesis about the mechanisms of development of both carboxylates and oxalates are presented.

Advantages of the use of SR-FT-IR microspectroscopy: applications to cultural heritage.

Synchrotron radiation Fourier transform infrared (SR-FT-IR) microspectroscopy represents an advance over conventional FT-IR spectroscopy because it gives a higher signal/noise ratio at the highest spatial resolution due to the high brightness and collimation of synchrotron radiation. It has been successfully applied to the study of ancient paintings, alteration and corrosion layers which are heterogeneous microlayered materials made by complex mixtures of organic and inorganic compounds. Moreover, the high brightness attribute allows FT-IR spectra to be routinely obtained directly on the surfaces of the objects and opens the possibility for nondestructive testing of museum objects. We present in this paper a selection of applications of SR-FT-IR to the analysis of ancient paintings, alteration and corrosion layers where the technique has proven to be especially useful: first, the separation and identification of pigment microparticles from ancient Roman wall paintings; second, the determination of the binding media and the byproducts resulting from the interaction between binders and pigments from medieval altarpieces; and third, the study of the surface corrosion layers of a bronze helmet by means of direct analysis of the surface.