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0 引言
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我国古代壁画主要分布在石窟寺、殿堂和墓葬,其长期受自然因素和人为因素影响产生如起甲、脱落、酥碱、空鼓、粉化和烟熏等多种病害,严重威胁其安全与价值[1]。其中因在寺庙内从事宗教活动时焚烧香、蜡、油灯,或取暖、烧火做饭等造成的烟熏污染,严重遮蔽画面内容和视觉效果。针对壁画烟熏层去除,早在20世纪80年代段修业[2-4]、王进玉[5]采用碳酸钠清除烟熏层后,再用柠檬酸中和过量碳酸钠和双氧水清除缝隙中碳颗粒的方法,对莫高窟第71窟、235窟和261窟烟熏层进行清洗,并探讨了清洗机理。但以水或有机溶剂清洁会导致溶解的污渍流动扩散,不易控制,清洁剂残留带来潜在风险,渗入后可能造成进一步腐蚀[6]。20世纪80年代末,Wolbers[7]采用“溶剂凝胶”的方法,以凝胶基质中最小化液体溶剂接触污渍,去除污渍的同时减少了绘画层的浸润和膨胀。凝胶材料因易成形、易于控制,可控释放和避免清洗组分过度渗透等应用优势[8-9]应用广泛,包括清洁建筑彩画[9-11]、石质造像[12-13]、青铜器[14-16]、油画帆布[17-18]、书画纸张[19-25]以及壁画[26-31]等文物表面油烟渍和其他沉积物。其在清洁石质和青铜等硬质文物表面污渍以及故宫倦勤斋内檐斑竹纹彩画、漆装饰表面、符望阁迎风板背部彩画时效果较好,而在清洁帆布、纸张和壁画等脆弱质多孔类文物时表面易残留[31-33]。近期,Al-Emam[34-35]等采用聚乙烯醇-硼砂琼脂糖(PVA-B/AG)混合水凝胶成功去除了古埃及阿比多斯塞提一世神庙(the Temple of Seti I)壁画表面烟熏层和Paraloid B72固化层,主要组分为双网状凝胶PVA-B/AG加载5%氨水、0.3%碳酸铵和0.3%EDTA,然而未用仪器检测残留情况。Segel采用纳米Peggy 6凝胶去除石灰基壁画污垢时效果良好,无残留物和造成损伤,并对Hoejelse Church壁画开展清洁试验,视觉评估效果较好[29]。
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目前尚未见到利用凝胶材料清洁仿真壁画烟熏的报道。为了探讨凝胶材料对壁画烟熏污渍的清洁能力和效果,本研究以文物保护中常用的卡波姆凝胶(Carbopol)、结冷胶(Gellan gum)和琼脂凝胶(Agar)开展烟熏污染去除试验。以高仿真的四种壁画试块为基底,制作了轻度、中度和重度烟熏污染,利用显微镜观测清洁效果,并用FTIR评测清洁效果。
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1 材料与方法
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1.1 仪器和试剂
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仪器:恒温磁力搅拌器S10-2(上海司乐)、Nicolet iN10MX显微红外成像光谱仪FT-IR(Thermo Scientific,美国)、超景深三维显微镜(Keyence VHX-1000,日本)、偏光显微镜(Leica DMLP,德国)、磨抛机(MetaServ250,美国)。
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试剂:卡波姆940(Noven,Inc公司)、氨水、N,N’-二(2-羟乙基)草酰胺、明胶、硅藻土(阿拉丁)、吐温80(天津凯信化工)、蒸馏水、Gellan水凝胶(PhytoTechnology Laboratories,美国)、乙酸钙(天津市大茂)、琼脂(Solarbio),所用试剂均为分析纯级。矿物颜料石青、石绿、铅丹、土红和朱砂购于西藏“扎西彩虹”,麦草和麻丝市售,醇酸树脂购于郑州双塔涂料。
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1.2 模拟仿真壁画制作
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1.2.1 无涂层壁画试块
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制作了三种试块。
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1)新制壁画试块。依据古代壁画制作材料和工艺,壁画试块(尺寸24cm×24cm×1cm)由2/3的粗泥层和1/3的细泥层及约0.5mm厚的底色层构成。粗泥层采用黏土和沙质量比为2∶1,加入质量比3%长约1cm的麦草。细泥层采用黏土和沙质量比2∶1,加入质量比3%的麻丝组成。待试块干燥后,用方解石粉和5%的明胶混匀涂在细泥层表面作为底色层,在底色层上以矿物颜料绘制莲花等图案。
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2)已老化的无盐分壁画试块。选用2006年模拟起甲病害的试块,其历经120次湿热老化循环(温度25℃、相对湿度RH 80%下持续4h;55℃、RH 30%下持续4h),老化后放置至今约15年,表面有数条开裂纹,局部有轻微龟裂起甲和粉化病害。
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3)已老化的含盐分壁画试块。选用2006年经90次老化循环(25℃、RH 75%持续4h;55℃、RH 25%持续4h)模拟粉化病害的含少量盐分试块,该试块含盐总量达0.6%,为1∶1∶1添加硫酸钠、硝酸钾、氯化钠三种盐,老化后至今约15年,表面颜料呈轻度粉化和数条裂纹。
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1.2.2 涂层壁画试块
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制作试块干燥后,在表面涂刷醇酸树脂的清漆层,涂刷2遍,干燥后清漆层厚约0.5mm。
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制作的壁画试块如图1所示。
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图1 高仿真壁画模拟试块
