## 3 Results and Discussion
### The Firing Temperature Analysis Using FTIR and XRD  
The IR spectra of dioctahedral and trioctahedral layer silicates display a very complex band system in the OH stretching region [40]. In the present study, the absorption bands at 3671-3851 cm\({}^{-1}\) and 3460-3496 cm\({}^{-1}\) were assigned to the OH stretching of water molecules and OH stretching of the inner hydroxyl group, respectively. However, upon heating the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Frequency with Relative Intensity (cm\({}^{-1}\))**} \\ \hline
**Assigned** & \multicolumn{3}{c}{**Sample No with Absorption Frequency and Relative Intensity (cm\({}^{-1}\))**} & \multicolumn{3}{c}{**Reference**} \\ \cline{2-7}
**Mineralogy/** & \multicolumn{3}{c}{**Mouramora-1**} & **Mouramora-2** & **Mouramora-3** & **Padumpukhuri** & **Pratima Garh** & **Rukhini Garh** & \\ \hline Hematite & - & - & - & - & 531W & - & [28; 51] \\  & 2922 VW & 2922 VW & 2919 VW & 2919 VW & 2919 VW & 2928 VW & \\  & 2851 VW & 2851 VW & 2854 VW & 2854 VW & 2854 VW & - & \\ Organic compound & - & - & - & - & 2356 VW & - & [27; 28; 57; 58; 59] \\  & - & - & - & 2004 VW & - & - & \\  & 1998 VW & - & 1998 VW & - & - & 1984 VW & \\  & - & 1878 VW & 1879 VW & 1879 VW & - & - & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: \(Cont.\)  
Figure 5: XRD pattern of studied brick samples (Q—Quartz, Feldspar (Or—Orthoclase, Mc—Microcline, Olg—Oligoclase, Sa—Sanidine, Ab—Albite), Arag—Aragonite, Dol—Dolomite, Lmt—Laumontite, Ilm—Ilmenite, Sd—Siderite, Wo—Wollastonite, Hm—Hematite, Chl—Chlorite, T—Trona, Py—Pyrite, Mgs—Magnesite, Gth—Geothite, and GI—Glauconite). samples, they underwent dehydration at 200 \({}^{\circ}\)C, continuing until 380 \({}^{\circ}\)C, resulting in the weakening of the 3400 cm\({}^{-1}\) broad band and the breaking of the crystal structure at 500 \({}^{\circ}\)C [40]. In addition, disorder in the octahedral and tetrahedral sheets is notable, with partial removal of the 535 cm\({}^{-1}\) Si-O bending band and broadening of the Si-O stretching bands in the 1100-1000 cm\({}^{-1}\) region [40]. This phenomenon implies a firing temperature of 500 \({}^{\circ}\)C. Additionally, at 500 \({}^{\circ}\)C, the absorption peak at 915 cm\({}^{-1}\) indicates that Al (OH) vibrations in the octahedral sheet structure begin to collapse [27; 40; 60; 61]. The absence of 915 cm\({}^{-1}\) spectra in the studied samples suggests that all the samples were fired above 500 \({}^{\circ}\)C.  
By increasing the temperature, the Si-O stretching band shifted toward higher frequencies and broadened, with the maximum intensity at around 1025 cm\({}^{-1}\) at 100 \({}^{\circ}\)C and shifting to 1036 cm\({}^{-1}\) at 600 \({}^{\circ}\)C [27; 62]. The silicate structure disappeared when the clay was fired to 650 \({}^{\circ}\)C, and a broad symmetry band was detected at 1035 cm\({}^{-1}\) for red clay and 1080 cm\({}^{-1}\) for white clay [27; 63]. The presence of an absorption band in five samples at around 1080-1084 cm\({}^{-1}\) indicates that white clay of kaolinite composition was used in the investigated brick samples, which were fired at 650 \({}^{\circ}\)C.  
The presence of hematite and magnetite, characterized by a peak at 540 cm\({}^{-1}\), is an indication of iron oxides formed during fabrication firing processes above 650 \({}^{\circ}\)C [27; 29; 64]. However, the presence of hematite implies that they were fired in the open air or in a completely oxidizing atmosphere at the time of manufacture [65; 66]. The presence of hematite and magnetite in the studied Pratima Garh sample confirms these manufacturing conditions, as well as the type of clay used, which is plausibly red clay. It can also be inferred that the artisans of Pratima Garh were well aware of the technology implemented in firing the bricks under both oxidizing and reducing atmosphere, which is inferred by the presence of peaks of 531 and 576 cm\({}^{-1}\), respectively.