## 3 Results and Discussion
### The Firing Temperature Analysis Using FTIR and XRD
During the firing of clay, remarkable changes are noticed from 1000 to 1200 \({}^{\circ}\)C, where the X-ray diffractogram of quartz disappears. Quartz gradually decomposes from 800 to 1100 \({}^{\circ}\)C, peaking at 1100 \({}^{\circ}\)C and then disappearing. Up to 800 \({}^{\circ}\)C, all the decomposed phases contribute to the formation of a vitreous phase. At this point, the clay would be mostly amorphous, with some residual grains of quartz, neo-formed mulite, hematite, and other high-temperature mineral phases, like microcline. As a result, except for 1200 \({}^{\circ}\)C, quartz was a residual phase, i.e., a component of the raw clay that did not undergo chemical transformations during the firing stage. On the contrary, during the firing process, mulite, hematite, and other silicates were formed. At temperatures ranging from 700 to 900 \({}^{\circ}\)C, clays showed phyllosilicate destruction followed by vitrification, which was significant at temperatures above 1000 \({}^{\circ}\)C [67]. The presence of orthoclase, microcline, albite, hematite, and magnetite in the Pratima Garh, Rukmini Garh, Padumpukhuri, and Mouramora bricks supports the high firing temperature at more than 800 \({}^{\circ}\)C.  
Clay deposits typically contain some organic material, but artisans can also add organic compounds to the ceramic paste during the preparation process to increase plasticity [68]. At 400-650 \({}^{\circ}\)C, depending on the type of flux and other raw materials used, the initial ingredients decompose, and CO\({}_{2}\), S, and hydrocarbons are lost. Dehydration occurs at \(\sim\)100 \({}^{\circ}\)C, followed by dehydroxylation of clay minerals at temperatures ranging from \(\sim\)450 to 750 \({}^{\circ}\)C, depending on the type and purity of the clay [69]. Dehydration and dehydroxylation of relatively pure clay minerals occur at various temperatures depending on the clay minerals present. Chlorite loses water at a relatively high temperature, usually starting at \(\sim\)750 \({}^{\circ}\)C, but this temperature is determined by the mineral chemistry. Illite (mica) transformation begins with dehydration at \(\sim\)600-700 \({}^{\circ}\)C. When carbonate minerals are heated, they release carbon dioxide; for example, calcite decomposition begins at 650 \({}^{\circ}\)C. The temperature of phase transformation varies from sample to sample due to a variety of factors, such as mineral grain size, heating rate, and how easily the evolved carbon dioxide can be lost to the atmosphere [70]. At \(\sim\)573 \({}^{\circ}\)C, alpha/low quartz structurally transforms into beta/high quartz. Because this transition involves rotation rather than bond breaking, the reaction is reversible, and any high-quality quartz that remains after the high-temperature soak will invert to low-quality quartz during the cooling phase of the firing cycle. The brick expands as the temperature rises; however, if the cooling rate is too fast, highly damaging microcracking can occur [71]. The firing minerals that are generated during the production of pottery depend on a number of factors, including the composition of the clay, the materials added to it, the atmosphere, and the duration of the firing process. High-temperature minerals such as gehlenite and diopside are formed from the decomposition products of illite and calcite [10; 11; 12]. At 650 \({}^{\circ}\)C, calcite starts to break down; at 850 \({}^{\circ}\)C, high-temperature calc-silicates and alumonic-calc-silicates, such as diopside and anorthite, are the products of this process. At temperatures higher than 830 \({}^{\circ}\)C, wollastonite--which is created when quartz and lime react from approximately 920 to 1000 \({}^{\circ}\)C--replaces gehlenite in systems that contain gehlenite [13].