## 3 Test Results
### Appearance Change of the Rock Sample  
The type and content of minerals in rocks determine their color. The color can be used to judge the transformation of rock minerals with temperature. The chromaticity measurement was used to accurately quantify the change in sandstone surface color with heating temperature. The test basis is shown in the CIELab uniform color coordinate system (Figure 2), and the roughness, knocking sound, and other appearance forms of sandstone heated to various temperatures were recorded. In the CIELAB coordinate system, \(L\), \(a\)\({}^{*}\), and \(b\)\({}^{*}\) values represent the brightness, red-green chroma, and yellow-green chroma of a color, respectively. The larger the \(L\) value is, the higher the brightness of the color. An \(L\) value of 0 is black, and 100 is white. The value of \(a\) is between \(-80\) and 100. When \(a>0\), the color is red, and the larger the value of \(a\) is, the redder the color. When \(a<0\), it indicates that the color is green. The \(b\) value is also between \(-80\) and 100, where \(b>0\) is yellow, and \(b<0\) is cyan. \(\Delta E\) is the total color difference, and the calculation method is shown in Formula (1). \[\Delta E=\sqrt{(\Delta L)^{2}+(\Delta a)^{2}+(\Delta b)^{2}} \tag{1}\]  
The appearance and morphology changes of sandstone samples before and after heating are shown in Figure 3. When the sample was not heated, the color was purple, the knocking sound was clear, and the surface was smooth. After heating to 200 \({}^{\circ}\)C, the color became slightly white, the knocking sound was crisp, and the surface was smooth. After heating to 400 \({}^{\circ}\)C, the color became slightly red, the knocking sound became turbid, and the surface was smooth. After heating at 600 \({}^{\circ}\)C, the overall color changed from purple to red, the knocking sound became turbid, and the surface was rougher than before. After heating to 800 \({}^{\circ}\)C, the red color was lighter than that at 600 \({}^{\circ}\)C, black matter appeared locally (it is speculated that this was the surface of dark minerals melting and precipitating), the knocking sound was turbid, the surface was rougher, and particles were difficult to rub down. After heating to 1000 \({}^{\circ}\)C, the red color was deeper than that at 800 \({}^{\circ}\)C, the black  
Figure 2: CIELAB uniform color coordinate system. matter disappeared, the knocking sound was very muddy, the surface was very rough, and particles could be rubbed off. Table 1 shows that with increasing temperature, the \(L\) value increased slowly, from 54.30 at 25 \({}^{\circ}\)C to 60.98 at 1000 \({}^{\circ}\)C, indicating that the color brightness of the sandstone gradually increased and became brighter. The change trends of the \(a\) and \(b\) values are similar. With increasing temperature, the \(a\) and \(b\) values first slowly increased at 25-200 \({}^{\circ}\)C and 200-400 \({}^{\circ}\)C, then sharply increased at 400-600 \({}^{\circ}\)C, but decreased at 600-800 \({}^{\circ}\)C, and finally slightly increased at 800-1000 \({}^{\circ}\)C. The change rule of the total color difference \(\Delta E\) is similar to that of \(a\) and \(b\), which increased with increasing temperature, decreased slightly at 800 \({}^{\circ}\)C, but increased to \(a\) maximum at 1000 \({}^{\circ}\)C.  
With increasing temperature, the \(L\) value increased (the appearance turned white) because the calcium oxide generated by the thermal decomposition of calcite on the surface of sandstone reacted with the water in the air to produce white calcium hydroxide. The values of a (red) and \(b\) (yellow) at 600 \({}^{\circ}\)C and above were significantly higher than those at room temperature because the iron ions of sandstone iron-bearing minerals are oxidized to Fe\({}^{3+}\) and turn red at 600 \({}^{\circ}\)C and above.