## 2 Materials and Methods
In our case, the CIE L*a*b* color space is perfectly suited for this  
Figure 2: Cylindrical samples are showing color changes from 22 \({}^{\circ}\)C, 250 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 750 \({}^{\circ}\)C. Note the macro cracks in sample 750 \({}^{\circ}\)C.  
purpose [42]. It should be noted that the CIE L*a*b* color space is not perfectly uniform perceptually, but it is still useful in the industry for detecting small color differences. From the CIE XYZ coordinates and a reference, which in our case is the reference light source D65, we can determine the L* lightness and the chromaticity, which is characterized by the values a* extends from green (\(-\)a*) to red (\(+\)a*) and b* extend from blue (\(-\)b*) to yellow (\(+\)b*). These can be used to determine the color difference between two surfaces with different spectral reflectance under a given reference illumination. Since this color difference corresponds to human perception, it can be compared with other color differences determined under such conditions. The reflectance spectra of the investigated surfaces were measured with a Konica Minolta CM-2600d spectrophotometer. The instrument parameters are 8 mm head diameter, SCI (specular component included) measurement mode, and CIE 1964/10\({}^{\circ}\) geometry. Using the spectrophotometer calibration process, absolute reflectance spectral distributions were measured. As it was mentioned earlier, D65 was used as the reference light source for the calculations. From the CIE L*a*b* colorimetric characteristics determined from the reflectance spectra of the investigated surfaces, the colorimetric transformation due to temperature stress could be quantified. The physical parameters were tested following the guidelines given by European Norms: Bulk density (EN 1936:2006) [43], the propagation speed of the ultrasonic wave (EN 14579:2005) [44]. For measuring the P- and S-wave velocities, a PUNDIT and a GEOTRON device were used. The PUNDIT (Portable Ultrasonic Non-destructive Digital Indicating Tester) is an automatic P-wave propagation time tester instrument. It is suitable for easy and quick measurements, however less accurate than a more precise instrument (e.g., GEOTRON). To increase accuracy, plasticine was used as a coupling agent to fill the gap between the sample and the transducers. The frequency of the transducers was 50 kHz. The GEOTRON instrument is a more sophisticated design than PUNDIT, which allows more accurate detection of wave propagation velocity at a longer measurement time. This instrument is capable of detecting the speed of S waves in addition to P waves. UP-SW transducers with an 80 kHz frequency were used for measurements. For the identification of the first onset of P- and S waves, the selected amplitudes of 100 mV were applied. On 750 \({}^{\circ}\)C groups, P- and S-wave velocity with the GEOTRON device could not be carried out due to the highly cracked sample and the sensitivity of the device. Instead, a non-standardized test, the Duroskop, was used to determine the surface hardness of the samples. This instrument was initially used to test the surface hardness of metals. It has recently been used to test rock specimens [45; 46]. Duroskop is a powerful tool for measuring small surfaces and minor changes in surface strength as it has a small-pointed mass that rebounded from the surface [47]. The device is primarily a portable tool, but a frame was used in the laboratory to test cylindrical specimens. The test numbers of measurements used in statistical results can be seen in Table 1.  
Testing for physical parameters in most cases was performed before and after the thermal treatments. However, in some cases, the GEOTRON ultrasound velocity measurements and spectral recordings were carried out only on heat-treated samples.