## 1 Introduction  
It is known that the artificial and natural aging of writing inks printed on paper is an important forensic, criminalistics, and social problem. The most pressing issue in this area is establishing the actual age of a document or its parts since the most common way of document forgery is adding and writing extra inscriptions, signatures, and fake dates with ballpoint, gel, and technical pens. In a variety of cases, in order to hide the fact of falsification, documents are heated or exposed to artificial or natural light sources. Thus, the disclosure of such falsification is an important legal and social task [1; 2]. All writing inks consist of a dye component or a mixture of dyes, as well as a binding substance known as a vehicle composed of several solvents and additives responsible for the physicochemical properties of inks (including lubricants, corrosion inhibitors, emulsifying agents, buffers, surfactants, and various other components) [2; 3; 4]. All ingredients in the ink mixture can yield important analytical information to the forensic document examiner. Therefore, it is better to use analytical techniques that can identify the variety of chemicals from which writing inks are composed [4]. The following dyes are most commonly used as a part of blue writing ink: victoria blue (VB), rhodamine B and 6G, the methyl violet group of dyes (pararoasaniline with four, five or six methyl groups), crystal violet (CV), methyl violet (MV), tetramethyl-pararosaniline (TPR) and copper phthalocyanine, which has been used in ink dye manufacturing since 1954 [5]. Initially, oleic, castor, and mineral (petroleum) oils were used as solvents. Glycols have been predominantly used as solvents since 1950. In turn, the most widespread solvents currently used are glycerol, tri-, tetra-, pentaethylene glycols, fenoxyetanol, phenoxyetoxy ethanol, benzyl alcohol, 2-pyrrolidone and butylene glycol [2; 6]. The investigation of the aging processes of ink is a challenging task because one has to take into account that each compound begins to decompose after an ink is deposited on paper through different mechanisms. Thus, a variety of processes occurs simultaneously when writing inks are printed on paper. Writing inks dry due to solvent evaporation; they get dull due to dye degradation and solidify due to resin polymerization. From this perspective, the degradation of dye as a process, which is the most resistant to external influence, attracts the most interest. The mechanisms of the temporal degradation of dyes are described in detail elsewhere [7; 8; 9; 10; 11]. In order to analyze the events that occur during the degradation of writing ink, one should minimize changes in the document in question caused by inappropriate ink sampling [12]. As a result, non-invasive methods of analysis are preferable for the forensic examination of documents. In this regard, spectral methods are the most effective ones; however, most of them have significant limits. Methods of attenuated total reflectance Fourier transform infrared spectroscopy (ART-FTIR) are also often used in this regard. However, the FTIR technique requires prior time-consuming sample preparation when used for the analysis of writing inks. Moreover, there are some difficulties related to the presence of strong absorption bands caused by paper overlapping with the absorption bands of an ink [13]. The most promising approach for investigation of dye degradation processes at this time is the Raman spectroscopy method. Raman spectroscopy is a unique tool for the explicit analysis of any material [14]. The main advantage of Raman spectroscopy is its non-destructive and non-invasive effect on a sample, which allows a back-to-back analysis to be conducted if necessary. In addition, this method is a fast and efficient way to conclusively identify both organic and inorganic substances, which indicates its high chemical selectivity [15]; moreover, the samples analyzed by Raman spectroscopy do not require preliminary preparation.