唐卡主色矿物颜料光谱分析

Spectral Analysis of Main Mineral Pigments in Thangka

唐卡作为一件艺术品,具有较高的历史价值和艺术价值。对唐卡的矿物颜料进行鉴别分析,对唐卡的鉴定、修复、数字化存档、再现等具有非常重要的意义。该研究对唐卡主色矿物颜料进行体系性的光谱分析,选用唐卡绘制过程中5种主色常用的矿物颜料,深入分析矿物颜料的可见光、近红外、短波红外光谱特征产生机理,总结了不同色系矿物颜料可见光、近红外、短波红外谱段光谱特征。通过分析同一矿物颜料粉末、调和骨胶颜料以及颜料上布色卡光谱特征,发现粉状颜料调和骨胶后,反射率整体下降,在1 447和1 928 nm附近出现两个水的强吸收特征。而当骨胶溶物涂绘上布后,随着膏状颜料中水分的减少,上述两个吸收特征均变弱,个别颜料在1 447 nm处的吸收特征甚至消失。因此,矿物颜料粉末和颜料上布色卡光谱极为接近,可以在后期的唐卡颜料分析中直接利用唐卡矿物颜料粉末光谱进行匹配分析。唐卡红色矿物颜料为朱砂,矿物成分主要为HgS,其光谱在可见光波段先降后升, 500 nm附近形成一个较深的吸收特征,且吸收峰较宽(430～530 nm),红光谱段附近反射率急速升高,近红外波段反射率变化较为平直,在1 940和2 250 nm附近有弱吸收特征。唐卡黄色矿物颜料主要有三种:土黄(雄黄、雌黄),赭石及金箔,主要成分分别为硫化砷、氧化铁及金。其特征光谱在可见光谱段集中在400～500 nm之间,不同颜料的吸收特征位置和吸收深度均不同。赭石在近红外波段的反射率整体较低,且860 nm附近还出现了吸收特征;而雄黄、雌黄和土黄则在近红外和短波红外谱段表现出反射率较高且波形平直,在1 890和2 230 nm附近有弱吸收特征;金箔在可见光波段的吸收特征窄浅,可作为区分的依据。唐卡蓝色矿物颜料为石青,主要矿物成分为蓝铜矿,其光谱在500～1 000, 1 500, 2 040, 2 285和2 350 nm附近均有较强吸收特征,而在1 885和1 980 nm处有弱吸收特征。唐卡绿色矿物颜料为石绿,主要矿物成分为孔雀石,其光谱在550～1 000 nm有较强的宽吸收特征,在2 270和2 350 nm有明显吸收特征。尽管石青和石绿主要矿物成分皆为碳酸铜,但石绿在900～1 900 nm红-近红外谱段反射率增加较缓, 1 500 nm无吸收特征,可以作为区分石青和石绿的依据。唐卡白色矿物颜料为砗磲和白土,主要矿物成分分别为碳酸钙和高岭土。在可见光谱段范围,砗磲在370 nm有弱吸收特征,而白土则在370和730 nm处有两个明显的吸收特征,可作为区分。在短波红外和近红外谱段,白土在1 425, 1 930和2 230 nm均具有明显吸收特征,砗磲则在1 930和2 320 nm有明显吸收特征, 1 440 nm处吸收特征较弱。且同种矿物颜料粉末,矿物粉末颗粒越大,颜料颜色越深,其光谱特征反射率越低。

As a work of art, Thangka has both high historical value and artistic value. The identification and analysis of the mineral pigments of the Thangka are of great significance to the identification, repair, digital archiving and reproduction of the Thangka. This is the first systematic spectral analysis of five kinds of main Thangka mineral pigment, commonly used in the process. Through an in-depth analysis of the spectral characteristics of mineral pigments, we summarized the spectral features of Thangka’s main mineral pigments. By comparing the spectral characteristics of the same mineral pigment powder, blend bone glue and pigment on the pigment, we found that the reflectance of the powder pigment decreases after the blending of bone glue, and there are two strong absorption peaks near 1 447 and 1 928 nm. When you paint the glue soluble on the cloth, with the reduction of water paste in the paint, the two peaks become weaker, and the absorption peak at 1 447 nm or even disappears. Therefore, the spectra of mineral pigment powder and pigment on the cloth are very close. Mineral pigment powder can be directly used in the analysis of the Thangka pigment spectra match and analysis in the later period. Red mineral pigments on Thangka are cinnabar, whose mineral composition is HgS. The reflectance in the visible band rises after the first drop, and there is a deep absorption valley near 500 nm(430~530 nm). After the rapid rise of red, the reflectance curve near infrared changes slowly, and there are weak absorption valleys in the vicinity of 1 940 and 2 250 nm. There are three main types of Thangka yellow mineral pigments: desert tan(realgar, orpiment), ochre and gold, whose main components are arsenic sulfide, iron oxide and gold. Their spectral characteristics are concentrated in the visible spectrum between 400~500 nm, and the absorption valley position and absorption depth of different pigments are different. Near infrared reflectance Ochre is low, and the 860 nm has appeared near the absorption peak;while desert tan, realgar and orpiment in near infrared and shortwave infrared spectrum show high values and flat curves, with two weak absorptions in the valley near 1 890 and 2 230 nm. The absorption valley of gold in visible band is narrow and shallow, which can be used as the basis to distinguish it. Thangka’s blue mineral pigment is azurite, which has strong absorption characteristics in 500~1 000, 1 500, 2 040, 2 285 nm and near 2 350 nm, and weak absorption characteristics in 1 885 and 1 980 nm. Thangka’s green mineral pigment is malachite, and the spectrum has a strong broad absorption feature in 550~1 000, 2 270 and 2 350 nm. Although the main mineral compositions of malachite and azurite are both copper carbonate, but the reflectance value of malachite in 900~1 900 nm increases slowly, and there is no absorption characteristic at 1500 nm, which can be used to distinguish them. Thangka’s white mineral pigments are mainly clay and clam, respectively, calcium carbonate and kaolin-clay. In the visible spectral range, clam has a weak absorption characteristic in 370 nm, and the clay has two obvious absorption characteristics in the 370 nm and 730 nm, which can be used to distinguish them. In the short wave infrared and near-infrared spectrum, clay has obvious absorption characteristics in 1 425, 1 930 and 2 230 nm, while clam has obvious absorption characteristics in 1 930 and 2 320 nm, plus a weak absorption characteristics in 1 440 nm. As for the same mineral pigment powder, the larger the mineral powder particle is, the darker the color of the pigment will be, and the lower the reflectance of the spectral characteristics will be.