Influence of firing conditions on the rock magnetic properties. Preliminary results from experimental heating experiments

Archaeomagnetism deals with baked clay materials carrying a thermoremanent magnetization acquired in the Earth`s magnetic field, which determines its importance for two scientific fields – geophysics and archaeology. It is well known that the success of an archaeomagnetic study is closely related to the magnetic properties of the materials used. In turn, the magnetic properties depend on the initial clay mineralogy, firing conditions and burial history. In order to get more information about the influence of the firing process, samples prepared of raw clays (taken from six different sources) were subjected to the successive experimental baking in three experimental combustion structures: open-hearth, single-chamber round furnace and double-chamber rectangular kiln. Heating and cooling temperatures in the various parts of the structures were constantly monitored. Rock magnetic measurements and analyses were carried out prior to, after the first and after the fourth experimental firing.

The heating/cooling cycle in the single-chamber furnace was the most prolonged. The temperatures achieved vary from 400 to 540°C displaying very uneven distribution after 400 °C. Maximum temperatures of about 850 – 900°C were reached in the hearth and in the double-chamber kiln but they were retained for a relatively short time (5 – 10 min) whether or not extra fuel was added. The heating and especially the cooling were the most homogeneous in the double-chamber kiln, where the cooling temperatures in its different parts varied within 50°C. In contrast, these temperatures differ by about 250°C in the single-chamber furnace and almost 400°C in the hearth.

X-ray diffraction analyses classify the chosen six clays as calcareous (all grayish clays) and non-calcareous (all brownish clays).  Magnetic susceptibility behaviour monitored during stepwise heating and the shape of alternative field demagnetization curves of laboratory induced isothermal magnetization divided clays into three groups. Remanence and magnetic susceptibility measured after the first experimental firing are quite variable according to the clay type, structure and samples position, but some trends are obvious. The lowest magnetic properties generally correspond to the samples heated in the single-chamber furnace where the lowest firing temperatures developed. However, in many cases the measurements for samples baked in the hearth and/or in the kiln are very close. The highest magnetic enhancement was always achieved in the double-chamber kiln but only in the parts farthest from the entrance. The reheating increases (except for one clay) and homogenizes the magnetic properties of the kiln samples but this pattern is not systematically observed for the hearth. Magnetically soft minerals dominate. Presence of a high-coercivity carrier (probably hematite) is supposed for three clays single-baked in the hearth and the single-chamber furnace (but only when the samples were placed in the parts with the most oxygen access). During the multiple experimental firing, some samples disintegrated in different extent.

This study is funded by the grant KP-06-N30/2 from the Bulgarian National Science Fund. The support by Russian Foundation of the Basic Research grant 19-55-18006 is also acknowledged.