## 3 Results and Discussion  
The monzogranite samples showed apparent and significant physical changes after heat treatment. The colors and the texture of the specimens were also changed in addition to the specified factors (Figure 3). The standard 22 \({}^{\circ}\)C groups had a reddish-dark grey base color, which is typical of the examined monzogranite. Small yellowish-brown patches emerged on the surface of the specimens after heat treatment at 250 \({}^{\circ}\)C. The entire sample exhibited a pale yellowish-brown base color after being heated from 22 \({}^{\circ}\)C to 500 \({}^{\circ}\)C. After 750 \({}^{\circ}\)C of thermal treatment, the most substantial alteration occurred. The specimens had a yellowish-reddish-brown color transition, and visible inter- and intragranular macro-cracks appeared on the surface. The whitening of the grey and light pale pink feldspars appeared among the primary phases of the rock, separating the light quartz and feldspar from the biotitle phases and transforming them into dark red. According to the distribution of the spectral reflectance spectrum (Figure 4), it can be clearly stated that the curves shift upwards due to the rising temperature, so the samples become brighter. It can also be seen that in the range of longer wavelengths, the curves rise more strongly, so their color shifts in the direction of yellow. Figure 3: Texture and color change of heat-treated granite specimens. Samples representing the same temperature are shown in rows. Sample ID-s from left to right increase from 1 to 6. However, the CIE L*a*b* system is suitable for calculating the extent of the color difference as its preferred method for more sophisticated tracking of color change. It is known that it converts the reflectance spectrum into a uniform color space. In our case, the illumination was a D65 standard light source corresponding to sunlight. The plot of the samples in CIE L*a*b* color space (Figure 5) shows that the measurement results for each temperature are well separated in terms of both lightness and color, and the trend of changes is also well identified. This is also supported by the projection of the CIE L*a*b* color space data onto the a*-b* plane, the direction of color change as a function of heat impact is clear. As mentioned earlier, lightness increases with temperature have also been proven [48]. The color changes from green to red and from 'blue' to yellow are represented in the a* vs. b* diagram (Figure 5). Our results show an agreement with previous findings, namely that the color and brightness values vary with the composition of the material and the degree of heat impact [49; 50]. The CIE L*a*b* diagram suggests a significant change in the colorimetric parameters between 250 and 500 \({}^{\circ}\)C, with little change at temperatures below and above (Figure 6  
It has been shown that color changes are good indicators of the thermal decay of granitic rocks [51]. Our results also support the previous findings that non-destructive measurement of the change in colorimetric parameters is an excellent tool for estimating heat damage [48]. Because the color change due to heating correlates with changes in other material properties, the color change can be used to detect the exposure temperature, from which the corresponding fire damage can be estimated [49; 50; 52]. Physical properties such as mass, volume, bulk density, P-wave velocity, S-wave velocity, and surface hardness show changes with temperature (Table 3). The mean bulk density of 2716.0 kg/m\({}^{3}\), with a standard deviation of 23.6 at room temperature, decreased to 2678.1 kg/m\({}^{3}\) at 250 \({}^{\circ}\)C. Surprisingly, at 500 \({}^{\circ}\)C, a slight increase of 0.62% in density (mean value 2694.7 kg/m\({}^{3}\)) was found compared to 250 \({}^{\circ}\)C bulk density values However, comparing 22 \({}^{\circ}\)C and 500 \({}^{\circ}\)C density values, bulk density decreases slightly with heat-treatment. Further heating to 750 \({}^{\circ}\)C caused a significant drop (\(-\)7.47%) in mean bulk density.