## 1 Introduction  
The history of human civilization is inherently connected to the use of bricks as a basic building material. Since the third century BCE, there have been numerous structures that use fired or burned bricks [1]. The chronicles of ancient civilizations give evidence to the enduring legacy of brick creation, and several studies infer that the quality and durability of bricks are closely related to the quality of raw materials, manufacturing techniques such as firing temperature, and many other factors [2; 3; 4; 5]. Firing temperature is recognized as an important factor in brick production because burning triggers significant changes in mineralogy, texture, and physical properties, impacting the porosity and pore dispersion of bricks [6; 7; 8; 9].  
When clays are fired to make pottery, new minerals called firing minerals are formed. These products are primarily determined by the composition of the clays, the additives used to make pottery, the atmospheric conditions, and the duration of firing during pottery making. Gehlenite and diopside are high-temperature firing minerals. Gehlenite is formed at 800-850 \({}^{\circ}\)C as illite and calcite decomposition products [10]; calcite decomposition begins at 650 \({}^{\circ}\)C and ends at 850 \({}^{\circ}\)C, generating new high-temperature calc-silicates and aluminum-calcium silicates such as pyroxene group members (diopside) and plagioclase feldspars (anorthite) [11; 12]. The reaction between quartz and lime at 920 or 1000 \({}^{\circ}\)C, depending on the quartz content, produces wollastonite. Wollastonite was discovered to replace gehlenite in the gehlenite bearing system at temperatures above 830 \({}^{\circ}\)C [13]. Multidimensional approaches like X-ray powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Differential Thermal Analysis (DTA), and Scanning Electron Microscopy (SEM) have been demonstrated to be very effective for understanding the firing temperature [5; 14; 15; 16; 17; 18; 19]. Moreover, the Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) dating techniques are essential for establishing the time elapsed since the last high-temperature event, which usually coincides with the bricks' firing in a kiln [20; 21]. TL and OSL exploit the accumulation of charge carriers in mineral defects, providing a timeline for brick usage and reuse [22; 23].  
In India, most investigations have focused on the ancient architectures of the Southern part of the country, using similar methodologies to understand the effects of firing temperature and the physical-mechanical properties of ancient bricks [19; 24; 25; 26; 27; 28; 29; 30; 31]. However, only one notable study analyzed the bricks of the Kamakhya Temple using XRD and FTIR techniques, revealing variations in firing temperatures (above and below 500 \({}^{\circ}\)C) [32]. Except for this, no archaeometric and dating studies of ancient artifacts from the region have been carried out so far. Hence, the present study aimed to fill this gap by focusing on different archaeological sites of Upper Assam to estimate technological parameters of brick production, such as raw materials and the firing temperature used in manufacturing, employing FTIR, XRD, and SEM-EDX techniques. Additionally, TL, OSL, and IRSL dating methods were employed to determine the chronology and historical context of the Dihing Valley and nearby areas of Upper Assam. In this way, it was possible to establish whether different brick production technologies exist and, thanks to the determination of an absolute chronology, determine the technological progress that has occurred in brick production.