## 1 Introduction  
Textile materials, including constituent fibres, can be subjected to degradation under the influence of various factors that occur during textile use. Degradation is the process of structural changes as a result of physical or chemical transformation that occurs in polymer materials under the influence of external factors. For textiles, the most common of these factors are light, high temperature, adverse environmental conditions (moisture, microorganisms, UV radiation) and chemicals used in the washing process (detergents, bleaching agents, enzymes, organic solvents) [1]. Thermal degradation of fibre occurs when its chemical properties alter under the influence of high temperature, i.e., there is a change in the molecular structure in the form of the partial depolymerisation or cross-linking of fibre polymer [2, 3]. The behaviour of fibres at high temperatures depends on their type. Natural (i.e., cotton, flax, hemp) and man-made (viscose, modal and lycoll) cellulosic fibres react substantially differently than synthetic fibres, e.g., polyesters, when exposed to a heat source. The heating of cellulose fibres in the presence of oxygen causes some amount of hydrolysis and dehydration, the extent of which is directly proportional to the temperature [3,4]. It should be noted that even a high degree of thermal destruction of cellulosic fibres does not usually cause clearly visible changes in their morphological structure [5,6]. However, synthetic fibres, mainly thermoplastic in nature, become soft at high temperatures. Textiles made of polyester, polyamide or polypropylene fibres shrink and deform when exposed to heat. Subsequently, these fibres tend to melt. The thermal degradation of cellulose fibres is an important issue in the production processes of plant fibre-reinforced composites as well as in the manufacturing of flame retardant textiles [7,8]. In the processes of finishing textile products, in order to achieve the desired aesthetics and performance, dyes and various chemicals are introduced into the structure of the fibres. In addition, as a result of use, textile products can contain various types of pollution. Under high temperatures, many of these substances may also speed up the degradation of fibres [3,8]. An accurate determination of damage to individual fibres in a variety of textile products is very important, both from a scientific and a practical point of view. This is necessary, for example, when assessing the state of preservation of antiquie fabrics [9,10] or for determining the degree of wear of thermal protective clothing [11,12]. Identification of thermal changes observed in single fibres is very important from the forensic science point of view [13,14]. High-temperature effects can be often observed during examination of evidential material in the form of fragments of single fibres, secured, for example, at a crime scene (arson or car accident). By studying single fibres that have changed thermally, one can evaluate their unique characteristics and subsequently relate an individual to a specific criminal event. The heat not only causes irreversible changes in the morphological and chemical structure of the fibre but is often the cause of its total destruction. The task of the forensic expert examining thermally altered fibres is to define the properties of the textile material before the changes that occurred under the influence of heat [15]. Many modern instrumental analysis methods allow fibres altered in such a way to be examined to some extent. The use of microscopic methods--optical and electron [6,16,17] and FTIR spectroscopy [18,19,20]--can yield very valuable research results. A new effective tool for non-destructive testing of the internal microstructure of fibres and fibrous materials is a method of X-ray microtomography [21,22]. This publication addresses a part of this broad issue related to the study of thermally altered fibres, i.e., identification of changes caused by the action of heat radiation.