Phase changes under heat treatment
The spectra of the raw bovine bone and ivory dentin both (shown in Fig. 1) contain absorption bands of amide I at 1652 cm−1, amide II at 1548 cm−1, ν3(CO32−) at 1454 cm−1 and 1414 cm−1, ν3(PO43−) at 1088 cm−1 and 1018 cm−1, ν1(PO43−) at 963 cm−1, ν2(CO32−) at 873 cm−1, and ν4(PO43−) at 601 cm−1 and 561 cm−1, representing composites of collagen and carbonated hydroxyapatite (CHAp). The main difference between the two materials before burning lie in the C=O stretching vibration band at 1745 cm−1 suggesting the presence of lipids in bones and absence in ivories. With the increase of the burning temperatures from 200 to 1000 °C, the absorption bands of organics and CO32− of both materials decrease as they decompose, and the IRSF increase shown in Table 1, which suggests the recrystallization process starts at around 600 °C as previously reported [1, 2, 8].
After being burnt at temperatures between 800 and 1000 °C, notable phase changes take place and vary between bones burnt in oxidizing atmosphere and reducing atmosphere, and also between bones and ivories. The spectra in Fig. 1a show the transformation of CHAp to well-crystallized HAp according to the triplet ν4(PO43−) band  and much higher IRSFs of the bones burnt at temperatures between 800 and 1000 °C. The absorption bands at 2017 cm−1 and 701 cm−1 in Fig. 1b suggest the formation of CN22− group and corresponding cyanamidapatite (Ca10(PO4)6CN2) [22, 23] in bones burnt at temperatures between 800 and 1000 °C in reducing atmosphere. The replacement of OH− by CN22− may also impede the recrystallization judging by the lower IRSFs of bones burnt at temperatures between 800 and 1000 °C in reducing atmosphere compared to those in oxidizing atmosphere. However, these bands of CN22− group are absent in the spectra of ivories burnt in reducing atmosphere (shown in Fig. 1d) as another phase forms. The ivories burnt at temperatures between 800 and 1000 °C show a new FTIR pattern with ν3(PO43−) bands shifting to 1121 cm−1, 1034 cm−1, 992 cm−1, ν1(PO43−) bands shifting to 946 cm−1, and ν4(PO43−) bands shifting to 597 cm−1 and 557 cm−1, which match the spectrum of magnesium-substituted β-tricalcium phosphate (Mg-TCP) [24, 25]. The XRD spectra of the ivories burnt at 800 °C (shown in Additional file 1: Fig. S1) also matches well the spectrum of whitlockite Ca18Mg2(HPO4)2(PO4)12 (JCDPS No. 70-2064). However, it is hard to distinguish whitlockite from Mg-TCP with high Mg content by XRD . Although some of the bands such as 992 cm−1 fit the characteristic bands of whitlockite , the key band of P–O–H stretching in HPO42− at 917 cm−1 is absent. Accordingly, the phase of the ivories burnt at temperatures between 800 and 1000 °C cannot be determined as whitlockite but Mg-TCP instead. Since the transformation of Mg-HAp to Mg-TCP coincides with the temperature range of CN22− formation, CN22− group can no longer be kept in the lattice as it does in apatite. Therefore, the burning atmosphere has little influence on ivories unlike bovine bones. It is also noted that the IRSFs drops when new phases such as Mg-TCP or cyanamidapatite are formed at 800 °C, which suggests the burning temperature cannot be solely determined by IRSF.
Many bioapatite materials are known to more easily transform to β-tricalcium phosphate (TCP) or Mg-TCP. For instance, TCP or Mg-TCP was found in fish bones of some species burnt at temperatures as low as 600 °C while samples of mammals and birds did not form this phase at temperatures below 1000 °C [28, 29]. TCP was found in deer antler burnt at temperatures between 800 and 1000 °C while whale tympanic bulla only formed CaO and HAP under the same conditions . Additionally, mammoth tusk was reported to transform from Mg-HAp to whitlockite during thermal treatment from 600 to 1000 °C . Human teeth were also observed the occurrence of TCP at temperatures as low as 750 °C . It is found that the magnesium ion destabilizes the structure of hydroxyapatite, which reduces the temperature of the hydroxyapatite–whitlockite transition as its content increases . As whitlockite shares a very closed XRD pattern with Mg-TCP, the XRD-determined whitlockite is likely to be Mg-TCP as well. Elemental analysis of the raw bone and ivory dentin shows they share a similar (Mg + Ca)/P ratio while the Mg/(Mg + Ca) ratio in the ivory dentin (0.112) is much higher than that in the bone (0.031) (shown in Table 2). FTIR spectra shown in Fig. 2a, b also confirms that the transition of the prepared Mg-HAp to Mg-TCP takes place at a lower temperature (600 °C) compared to that of HAp to TCP (800 °C). Therefore, it is believed that the higher magnesium content in ivories destabilize the HAp lattice, allowing it more easily to transform to the TCP structure. Additionally, when CO32− substitution is increased by 2% in HAp, it requires a temperature as high as 1000 °C for CHAp to partially transform to TCP (shown in Fig. 2c). The substitution of PO43− by CO32− would cause the (Ca + Mg)/P ratio to deviate more from the theoretical value of TCP (1.5), creating the barrier for the transformation. For the same reason, bovine bones with even higher CO32− substitution in HAp cannot form TCP when burnt below 1000 °C.
