## 1 Introduction  
In the last decades, Raman spectroscopy has become one of the most important analytical tools for a wide range of research areas in all sub-disciplines of physics, chemistry, biology, geosciences, and medicine. Initially, Raman spectroscopy was limited to scientific applications, but a low preparation effort, the ease of implementation, and developments in measurement automation have made Raman spectroscopy also interesting to users other than pure scientists. In particular, Raman spectroscopy is used for quality and process control applications [1]. In an early stage of Raman spectroscopy, the applications were limited due to generally weak Raman signals and interfering fluorescence caused by impurities of (natural) samples. However, the development of new laser technologies, filters, spectrometers, detection devices has enabled the investigation of all kinds of condensed matter. The resulting sensitivity of modern Raman spectrometer systems, combined with the high spatial resolution of confocal optical microscopes, nowadays also facilitates to image certain properties of solid materials or liquids at the micrometer scale. In many industries, including food processing pharmaceuticals, and biomedicine, the demand for multidimensional assessment of the sample composition has increased dramatically [2]. Hyperspectral Raman imaging enables a rapid, routinely practicable, non-destructive food quality and safety evaluation [3] and has turned into a novel clinical diagnostic tool in biomedical applications [4]. It facilitates the investigation of heterogeneous systems and thereby reveals a wealth of chemical and physical information about the chemical composition, short-range atomic structure, structural strain, crystallite orientation [5]. This information is provided with a spatial resolution down to the micrometer length scale. In earth and materials sciences, Raman imaging has been used, for example, (i) for (mineral) phase identification [6], (ii) to study internal growth structures of minerals such as zonation, (iii) to investigate the substitution of elements in solid solutions [7; 8], (iv) to investigate isotope substitution and mineral replacement mechanisms [9], and (v) to _in situ_ study transport and reaction phenomena during solid-water interaction at elevated temperature [10]. With the development of diamond anvil cells and heating devices, the investigation of mineral transformation reactions and their kinetics at high-temperatures and pressures became also possible. _In situ_ high-temperature Raman spectroscopy has been used to study the temperature dependence of first-order phonon bands [11; 12; 13; 14], pre-melting effects [15; 16], the structure of melts [17; 18; 19; 20; 15], as well as solid-state phase transitions [22; 23]. In this work, we combined the advantages of _in situ_ high-temperature Raman spectroscopy with the possibility of 2-dimensional Raman imaging with a micrometer-scale resolution as a powerful tool for the _in situ_ investigation of solid state sintering processes. In a recently published study, we reported the first results of sintering experiments using confocal hyperspectral Raman imaging (CHRI) to _in situ_ study high-temperature, solid-state reactions in kaolin-based green bodies [24]. The most striking results of this study were (i) the formation of pseudowollastonite occurring already at temperatures as low as 840 \({}^{\circ}\)C, which is about 290 \({}^{\circ}\)C below the critical temperature for the wollastonite-pseudowollastonite transformation, and (ii) the observation that both polymorphs formed in direct contact to each other. These observations imply that pseudowollastonite can form metastably, which, for instance, raises doubts on the interpretation of the temperature conditions of pyrometamorphic rocks that is based on the presence or absence of pseudowollastonite in a rock [25].