颜料层微观形貌对古代壁画褪色及显现影响

文物壁画在自然环境下产生褪色现象具有复杂性，严格意义上讲，人们无法重现自然环境下这种褪色发生的条件，但这并不妨碍人们研究其褪色原因的可能性和途径。通过实验来验证所提出机制的合理性，这也符合自然科学普遍采用的研究方法。提出颜料层微观形貌变化是引起壁画褪色的一种重要原因。胶料作为颜料层的连续相，当胶料分解时，颜料层微观形貌必然产生变化，为证明这种变化会引起颜料层褪色，而非颜料变化所致。建立了以具有较高热稳定性的赭石为颜料，以明胶作为粘接剂制成模拟的壁画颜料层。然后通过加热处理使胶料发生氧化分解，从而造成颜料层微观形貌发生变化，但赭石颜料依然稳定，期望证明在颜料赭石不发生变化时，由于颜料层微观形貌变化也可引起颜料褪色。实验通过煅烧壁画颜料层模拟样获得胶料降解的壁画模型，分别以SEM及多角度反射率光谱等技术，考察模拟样胶料降解前后表面微观形貌和光学性质变化。结果表明：壁画模拟样煅烧后，颜料层表面产生空隙结构，样品表面多角度反射率光谱表明煅烧后样品颜料层对可见光的反射增加而吸收降低，并观察到模拟样颜色淡化。以无色，难挥发，且不与颜料层发生化学反应的离子液体涂覆褪色样品表面，以上述技术表征涂覆处理前后颜料层表面微观形貌和表面光学性质变化，结果表明：离子液体填充了褪色样品表面颗粒间空隙，检测到样品表面对可见光的反射降低而吸收增加，观察到褪色样品的颜色加深。以上实验结果证明了推测合理性，即微观形貌变化确实会引起颜色变化。依据此原理以唐代墓室褪色壁画为例，进行了显现修复应用研究，其颜色显现效果显著。

Effect of Surface Morphology on the Fading and Showing of Ancient Mural Painting

Fading phenomenon under natural environment is complicated. Strictly, it is impossible to reproduce above phenomenon. However, it is possible to reveal a reasonable mechanism for fading ancient mural, and the rationality proposed mechanism could be verified by experimental method, which is a basic point of view of natural science. In this paper, the viewpoint that the fading of mural painting caused by degradation of binder rather than change of pigments, was proposed. To verify this viewpoint, the following experiments were conducted. To construct the simulated mural painting with stable pigments and degradable binder, thermally stable ocher and thermally degradation gelatin were used in preparation of simulated mural painting. The composition and surface of simulated painting before and after calcination were characterized by TG, IR, SEM and reflectance spectroscopy, respectively. The following results were obtained. After calcination, pore structure emerged in painting layer. From multiple-angled reflectance spectra, the reflectance of visible light increased and the absorption decreased, and the color fading of the simulated sample was observed. The ionic liquid, colorless, nonvolatile and no reaction with painting layer, was coated on the surface of faded sample. Taking the same characterization method to investigate the surface morphology and optical properties changes before and after ionic liquid treatment, the results indicate that the pore in faded painting layer was filled by ionic liquid. Furthermore, reflectance for treated painting layer decreased, the absorption improved and the color of faded painting layer changed to deep color. In fact, this experiment result could verify the conjecture that morphology changes in painting layer could cause the fading. According to this results, a real faded ancient mural painting was restored using the same method and the restoration is remarkable.