Applying thermal demagnetization to archaeological materials: A tool for detecting burnt clay and estimating its firing temperature

Abstract

Burnt materials are very common in the archaeological record. Their identification and the reconstruction of their firing history are crucial for reliable archaeological interpretations. Commonly used methods are limited in their ability to identify and estimate heating temperatures below ~500⁰C and cannot reconstruct the orientation in which these materials were burnt. Stepwise thermal demagnetization is widely used in archaeomagnetism, but its use for identifying burnt materials and reconstructing paleotemperatures requires further experimental verification. Here we present an experimental test that has indicated that this method is useful for identifying the firing of mud bricks to 190⁰C or higher. Application of the method to oriented samples also enables reconstruction of the position in which they cooled down. Our algorithm for interpreting thermal demagnetization results was tested on 49 miniature sun-dried “mud bricks”, 46 of which were heated to a range of temperatures between 100⁰C to 700⁰C under a controlled magnetic field and three “bricks” which were not heated and used as a control group. The results enabled distinguishing between unheated material and material heated to at least 190⁰C and accurately recovering the minimum heating temperature of the latter. Fourier-Transform Infrared Spectroscopy (FTIR) on the same materials demonstrated how the two methods complement each other. We implemented the thermal demagnetization method on burnt materials from an Iron Age structure at Tell es-Safi/Gath (central Israel), which led to a revision of the previously published understanding of this archaeological context. We demonstrated that the conflagration occurred within the structure, and not only in its vicinity as previously suggested. We also showed that a previously published hypothesis that bricks were fired in a kiln prior to construction is very unlikely. Finally, we conclude that the destruction of the structure occurred in a single event and not in stages over several decades.

Archaeomagnetic Dating of the Outer Revetment Wall at Tel Lachish

“…This observation indicates that the archaeomagnetic signal represents the time of a conflagration that took place after the wall had been built and rules out the possibility that it predated the construction of the wall. Our results further support the recent suggestion (Vaknin et al 2023 ) to re-evaluate previous hypotheses regarding the use of pre-fired bricks in the Southern Levant before the Roman period (e.g., Namdar et al 2011 : 3477; Faust et al 2017 : 146).…”

Plant-related Philistine ritual practices at biblical Gath

“…It should be stressed that both in the Stratum D3 Temple and in other parts of the city, a thick and largely undisturbed destruction level was revealed (ca. 830 BCE), and the finds discussed here come from clearly undisturbed contexts relating to this destruction (without later disturbances) 26 , 27 . The studied temples include inner rooms with rich votives, and numerous loom weights as evidence of weaving at the temples; adjacent courtyards with cooking facilities, and a metallurgic industrial zone in the immediate vicinity (Figs.…”