Bromine volatilization during firing of calcareous and non-calcareous clays: Archaeometric implications

The influence of carbonates in the temperature of Br volatilization during firing of clays and its implications in archaeometric studies are investigated. The main goal is to determine the circumstances in which Br content of fired products (“pottery”) can be considered inherited from raw material or added to ceramic pastes after production. Seven samples representing different types of clays (non-calcareous clays, calcite-rich clays and dolomite-rich clays) from Algarve region (South Portugal) were studied before and after firing at temperatures ranging from 300 °C to 1100 °C by steps of 100 °C. Original clays were characterized by major element geochemistry, obtained by X-ray fluorescence, and by mineralogy of bulk rock and < 2 μm fraction, using X-ray diffraction. The chemical composition of the test pieces (unfired and fired at various temperatures) of each clay was determined by instrumental neutron activation analysis, as it enables obtaining Br concentration with high precision and accuracy. Thermogravimetric analysis was done to better characterize mass changes after firing. The results confirm the influence of clays composition, especially the presence of carbonates rather than the clay minerals associations, on temperature of Br volatilization: i) in non-calcareous clays Br volatilized more intensely up to 600 °C, suggesting its association with combustion of organic matter and dehydroxylation of clay minerals; ii) in calcareous clays Br volatilized more strongly after 800―900 °C, suggesting that at least part of the process is associated with decarbonation reactions. As a result, this study contributes to elucidate that not all Br existing in pottery can be interpreted as a contamination product. Instead, Br can be inherited from raw material and its presence in pottery, particularly in carbonate-rich pastes, may simply indicate that the temperature attained in the kiln was not enough for its complete volatilization. Extending the Algarve study to other clay materials used in ancient ceramics, the conclusions of this study may assist broad archaeometric studies.

Research highlights
► Influence of clays composition on temperature of bromine volatilization.
► In calcareous clays Br volatilize at higher T associated to decarbonation reaction.
► Bromine in pottery can be added after production or inherited from raw material.
► Br in calcareous pastes may indicate T was not high enough to its volatilization.