## 3 Results and Discussion
### Experimental Analysis
Apparently, B-type carbonates were not lost in anaerobically burnt bones as much as in the case of their aerobic counterparts [21, 40]. This is coherent with the differential heat-related degradation of organic components in anaerobic and aerobic burns, which appears to take place at considerably lower temperatures for the latter [21]. However, interpretations must be carried out with caution, since a C/C of 1.22 was obtained at 800 \({}^{\circ}\)C which was an abnormal index given the much smaller valuesobtained for all other temperatures, both lower and higher. The appearance of a shoulder at 630 cm\({}^{-1}\) may suggest that we failed to completely seal the chamber, and therefore the burning process may not have been totally anaerobic. Regarding the amount of cyanamide, this finding has seldom been reported in aerabically burnt bones. Using the CN/P ratio, [PERSON] et al. [19] reported the presence of this organic compound in bones burnt at 800 \({}^{\circ}\)C. To that purpose, these authors [19] took only into account values above 0.25. Therefore, it was not a consistent find during their experimental work, contrary to what was observed in our own anaerbly burnt bones experiments. This was especially clear for bones burnt at higher temperatures which corroborated the results of [PERSON] et al. [21]. Cyanamide was present in all samples burnt at temperatures equal to or higher than 700 \({}^{\circ}\)C and this band became progressively more intense along with increasing temperature suggestive of a correlation between these two variables (Figure 5). At 900 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, it became even more intense than the 605 cm\({}^{-1}\) phosphate band (Figure 3), which is reflected in the CN/P ratios higher than 1. If the bone heat-induced changes under anaerobic conditions depicted here are demonstrated to be recurrent, spectral changes may then be able to discriminate between aerobically and anaerbly burnt bone. Summing up our results, the presence of anaerbly burnt bones can be suspected if bone presents a dark colouration, a high CI value (>4.00), the absence of an OH libration infrared band and intense bands associated with the presence of cyanamide (700 cm\({}^{-1}\), 2010 cm\({}^{-1}\)). These four criteria are especially helpful in the case of bones burnt anaerbly (in a sealed environment) at relatively high temperatures  
Figure 3: Selected FTIR-ATR spectra of human femur experimentally burnt at different temperatures (400 to 1000 \({}^{\circ}\)C) under anaerobic conditions. (**A**) OH libration at 630 cm\({}^{-1}\) and cyanamide at 700 cm\({}^{-1}\); (**B**) cyanamide at 2010 cm\({}^{-1}\); (**C**) OH stretching at 3572 cm\({}^{-1}\). (\(\geq\)800 \({}^{\circ}\)C), since those burnt at lower temperatures may present characteristics somewhat similar to bones burnt aerobically in the same temperature range. For bone samples subjected to low temperatures (<ca. 200 \({}^{\circ}\)C) the characteristic water stretching may explain the large band found at ca. 3400 cm\({}^{-1}\), so caution is advisable when interpreting the infrared spectra. In fact, this may occur in bones burnt at higher temperatures that have been exposed to high moisture conditions, since water may then adsorb to the bone surface. Both hydration and dehydration are frequent occurrences in inhumed materials and are prone to affect the corresponding infrared profile [41; 42]. The same can be said for B-type carbonates [43] thus rendering the usefulness of the BPI and C/C ratios arguable. Of course, it is not possible to state that taphonomy does not affect the four abovementioned criteria. For example, as previously discussed, bone discolouration can be caused by multiple taphonomic factors [9; 10; 11; 12; 13] and the CI is known to increase with post-depositional time [44]. Therefore, the interpretation of archaeological materials must have these factors in consideration. Figure 4: FTIR-ATR spectra of human bone samples, unburnt and experimentally burnt from 400 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C in aerobic conditions.