Physical and mineralogical properties of experimentally heated chaff-tempered mud bricks: Implications for reconstruction of environmental factors influencing the appearance of mud bricks in archaeological conflagration events

Abstract

Sun-dried mud bricks are used around the world and have been found in the Levant and Mesopotamia since the Neolithic period. Their form and composition lend important information pertaining to social and technological meaning in human cultures. Fired mud bricks are well known in the southern Levant, often identified in Bronze and Iron Age strata and used as a marker for destruction by fire (conflagration events). Only a few studies have attempted to reconstruct conflagration conditions from fired mud bricks because many variables impact the formation of the final fired brick. These include brick composition, heat intensity (i.e., maximum temperature), heat duration and firing atmosphere (i.e., oxidizing vs. reducing). The myriad combinations of these factors may result in different appearance of fired bricks. Infrared spectroscopy is one method that has been exploited quite extensively in relation to fired clay-based materials: studies were conducted on powdered sediment samples for a fixed duration and in oxidizing conditions, producing calibration curves that were then utilized for reconstruction of past maximal heat. Here we report on an experimental study of the thermal behavior of mud bricks under differing composition, heat intensity, heat duration and firing atmosphere. We carried out experiments in a furnace oven using micro-thermocouples which allowed us to simultaneously measure heat across bricks, from edge to core. The resulting mud bricks were analyzed using Fourier Transform Infrared (FTIR) spectroscopy. We identify a previously unknown thermal effect that occurs in bricks tempered with organic material while they are fired; namely a correlation between the amount of organic temper and elevation of temperatures up to 100°C above the oven chamber temperature. We record the color patterns obtained at different temperatures and duration of heating, as well as the colors obtained from heating in different atmospheres. We report that the FTIR spectrum of bricks heated in oxidizing conditions differs from that of bricks heated in reducing conditions at the same temperature. We note that the position of the main clay absorbance band cannot be used alone to infer firing temperature as its shift is not systematic. We show that combining this parameter with the width of the same band in the FTIR spectrum makes it possible to achieve better temperature reconstructions from fired bricks. Lastly, we report a small scale case study in which we tested the applicability of the experimental results to the remains of a mud brick wall unearthed within the largest known destruction event in the ancient city of Megiddo, i.e., Tel Megiddo Stratum VIA. We show that this wall was burnt as one unit, having a reduced core and oxidized outer part, where the core experienced temperatures in the range of 500–600°C and the edge 600–700°C. The detailed analysis of brick compositions carried out in this study further allows us to reconstruct the ancient bricks' preparation recipe. The results of this study bear important implications for future studies of archaeological conflagration events, and the destruction phenomenon in general.