1.1. Introduction
The study of objects of cultural heritage as particularly valuable products of human civilization development has been regarded as a matter of great importance. Once a prehistoric human discovered the first unusual artifact, a collecting phenomenon arose and the process of accumulating values created by skilled craftsmen of different eras began. Initially, scientific research in the area was stimulated by the need for restoration and understanding the degradation processes occurring in the objects of cultural heritage. As any human creation, whether ancient ceramics, jewelry, artwork, or architectural masterpiece, tends to deteriorate in course of time, the issue of scientific restoration becomes increasingly important. For successful restoration it is necessary to characterize the quantitative and qualitative changes that occurred in the material structure of the artifact under the influence of physical, chemical, biological, and other factors that determine the degree of its preservation or destruction. The researcher also needs to answer the questions related to the processes of the object creation and describe the substance from which the monument was made, determining its composition and mechanical properties, as well as the technical and technological methods used to create the artifact. The urgency of these tasks determines the growing need for analytical studies of cultural heritage objects.
Most of the objects of cultural heritage are unique and invaluable; therefore one of the main requirements for the study of such artifacts is the minimal destruction during sample preparation; ideally the sample preparation should be nondestructive. It is not surprising then, that sooner or later a microscope becomes one of the necessary elements of almost any system of research of cultural artifacts. It is not an absolutely nondestructive technique because usually it also requires sampling, but the sample in many cases is not completely destroyed during microscopic investigation and can be thoroughly studied by sequential application of different methods (Trentelman, 2017).
Basically, any optical microscope contains a source of illumination, a condenser system, an objective lens, an eyepiece, and an image recording system. The main characteristic of the microscope is its resolving powerâthe ability to give a separate image of two closely located points. The theoretical resolution of the light microscope is 0.2â0.3 Îźm when using the visible spectral region, and 0.13â0.14 Îźm when using light in the UV range (the resolution of the human eye is only 0.1 mm). The development of an electron microscope increased the possible resolving power to fractions of a nanometer. Another high-resolution microscopy system with a completely different principle of operation is atomic force microscopy; there the sample surface is scanned by a thin-tip probe, providing topographical information reaching incredible, up-to-atomic resolution. In addition, atomic force microscopy, as a sample-touching technique, provides nanomechanical data from the samples. There are other types of microscopy systems, but not all of them are currently used to study cultural artifacts.
In general, microscopic methods enable investigation of the structure of objects at various magnifications, but being integrated with other physical methods of analysis, they also allow study of the chemical composition of the objects, eventually drawing conclusions about the origin of the object or its initial components, method or time of its creation, author(s), purpose, authenticity, mechanisms of degradation, and ways of further conservation. At present, different kinds of microscopy can be applied to investigate a wide range of cultural heritage objects such as paintings, murals and frescoes, manuscripts and books, statues, precious stone products as well as various objects made of leather, fabric, stone, ceramics, glass, wood, or metal. One of the tendencies for future development in this area is associated with the desire for greater portability of research microscopic equipment, since many subjects of research cannot be moved to the laboratory, either because of their size or inseparability (for example, cave and mural paintings), or threat to their preservation. In this chapter we briefly overview the main microscopic methods used to study cultural heritage objects, indicating the scope of their application as well as the degree of portability. Here we omit detailed technical information on the functioning and manipulation with microscopes, which can be found elsewhere in numerous publications devoted to each particular microscopy technique. Instead, we try to introduce the reader to the application in cultural heritage nanoscale studies based on the use of various microscopic techniques, including the combinations of several techniques. Also, here we demonstrate several case studies for the application of microscopic methods for investigation of a series of cultural heritage objects found in the Republic of Tatarstan. We have successfully utilized correlated optical and scanning electron microscopies to investigate and characterize the following artifacts: the first case is Islamic medieval silver coins circulating in Volga Bulgary and Golden Horde during the 9th to 15th centuries AD. The second case is the fresco base (plaster substrate) and paint layer of a mural dating from the 16th century, preserved in the Orthodox ...