Researchers working at the ISIS Neutron and Muon Source in Oxfordshire, the only facility of its kind in the UK, have produced a catalogue of data which could be used to help scientific researchers analyse human bones that have been heated. The team, from the University of Coimbra in Portugal, says their results could be used by archaeologists to distinguish between skeletal remains that have been burned at high temperatures, as in cremation, and those that may have been heated at lower temperatures, as part of ritualistic or cannibalistic defleshing practices, for example.
Bones are made up of collagen, a mineral called hydroxyapatite, and water. If heated, they undergo structural changes at the molecular level that result in obvious alterations, including shrinkage and colour change. These visual indicators, though, do not tell us what temperature the bones were heated at, and, by extension, it can sometimes be difficult to determine the circumstances in which certain human remains were exposed to heat. The physical and chemical changes that can occur when bone is heated at low to moderate temperatures, moreover, can resemble bone that has been buried for a long time, further complicating matters for archaeologists keen to establish the context in which some skeletal remains may have been heated.
In order to generate reliable reference data that could be used to reveal clues about the circumstances in which a given bone may have been heated, the researchers used a non-destructive technique called ‘vibrational spectroscopy’ to chart the effects of low to moderate heat on human skeletal remains. At ISIS, this involved beaming neutrons on to samples of human bone that had been burned at known temperatures to measure the atomic structures within the samples. Describing vibrational spectroscopy, Professor Stewart Parker, a senior research scientist at ISIS, told CA: ‘All matter is basically made up of atoms and molecules, and you can think of a molecule as being atoms connected by springs. As you know, if you take a Slinky spring and prod it, you can actually see vibrations occurring in that spring. That’s essentially what happens at the atomic level: if you hit it with the right energy, you set the spring oscillating, and the energy you need to set that spring going tells you something about both the strength of the spring and the atomic weights on the end of it.’ Vibrational spectroscopy, then, can be used as a fingerprinting technique: ‘if you measure a material, you end up with a particular pattern of peaks and troughs, which you can then compare to a known material to see whether this material is the same, different, or similar,’ Stewart said.
Working within a temperature range of 200ºC to 650ºC, the researchers prepared samples of bone for analysis by heating them for three hours at regular intervals in both aerobic and anaerobic conditions. Then, at ISIS, the team used neutron beams (via a technique called ‘inelastic neutron scattering’, or INS), to ‘see’ the hydroxyapatite within the bone samples (this is difficult to observe by other means but crystallises when bone is heated, giving a clear signature of its presence). The analysis using neutron vibrational spectroscopy was complemented by optical vibrational spectroscopy analysis of the same samples at Coimbra, using the more widely accessible technique of Fourier-transform infrared spectroscopy (FTIR), generating parallel data.
The data gathered by the researchers allowed them to catalogue various heat-induced changes to the structure of human bone caused by heating at low to moderate temperatures, while validating FTIR as a method for analysing the impact of heat on bone. The team found that vibrational spectroscopy can be used accurately to detect heat-induced changes to the organic and inorganic structures of bone as a function of temperature. The main spectral differences were detected at 300º-400ºC in anaerobic conditions, and at 450º-500ºC in aerobic environments.
These latest investigations build on the team’s previous analysis of the impact of high temperatures (>600ºC) on human skeletal remains, so reference data has now been catalogued across the 200ºC to 1,000ºC range. It is possible, then, for archaeologists to measure the same data in a sample of human bone heated in unknown or uncertain circumstances, and to compare this with the data in the catalogue to determine the temperature at which the bone was heated. This, in turn, may offer clues about the context in which the bone was cooked or burned.
The group’s paper on the effects of moderate heating was published in the journal Royal Society Open Science. It is available at https://doi.org/10.1098/rsos.210774.