三星堆出土象牙多层级结构及失水过程研究

由于长期埋藏于潮湿的地下环境,三星堆遗址祭祀区出土的象牙含水率高,且存在本体断裂、表面开裂和剥落等问题,保存状况差。出土后的象牙极易快速失水,而发生开裂和粉化等不可逆的损害。为明确三星堆出土象牙的保存现状,采用光谱、能谱、电镜、纳米压痕等测试手段,深入解析了2021年三星堆遗址3、4、7和8号坑(K3、K4、K7、K8)发掘出土象牙的组成、结构及失水过程。结果表明:三星堆遗址四个坑出土象牙的主要物相为碳羟基磷灰石[Ca₁₀(PO₄)₃(CO₃)₃(OH)₂],有机纤维蛋白成分基本消失殆尽,形成了以介孔为主(孔径分布在3~50 nm)的孔隙结构,且孔隙被大量水分占位,其中,K3出土象牙(K3XY-54)和K4出土象牙(K4YW-289)热重所测含水率分别为43.8%和43.4%；四个坑出土象牙均存在片状和针状晶体结构,其中K3XY-54以针状晶体结构为主,并呈现出明显的取向生长,K4YW-289呈现晶体聚集,其晶体尺寸厚度明显增加；出土象牙表面和内部吸附水的挥发使象牙微观结构产生内应力,从而造成出土象牙干裂、粉化等不可逆损坏；采用纳米压痕技术,初步探索了出土象牙在糟朽状态下表面微观硬度的变化,其中K4YW-289经失水前后的表面微观硬度变化差异最大(饱水状态0.090±0.049 GPa,失水后1.553±0.312 GPa),这可能与其组分结构内吸附水含量较少和高泥化现象有关,结合K3XY-54、K7XY-43和K8XY-11-13研究可发现出土象牙在失水前后微观硬度和弹性模量受组分结构-失水速率关系的影响较大。对三星堆出土象牙多层级结构和失水过程的初步研究结果为其后期实施有效保存和本体加固提供有力的科学依据和参考数据。

Preliminary research on the multi-hierarchy structure and water loss process of excavated ivories at Sanxingdui

The high moisture content of excavated ivories excavated from Sanxingdui sacrificial pits is due to their long-term burial in the wet underground environment. There are also such problems as body fracture, glaze cracking and peeling, resulting from poor preservation conditions. It is very easy for excavated ivories to lose water quickly and suffer irreversible damages such as cracking and pulverization. To clarify the preservation status of excavated ivories from Sanxingdui site, the composition, structure and water loss process of ivories at Pits No.3,4, 7 and 8 (K3, K4, K7 and K8) of Sanxingdui site excavated in 2021 were analyzed by means of spectrometry, electron microscopy and nano-indentation, etc. The results show that the main phase of the excavated ivories from the four sacrificial pits was hydroxyapatite [Ca₁₀(PO₄)₃(CO₃)₃(OH)₂]. The organic components had largely disappeared, forming a mainly mesoporous pore structure (pore size distribution in the range of 3-50 nm). These pore structures were occupied by a large amount of adsorbed water and bound water, showing high adsorbent water contents. The moisture contents of K3-excavated ivory (K3XY-54) and K4-excavated ivory (K4YW-289) were 43.8% and 43.4%, respectively. All the four sacrificial pits had flake and needle-like crystal structures, among which, K3XY-54 showed obvious orientation growth with needle-like crystal structures, while K4YW-289 showed crystal aggregation and significant increase in thickness. The internal stress of microstructure caused by the volatilization of adsorbed water on the surface and inside of excavated ivories, resulted in irreversible damages such as cracking and pulverization. The surface micro-hardness changes of excavated ivories in the rotten state were preliminarily explored by nano-indentation. The surface micro-hardness difference of K4YW-289 before and after water loss was the largest (0.090±0.049 GPa in the waterlogged state, 1.55±0.31 GPa after water loss), which may be related to the low adsorbed water content in the component structure and high argillation phenomenon. In combination with the studies of K3XY-54, K7XY-43 and K8XY-11-13, it was found that the micro-hardness and elasticity modulus of excavated ivories before and after water loss was largely affected by the relationship between the component structure and water loss rate. These research results provide a powerful scientific basis and reference data for the subsequent effective implementation of preservation and reinforcement of excavated ivories.