## 2 Materials and Method
### Experimental Series  
Three types of experiments were performed in this work. The first experimental series (E1) involved heating studies with pure mineral phases that occur in silicate ceramics. These experiments were carried out to obtain high-temperature reference spectra that served as a basis for phase identification and the visualization of the Raman data from the _in situ_ sintering experiments. They were performed with both single crystals (E1A) and powder pellets (E1B). Pure phase Raman spectra were taken every 10 \({}^{\circ}\)C from RT to 1200 \({}^{\circ}\)C with a total acquisition time of 100 s. The heating rate was 10 \({}^{\circ}\)C/min. After having recorded a spectrum at maximum temperature, the sample was cooled to RT with a cooling rate of 10 \({}^{\circ}\)C/min and the last spectrum was recorded. At each temperature step, depth profiles were acquired (auto-focus function) after an equilibration time of 300 s to obtain (i) an optimal signal-to-noise ratio and (ii) a measure of reaction- and temperature-related shrinking or expansion effects. One example for the latter is the decomposition reaction of calcite to lime (CaO) and CO\({}_{2}\) between about 650 and 950 \({}^{\circ}\)C, which could be followed by spectral depth profiling (Figure 1).  
Figure 1: Dilatation of a pellet composed of calcite powder as a function of temperature. The dilatation was measured during the automated measurements by depth profiling. Note the dramatic shrinkage above 740 \({}^{\circ}\)C due to the decarbonation of calcite. The decarbonation is taking place between 700 and 950 \({}^{\circ}\)C, which can be followed by the shrinkage of the sample. At 804 \({}^{\circ}\)C, the calcite bands broaden and are shifted to lower wavenumbers (inset). Above 813 \({}^{\circ}\)C, calcite at the sample surface is fully decomposed to CaO (lime) that only shows a second order Raman spectrum. Note that the Raman spectra were recorded at the sample surface.  
The second series of experiments (E2) involved multi-component sintering experiments and were carried out under very similar conditions as the experiments with pure reference phases. After selecting a representative area of the sample surface, a hyperspectral Raman image was first recorded at RT. The sample was then fired with a heating rate of 10 \({}^{\circ}\)C/min to 700 \({}^{\circ}\)C (corrected temperature 650 \(\pm\) 5 \({}^{\circ}\)C, see below), where two images were subsequently recorded before heating the sample to the next temperature step. After having recorded the last high-temperature image at 1200 \({}^{\circ}\)C (corrected temperature 1100 \(\pm\) 5 \({}^{\circ}\)C) the sample was cooled to RT with a cooling rate of 10 \({}^{\circ}\)C/min and a final image was taken.  
The third series of experiments (E3) involved multi-component isothermal sintering experiments. The first image was recorded at RT to select a representative area. Then, the sample was fired with a heating rate of 10 \({}^{\circ}\)C/min to the target temperature of \(\sim\)850 \({}^{\circ}\)C. Within a dwell time of 24 h, 12 Raman images (\(\sim\)2 h per image) were recorded. The sample was then cooled down with a cooling rate of 10 \({}^{\circ}\)C/min before recording the last image at RT.