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Fig.1 High simulation mural test blocks
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1.3 仿真壁画烟熏病害模拟
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将壁画试块正面朝下放置在自制模拟烟熏装置上,下方距试块1m处中间部位点燃干湿混合的木材锯末屑(柏、松、杏、杨等),边缘位置点燃20枚小酥油灯(40mm×25mm)、6盏清油灯(菜籽油混合棉籽油)和佛香2把(2 000支)进行焚烧,分别经过120h、240h、360h后烟熏,制成轻度、中度、重度烟熏试块。
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为了模拟现实中烟熏污染后壁画的状态,且高度仿真烟熏病害,将熏制的试块放置在自然光下暴晒30d后,转入老化箱中进行湿热老化,湿热老化条件为25℃、RH 60%条件4h,50℃、RH 25%条件4h,经30个循环后在紫外光365nm条件下照射12h完成紫外老化。仿真壁画烟熏试块见图2。
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图2 壁画试块烟熏污染模拟
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Fig.2 Simulation results of soot pollution of mural test blocks
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2 清洁实验
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2.1 凝胶制备
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2.2 凝胶清洁方法
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用制备好的凝胶清洁四类表面为轻度、中度和重度烟熏污染的壁画试块。凝胶材料贴敷于实验区域20min,待其从无色透明的胶状逐渐变为不同程度的黑褐色后,揭取凝胶材料并立即用蘸去离子水的棉签进行滚动擦拭已被凝胶贴敷区域,棉签清理已溶解的黑褐色污渍层。卡波姆凝胶清洁时需在壁画表面铺设一层擦镜纸作为隔离层,张晓梅等[40]研究指出隔离层能有效防止卡波姆凝胶残留。
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3 结果与讨论
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3.1 烟熏成分与剖面特征
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在莫高窟第172窟甬道壁画较厚烟熏层部位采集1个样品,烟熏层与壁画为分层状态,用竹片刀分离片状烟熏薄层(图3),长约1.2cm,宽0.4cm。
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模拟烟熏和古代壁画有及无烟熏成分的FTIR如图4所示。三类样品经FTIR在675~4 000cm-1表征下,特征峰在2 850~2 927cm-1、1 730~1 750cm-1等多处显示,除涂层为醇酸类的树脂材料外,共同的油状物为饱和脂肪酸甘油酯或饱和脂肪酸,其余物质为部分长链的油类树脂、有机酸、烃类、羰基等多种复杂物质。真实壁画烟熏层中存在硅酸盐物质,可能为泥渍污染的成分。模拟烟熏无涂层样品特征峰中显示出碳酸钙、明胶外,其余特征峰与真实壁画烟熏层相似。
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图3 古代壁画表面烟熏层采集位置及其正反面
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Fig.3 Collection position and its front and back of the soot layer on the surface of an ancient wall painting
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图4 古代壁画烟熏与模拟壁画烟熏样品红外光谱对比
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Fig.4 Comparison of infrared spectra between actual and simulated mural soot samples
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对不同程度烟熏试块表层取样,经树脂包埋,制成切片在偏光显微镜下观察其剖面。图5显示了2#中度烟熏带涂层样品的中层和下层成像,上层为厚约114~130 μm的褐色烟熏层,呈絮状大颗粒堆积;中层为厚约26 μm黑色油状层,为烟熏油烟渍侵蚀变色的涂层;下层为厚约28 μm灰白色层,未受到烟熏侵蚀;剖面清晰呈现了烟熏对不同层位的侵蚀状态。图6显示3#重度烟熏无涂层颜料样品剖面,厚约75 μm的蓝色颜料层上覆盖着一层厚约32~38 μm的黑色油渍污染物,较致密。
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图5 2#中度烟熏的涂层颜料样品在200倍偏光显微镜下剖面成像
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Fig.5 Cross-sectional image of the No.2coated pigment sample with moderate soot pollution under a200× polarizing microscope
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图6 3#重度烟熏无涂层颜料样品偏光显微镜下成像
