# Research on the Organic Binders in Archaeological Wall Paintings
## 3 Techniques and Procedures to Characterize the Organic Binders in Wall Paintings
It has been used for many years in the study of cultural heritage, especially for the examination of materials of inorganic nature, but considered potentially usable for the study of organic media, as exposed by some papers carried out on pure materials and pigment/binder mixtures [28]. Some authors consider Raman microscopy the ideal technique for the investigation of materials used on works of art because it is very sensitive, reliable, specific, nondestructive, with high spatial resolution (typically ranging between 1 and 10 \(\upmu\)m) and can be used in situ, therefore avoiding any sampling and consequently any damage to the item under investigation [29]. Integrated multispectral imaging systems. This type of imaging system is based on the selection of specific wavelengths and the acquisition of black and white images, in order to underline the presence of non-homogeneous regions in the surface This technique provides non-invasive mapping and classification of materials with different chemical and optical properties. It is mentioned here, because it is very interesting, but there are no known applications that have allowed to characterize organic binders [30]. * Fiber optic reflectance spectroscopy (FORS). The FORS technique gives information about materials by analyzing their optical properties [31]. Very often to complete the study it is necessary to obtain specific information about chemical composition, the presence of degradation products or information about the stratigraphic distribution of organic binding media in the wall paintings. For this reason, after a general investigation performed by non-invasive methods, the taking of samples is necessary, in order to carry out a detailed characterization of analytes. For this reason, when we decide to arrive at an in-depth characterization of the organic binder, it is necessary to resort to laboratory investigation techniques on pictorial material, thus having to provide for a sampling of material from wall painting. When dealing with sampling of the paint fragments you need to be aware of important issues. A relevant limit in the study of organic materials in wall paintings is represented by the low quantity of these, compared to inorganic materials (mortar and pigments). Since not more than 1 mg of sample is generally obtained from sampling and being the organic fraction lower than the inorganic one (about 10% \(w\)/\(w\)), sometimes the quantification of analytes results very difficult, by obtaining a low signal to noise ratio and signals lower than the instrumental detection limit. Moreover, the possibility that organic components could be present in mixture and that they could be subjected to degradation phenomena in relation to their stability in time, their detection results very complex, above all if only some traces are present. Finally, the presence of materials used in past conservative interventions, i.e., fixatives based on egg, can alter the original composition of the paintings, and makes the identification of original organic materials more difficult. The selection of the most suitable regions where to take the samples requires a lot of attention and should be performed after careful observation of the surface, in relation to the information to be obtained. This is why the use of several complementary analytic techniques may be necessary. Further and more in- depth characterization of organic fraction contained in samples can be obtained by different invasive-techniques, some of which can be summarized as follows:  
* Fourier transform infrared spectroscopy (FTIR). The FTIR spectroscopy is a simple and fast method to obtain information about the organic components (by identifying the class of materials present) and the inorganic materials, through the interpretation of the characteristic vibrational modes of the functional groups when they interact with NIR light [32; 33]. Such technique can be used in transmission or attenuated total reflectance (ATR) mode, in relation to the physical morphological properties of samples.