An evaluation of the infrared 630 cm−1 OH libration band in bone mineral as evidence of fire in the archaeological record

Abstract

FTIR spectroscopy has played an important role in attempts to understand the use of fire in prehistory, particularly through the identification of heated bone. The presence of the OH libration band at ca. 630 cm−1 in the FTIR spectrum of archaeological bone has been assumed to be indicative of bone that has been altered by fire. However, no definitive research has explored what the effects of heating variables may be on the appearance of this band. This paper studied the nature of heated bone, with the understanding that the FTIR 630 cm−1 band is the result of the formation of pure hydroxyapatite through the loss of carbonates from the bone mineral, carbonated hydroxyapatite. A set of experiments was designed to determine the impact of temperature, duration, and bone size on the appearance of the 630 cm−1 band: using samples of cortical bone from micro- and macrofauna, heating experiments were carried out to explore the appearance and change in the OH libration band. Results showed that this band appeared in bone samples heated to temperatures above 537 °C, and demonstrated that microfauna were more sensitive to thermal alteration. Bones heated in reducing conditions and bones which are fluoridated do not express the 630 cm−1 OH libration band, therefore FTIR is underrepresenting burning in the archaeological record. A quantitative method “HATI” was created to objectively assess whether a bone sample has been heated above 537 °C.