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Fig.6 Cross-sectional image of the No.3uncoated pigment sample with severe pollution under a polarizing microscope
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3.2 表观清洁效果
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三种凝胶清洁轻度、中度和重度烟熏涂层壁画的结果见图7。对比清洁区域和未清洁区域,不论轻度、中度或重度烟熏污染,三种凝胶均能有效去除表面烟熏污渍,被清洁区域再现出壁画图案,三种凝胶对有涂层壁画表面烟熏污染去除效果均较好。
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三种凝胶在清洁无涂层壁画试块烟熏时,对于轻度和中度烟熏污染清洁效果较为理想,对于重度烟熏能在一定程度减轻烟熏污染。如图8所示,三种凝胶清洁轻度烟熏的含盐老化试块,其贴敷凝胶区域与未清洁周围对比明显,清洁效果较好;琼脂清洁中度烟熏的含盐老化试块,因颜料层中胶结材料老化失效,颜料颗粒黏结力较弱,在清洁过程中凝胶吸附污渍颗粒的同时也吸附了少量颜料颗粒,致使出现颜料颗粒脱落的痕迹,从整体凝胶贴敷的轮廓来看,清洁后分界线和轮廓较明显(图8D);用结冷胶清洁新制的重度烟熏试块(图8G)、卡波姆凝胶清洁重度烟熏不含盐老化试块(图8H)后,明显减轻了黑色油状污渍,增加了白度和亮度,但依然存在部分油烟渍;对比清洁区域与清洁边缘土褐色的污渍堆积以及未清洁区域,两种凝胶均有一定的清洁作用。
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图7 三种凝胶清洁涂层壁画试块烟熏污染
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Fig.7 Cleaning soot on the surface of a coated mural using three kinds of gels
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图8 三种凝胶材料清洁不同类型壁画试块烟熏效果
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Fig.8 Cleaning soot on the surface of different types of murals using three kinds of gels
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3.3 微观清洁效果
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3.3.1 显微镜观察
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为了分析清洁后壁画颜料颗粒的状况,利用便携式显微镜观察三种凝胶材料清洁轻度、中度和重度烟熏污染的微观特征(图9),从图9中a、b、d、f等均可明显看到清洁区域和未清洁区域的分界线,未清洁一侧整体呈现黑灰色,已清洁区域为整体亮红色,清洁区域红色颜料较密集,且亮红色颜料颗粒与部分未彻底去除的黑褐色污渍颗粒交替相间分布,未清洁区域密集分布着黑褐色颗粒。凝胶清洁后壁画整体明亮度提升,去污效果较好。从图9中g、h、i可见三种凝胶清洁后壁画表面黑色油渍呈分层堆积状,清洁边沿的污渍呈现由“黑褐色油渍堆积层—褐色堆积—灰色的薄层—无”的过程,油烟渍的堆积由厚到薄至无,颜色由黑褐色变为淡灰色,至还原壁画本色,三种凝胶均能不同程度减轻壁画烟熏。
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图9 凝胶清洁不同类型壁画试块表面烟熏微观特征
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Fig.9 Soot condition features on the surfaces of different types of murals using gel materials
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经多次实验,对三种凝胶清洁不同程度壁画烟熏的效果评估见表1。评述如下:
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1)三种凝胶对不同程度烟熏的涂层壁画清洁效果较好。
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2)对于无涂层壁画烟熏的去除,无论新制试块或老化的不含盐壁画试块,三种凝胶能有效减轻轻度和中度烟熏。
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3)对于老化的含盐壁画烟熏,三种凝胶仅对轻度烟熏有较好的清洁效果,在清洁中度和重度烟熏时均不同程度粘附颜料颗粒,清洁效果不佳;这也是凝胶应用到真实壁画上最大局限性所在,因真实壁画多数脆弱,含盐分或存在如起甲、粉化、疱疹、酥碱等复杂病害,在清洁此类壁画烟熏污染过程中极易造成颜料颗粒被吸附或颜料层被破坏的现象,故凝胶材料不适宜于清洁壁画颜料层已严重劣化的烟熏。
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4)对于重度烟熏的无涂层试块,三种凝胶均能不同程度减轻表面黑色油状污染,可从黝黑减缓至灰色,但无法恢复至画面清晰状态。比较三种凝胶去污效果,卡波姆最优,结冷胶次之,琼脂稍弱;究其原因,卡波姆为柔软性凝胶,相对保水量较多,结冷胶[37]和琼脂[40]为刚性凝胶,保水量少,且贴敷后与壁画表面贴敷度不严密,清洁效果不及柔软性卡波姆好,结冷胶稍软于琼脂。
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注:表中++++表示优,+++表示良,++表示一般,+表示弱。
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3.3.2 残留物检测