Distinction from the phase transition during burial
Since magnesium substitution in HAp destabilizes the lattice, Mg-HAp is also suspected to be less stable in burial environments at pH normally from 5.0 to 8.0. As shown in Fig. 3a, the prepared HAp shows identical FTIR spectra after treated at 95 °C and pH from 5.0 to 8.0 for 2d. Mg-HAp, however, is not as stable as HAp at pH 5.0 and shows new ν3(PO43−) bands at 1134 cm−1, 1060 cm−1, 993 cm−1, P–O–H stretching in HPO42− at 918 cm−1 and ν4(PO43−) bands shifting to 603 cm−1 and 560 cm−1 (shown in Fig. 3b), which match the spectrum of whitlockite. The XRD spectrum of the Mg-HAp treated at pH 5.0 (shown in Additional file 1: Fig. S2) matches whitlockite as well with reference to JCDPS No. 70-2064. Although the hydrothermal treatment of the prepared HAp and Mg-HAp cannot reproduce the actual burial outcomes of bones and ivories at archaeological sites, it is indicated that ivories are probably less stable in acidic environment due to the higher Mg content and the phase transition is possible. Supposing whitlockite is formed, it is easily distinguishable from the Mg-TCP of burnt ivories in FTIR spectrum based on the higher wavenumber of ν3(PO43−) band at 1134 cm−1 compared to 1121 cm−1 in the spectra of Mg-TCP and the presence of HPO42− band at around 918 cm−1. Nonetheless, the high content of Mg in ivories dissipates over time, at rates that differ with burial conditions. Elemental analysis of ivory specimens from modern to paleolithic periods found that ivories earlier than 2700 BP have lost the majority of Mg and are indistinguishable from other osseous materials with respect to Mg contents [32,33,34]. This fact indicates that phase transition of ivories during actual burials may be even harder due to a continuous loss of Mg compounds, making the compositions close to non-substituted HAp. Therefore, the possibility of phase transition of ivories during burials remain to be verified until more archaeological specimens are tested or better burial imitation methodology is established.
Ivory samples from the Sanxingdui site
Shown in Fig. 4, the 3 ivory samples from the Sanxingdui site display different appearances. Both XY4-1(1) and XY6-1 have whitish cementum and black dentin, while XY4-1(2) have black cementum and green dentin stained by Cu2+ from bronze rusts, which implies different burning temperatures.
The FTIR spectra of the 3 ivory samples are shown in Fig. 5 and the IRSF are shown in Table 3. Only the spectrum of XY6-1 dentin shows featured absorption bands of Mg-TCP at 1119 cm−1, 985 cm−1, 948 cm−1, and 557 cm−1, which indicates the transformation of Mg-HAp to Mg-TCP and the burning temperature was presumably around 800 °C. Both cementum samples of XY6-1 and XY4-1(1) show evident recrystallization of Mg-HAp judging by the high IRSFs over 6 (shown in Table 3) and decreases in the absorption bands of CO32− at around 1458 cm−1, 1413 cm−1, and 874 cm−1. With reference to Fig. 1, the burning temperature of them is determined to be also around 800 °C as CO32− bands marginally remain. In the spectra of the two samples, the tiny absorption bands at around 990 cm−1 indicate the transformation of Mg-HAp to Mg-TCP in cementum was very limited at around 800 °C due to the much lower Mg content in cementum than that in dentin . Because most of the HAp is preserved in cementum, the absence of absorption bands of CN22− indicates an oxidizing burning condition. The spectrum of XY4-1(1) dentin and the IRSF as 4.21 matches the features of ivory dentin burnt at around 600 °C. The spectra of both the dentin and cementum of XY4-1(2) show pronounced ν3(CO32−) absorption bands at 1454 cm−1 and 1414 cm−1, and the IRSFs lower than 3.5. Considering the light green dentin without carbonized features and black cementum, it was presumably burnt mildly. The slightly increased IRSF should mostly result from collagen decomposition .
The FTIR results of the 3 ivory samples from the Sanxingdui site suggests that these ivories were unevenly burnt at temperatures up to around 800 °C. Unlike bones, burnt ivories have nothing to do with cooking food. Ivories may serve as a symbol of power or raw materials for handcrafts. However, the loss of collagen in burnt ivories would cause a dramatic decrease in mechanical strength, making them less durable and processable. Therefore, the unevenly burnt ivories in the Sanxingdui site are possibly evidences for an accidental conflagration or a sacrificial custom.
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