## 3 Results and Discussion  
### Experimental Analysis  
All experimentally burnt bone sections displayed a charred appearance regardless of the maximum temperature of exposure, as opposed to the usual heat-induced colour sequence documented for aerobically burnt bone--natural colour to darker colour to whitish colour (e.g., [PERSON] et al. [18]). Infrared spectra for the experimentally burnt femur are given in Figure 3. Both presented similar basic features, the most noteworthy being:  
1. the absence of bands for the OH libration at \(\sim\)630 cm\({}^{-1}\) (Figure 3A), in clear contrast to the sharp peak usually observed in bone burnt aerobically at temperatures above 600 \({}^{\circ}\)C [21, 25, 34, 35, 38]. Possibly, this is explained by the OH libration being a lattice mode which is visible only when the system has other neighbouring OH groups (within a network). When considerable CN\({}_{2}\)\({}^{2-}\) for OH substitutions take place, only the OH stretching (at ca. 3500 cm\({}^{-1}\)) is visible. 2. the OH stretching band at \(\sim\)3572 cm\({}^{-1}\) was clearly present in spectra from bones burnt at all investigated temperature thresholds (Figure 3C). The spectra reported by [PERSON] et al. [20], showed no OH stretching under anaerobic conditions which is, contrastingly, a frequent observation in aerobically burnt bones (Figure 4). This suggests that some variability may be expected as well for the intensity of this feature or, that it instead reflects burns at shorter durations (60 min against the 120-211 mins durations used in the present study);
3. progressively more intense bands from CN\({}_{2}\)\({}^{2-}\) were present at ca. 700 cm\({}^{-1}\) (N-C\(\equiv\)N bending, Figure 3A) and 2009 cm\({}^{-1}\) (Figure 3B, C\(\equiv\)N stretching) in the spectra of bones burnt at temperatures above 650 \({}^{\circ}\)C. Cyanamide formation has been previously reported to be due to incomplete oxidation of organic matter [39], leading to the incorporation of CN\({}_{2}\)\({}^{2-}\) ions into bone's inorganic framework substituting for OH\({}^{-}\) and yielding cyanamidapatite (Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)CN\({}_{2}\)) [19, 20, 21, 22, 23];
4. additionally, between 600 and 700 \({}^{\circ}\)C, the formation of NH\({}_{3}\)/NH\({}_{4}\)\({}^{+}\) was observed which resulted in broad signals at ca. 1700 and 3300 cm\({}^{-1}\) due to NH deformation and stretching modes, respectively. As the temperature increased, more and more cyanamide was formed (from ammonia) and the characteristic NH signals gave way to those from cyanamide. Results for the calculation of the spectroscopic indices measured for the bone samples are comprised in Table 3. The crystallinity index (CI) of bones burnt under both aerobic conditions--obtained from [PERSON] et al. [24] and [PERSON] et al. [19]--and anaerobic settings (in a sealed chamber) presented somewhat similar values for temperature thresholds up to 600 \({}^{\circ}\)C. However, differences became much larger at higher temperatures, with anaerobically burnt bone presenting values considerably smaller than those obtained for aerobically burnt bone. As for the amount of B-type carbonates (BPI), it was always higher for anaerobically burnt bone, especially at the highest temperatures, with values considerably larger than the values obtained for aerobically burnt bone. In the case of the C/C ratio, values were quite similar between the two settings for temperature thresholds up to 700 \({}^{\circ}\)C, revealing a similar intensity of the features allocated to the dividend (Carbonates A + Carbonates B & collagen + lipids) and to the divisor (B-type carbonates) thus leading the value to be approximate to one, on average. At higher temperatures (at 900 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C), this remained to be the case for anaerobically burnt bones but, for aerobically burnt bones, B-type carbonates were proportionately less intense leading to higher C/C ratios.