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三种凝胶清洁无涂层壁画表面烟熏后,其FTIR检测结果见图10,表现为动物胶和碳酸钙的特征吸收。3 330cm-1为N-H振动吸收峰,2 927cm-1为亚甲基振动吸收峰,1 646cm-1为羰基振动吸收峰,1 539cm-1为N-H和C-N振动吸收峰,1 451cm-1为亚甲基振动吸收峰,均为动物胶特征吸收。2 523、1 435、878和713cm-1均为碳酸钙特征吸收。除此之外并未发现凝胶吸收峰,说明无残留。
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图10 凝胶清洁无涂层壁画后表面红外光谱图
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Fig.10 FTIR spectra after cleaning uncoated murals using gels
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凝胶清洁涂层壁画烟熏后,经FTIR检测表面残留(图11),有涂层试块经清洁后表现为醇酸树脂的特征吸收。2 924cm-1和2 854cm-1为亚甲基振动吸收峰,1 722cm-1为羰基振动吸收峰,1 253cm-1和1 117cm-1为酯基振动吸收峰,1 068cm-1和741cm-1为苯环特征吸收峰,均为醇酸树脂特征吸收。除此之外未发现凝胶吸收峰,说明无残留。
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图11 凝胶清洁涂层壁画后表面红外光谱图
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Fig.11 FTIR spectra after cleaning coated murals using gels
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3.3.3 讨论
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总体看,烟熏清洁效果受壁画有无涂层、老化程度、脆弱性病害、烟熏或灼烧程度、烟熏污染物与壁画颜料间结合程度、不同燃料导致的烟熏等多种耦合因素共同影响。因涂层壁画表面光滑,颜料颗粒被封护在涂层下,起到一定保护作用,虽烟熏污染物可渗透至涂层中,但极少渗透至颜料颗粒中,故较易清洁;无涂层壁画烟熏清洁效果取决于颜料层老化和烟熏程度,若颜料颗粒粘结紧密,其轻度和中度烟熏的凝胶清洁效果较好,而重度烟熏因污染物直接粘附甚至渗透于颜料颗粒间,故无法彻底清除,仅能减轻表面污染;若壁画老化严重或存在盐害、颜料层脆弱及颜料颗粒间粘结力弱,则不论其烟熏程度,凝胶清洁时均会粘附颜料颗粒,对此类壁画不宜进行清洁操作,包括凝胶清洁。
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凝胶材料残留是困扰文物保护界已久的问题,物理凝胶多为软凝胶,易在粗糙的文物表面或曲面上均匀地粘附,因凝胶内部分子间的粘聚力较弱,故在文物表面易残留;化学凝胶中的共价键使其类似于固体,粘聚力强,基本无残留[31]。卡波姆复合凝胶为柔软粘稠胶状的物理凝胶,在已老化的壁画表面极易残留[31-33],借鉴张晓梅等[40]研究结果实验前在清洁区域铺设隔离层,以防卡波姆粘附于颜料颗粒间隙中造成残留。本实验制备的结冷胶为化学凝胶,无残留[38,41],琼脂凝胶为刚性凝胶,有一定强度的固体状,经Scott[42]研究无残留。为防止清洁材料应用后残留,在凝胶材料中添加少量的表面活性剂,仅0.2g吐温80包裹在网络状凝胶中,起微弱清洁作用,因含量较少,所有样品检测均未检出残留物。说明清洁过程符合《中国文物古迹保护准则》中“最小介入”和“不改变文物原状”原则,可为壁画保护修复和烟熏清洁提供参考。在凝胶清洁后,用蘸水的棉签擦拭清洁区域这一操作会使少量水分引入,微观上可能会引起清洁区域颜料层轻微溶胀现象,但干燥后无颜料层开裂、起翘或粉化等现象。
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在三种凝胶清洁实验中,主要利用了凝胶网络材料的强吸附性和保水性,以及稳定的惰性[31-32],加载表面活性剂(吐温80)来清洁壁画表面烟熏污染。吐温80在水凝胶材料中以乳化作用减弱油状污渍粘附在颜料层的表面张力[43],作用时间内缓慢释放表面活性剂和水分,乳化后逐渐瓦解污渍团的结合能力,污渍团分开后污渍以小颗粒或团状被网络凝胶结构吸附在凝胶组织中,凝胶组织逐渐变灰至黑褐色。根据文献研究[32,44-45,64-69]绘制出本实验中凝胶材料加载吐温80的清洁三相图(图12)。
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图12 三相图显示凝胶材料中吐温80和其他常用溶剂的溶解度参数
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Fig.12 Teas diagram showing solubility parameters of Tween 80and other commonly used solvents in gel materials
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4 结论
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1)三种凝胶对轻度、中度和重度烟熏污染均有一定的清洁效果,能不同程度地减轻壁画烟熏污染,尤其对涂层壁画清洁效果较好。
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2)卡波姆清洁效果最佳,结冷胶次之,琼脂稍弱。
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3)经显微观察和FTIR检测对比分析,三种凝胶清洁去污后壁画表面无残留物,说明其达到绿色环保,无残留效果。
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5 问题与展望
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烟熏模拟过程中,因烟熏燃料点距离壁画试块较近,会导致其局部温度过高,部分重度烟熏受高温烘烤表面颜料与烟熏颗粒粘附更为牢固,整体颜料层变黑变黄,壁画颜色整体发黑,致使清洁后壁画表面颜色呈现一定的黑褐色或黄褐色。当前凝胶材料未加载生物酶类试剂(如脂肪酶)等活性组分[30],凝胶制备技术今后有待进一步改进,以提高凝胶清洁烟熏的效率。
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摘要
烟熏污染是古代壁画的主要病害之一,严重影响其艺术价值和视觉效果。针对石窟和殿堂壁画的烟熏病害,高仿真制作了轻度、中度和重度烟熏试块,采用卡波姆凝胶、琼脂凝胶和结冷胶开展烟熏清洁实验,并利用显微镜观察、剖面分析和红外光谱(FTIR)分析烟熏污渍形态、胶结状态及评估清洁效果。结果表明,三种凝胶材料对轻度和中度烟熏试块的清洁效果较好;并能不同程度地减轻重度烟熏,使黑褐色污染变为浅灰色,其中卡波姆凝胶清洁效果为佳。便携式显微镜观察清洁界面可见,未清洁区密集分布着黑褐色污渍颗粒,清洁区污渍颗粒大幅减少;相较而言,无涂层壁画经清洁后亦有少量污渍颗粒与颜料颗粒交替相间分布,而涂层壁画清洁后除重度烟熏试块表面有少数褐色小泡状结构外,其余均无污渍,清洁后可完全再现颜料本色,壁画亮度明显提升。清洁区经FTIR检测,表面无凝胶残留。然而,清洁效果受壁画有无涂层、老化程度、并发脆弱性病害、烟熏或灼烧程度、污渍与壁画颜料结合程度以及导致烟熏的燃料介质差异等多种耦合因素共同影响。当前凝胶清洁仅适宜于涂层壁画烟熏和无涂层壁画非重度烟熏的清洁。本研究为壁画烟熏病害的清洁提供了参考和借鉴。
Abstract
As one of the main diseases of ancient murals, soot pollution has seriously affected their artistic value and visual appearance. In view of the soot diseases of grotto and temple murals, we prepared mildly, moderately and heavily sooted test blocks using simulation techniques, and conducted soot-cleaning experiments with Carbomer gel, agar gel and gellan gum. The morphology, cementation state and cleaning efficiency of soot stains were analyzed using microscopic observation, profile analysis and Fourier transform infrared (FTIR) spectrometry. The results show that the three kinds of gel materials had better cleaning effects on mildly and moderately sooted test blocks and could relieve heavy soot to certain degrees, turning dark brown pollution into light grey. Among the three, Carbomer gel has the best cleaning effect. After observing interfaces with a portable microscope, we found that 1) black and brown stain particles were densely distributed in the uncleaned area, and that stain particles in the cleaned area were greatly reduced; 2) in contrast, after cleaning, small amounts of stain particles and pigment particles were alternately distributed in the uncoated murals, and the coated murals were free of stains except for only a few small brown bubble structures on the surface of the heavily sooted test blocks (the natural colors of pigments can be completely recovered, and the brightness of murals can be greatly improved). In addition, there was no gel residue on the wall paintings according to FTIR analysis. However, the soot-cleaning effect is affected by many coupling factors, such as whether a mural has a coating, degrees of aging, concurrent vulnerability to diseases, types of soot or burning, degrees of combination between soot stains and mural pigments, and differences between fuel media that lead to sooting. At present, cleaning with gel is only suitable for soot of coated murals and non-heavy soot of uncoated murals. This study may provide a reference for the cleaning of soot diseases of ancient murals.
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