石质文物表面墨迹的保护作用

在许多不可移动石质文物,尤其是石刻文字表面,因历代文人墨客的拓片留下了大量墨迹。这些墨迹是否对文物本体岩石有害?一直是保护工作者希望了解的问题之一。为此本研究开展了一系列调研和实验研究。通过对各地保存较完好的石刻表面的观察,可以发现石刻表面残留的墨迹具有某种保护作用;对模拟样块的SO2腐蚀实验结果表明,岩石表面的墨迹具有一定的阻止空气中腐蚀性气体侵蚀岩石的作用;盐迁移破坏实验也发现墨迹在某种程度上具有减小可溶盐结晶破坏岩石的作用。吸水率实验和电镜观察等结果显示,墨迹缓解岩石风化的机理主要是墨迹对岩石微裂隙具有填充和加固作用,使岩石的吸水系数降低,部分阻挡了可溶盐的入侵和聚集结晶。     

盐分在文物本体中的迁移及毁损机理

本文以盐分对文物本体的破坏机理为主题,分别评述了国内和国外的代表性研究成果,提出今后文化遗产盐害机理研究中需要重点关注的三个方面的问题。文物本体盐害的发生、发展与遗址本体中盐分的种类、含量、文物本体微结构以及环境变化密切相关,其盐害发生发展机理及盐分活动规律的研究是文化遗产保护的当然要求,也是文化遗产保护的难点和重点。以土遗址和陶质文物为主要研究对象,着眼于共性问题,研究文物本体环境现状、材料组成、病害调查、盐分组成、迁移、毛细吸附、结晶破坏及可溶盐沉淀分布等,归纳出了若干盐害毁损的机理和规律。鉴于硅酸盐质文化遗产盐害分布极其广泛、赋存环境条件千差万别、盐害成因复杂、表现形式多种多样、治理难度极其困难、盐害文物修复后的复发几率又比较频繁,所取得的研究进展还很难解释所有复杂的、千变万化的盐害现象。因此,如何建立一套符合文物盐害特点的研究方法,使得在该方法下获取的研究结果更加符合实际,也是很迫切的一项工作。     

实验室模拟硅酸盐质文物除盐控盐的可视化研究

针对目前广泛使用的材料吸附除盐和加湿喷雾控盐等方法对含盐硅酸盐质文物可能造成的损伤,采用扫描电子显微镜、电阻仪和高分辨摄影仪等实时在线记录具有显色性质硫酸铜溶液在吸附除盐和水雾控盐加湿干燥过程中文物内部盐带分布的变化。研究结果表明:吸附材料除盐法和表面喷雾控盐法均会加速样品内部盐离子的扩散迁移,在样品中形成双层或多层盐结晶带;所形成的盐结晶带与样品结构特性、盐溶液在样品中的运移和蒸发速率、加湿-干燥频率等因素有关,其中溶液毛细上升高度和样品透气性越好,以及环境温度和盐溶液浓度越高,加湿-干燥频率越快,样品内部的盐带分布变化越明显。材料吸附除盐和水雾控盐等方法加速了盐溶液在样品内部的扩散,促进多层盐带的形成。这些结果对于目前文物保护所采取的除盐控盐技术手段提出了新的挑战。     

茂陵石刻表面污染物和可溶盐的分析研究

为了厘清造成茂陵石刻文物劣化的原因和机理,对石刻表面的污染物和析出可溶盐进行了分析研究,以找出石刻文物劣化和污染物、可溶盐之间的关系,为制定科学有效的保护措施提供科学依据。采用X射线衍射仪(XRD)、扫描电镜-能谱仪(SEM-EDS)、傅里叶变换显微红外光谱仪(FTIR)等仪器对采集的茂陵石刻文物表面污染物和可溶盐进行了表征,用离子色谱(IC)分析了可溶盐的离子种类和含量。结果表明,“马踏匈奴”石刻雕像表面污染物的主要成分是CaSO4·2H2O,其形成原因是石材与空气污染物经长期化学反应形成的钙质结壳,或者是地下水中可溶盐的毛细运移造成的;“怪兽吃羊”石刻雕像表面析出的可溶盐有Na2SO4、NaNO3和CaSO4·2H2O,主要是由于地下水中可溶盐从砖石基座迁移到石刻文物本体,随温湿度变化反复结晶溶解所造成。可溶盐会引起石质文物表面酥粉、片状剥落、内部结构破坏等侵蚀,是茂陵石刻文物的主要病害之一。     

石窟壁画可溶盐破坏过程的实验室模拟研究

石窟壁画是十分珍贵的文化遗产,但是其保存一直受到可溶盐及相关病害的威胁。研究石窟壁画可溶盐破坏过程是治理和预防这类病害的基础工作,深入的基础研究既需要全面的现场勘测,也需要实验室的模拟研究,能否制作出反映实际情况的试验样品仍然是十分艰巨的挑战。为此,作为一系列应用基础研究的第一步,通过对敦煌莫高窟壁画典型制作工艺和病害情况的调研,在敦煌研究院工作的基础上,规范地制作出便于实验室研究的模拟样品。通过对样品的破坏循环实验表明,所做样品可以重现可溶盐的迁移过程、分布状况和破坏情形。同时,样品的各种性能变化参数能够比较方便准确地测定,呈现的病害状况基本符合文物现场的实际现象,初步实现了石窟壁画病害的实验室模拟研究。本工作建立的模拟方法为深入开展壁画保护的基础科学研究开辟了一种途径。     

石质文物表面憎水性化学保护的副作用研究

根据最小干预和不影响文物原貌的原则,使用表面化学防护和化学加固等保护材料被认为是保护石质文物的有效方法。目前,已试用于石质文物表面保护的化学材料主要是有机硅等有机化合物。这些有机材料能起到一定的防水和加固作用,但是也潜在不少问题,一些可能的危害目前还没有引起人们足够的重视。本工作以通用有机硅类防水加固剂为表面保护材料,以砂岩、凝灰岩、白云岩、大理岩和花岗岩为保护对象,模拟各种破坏因素,包括干湿循环、盐结晶、冻融和加热等的作用和联合作用,研究了表面化学处理后岩石样品在模拟环境中的变化情况,发现破坏现象严重。其中,化学保护剂在石材表层产生的亲水/憎水界面导致石材受某些应力的加速破坏是重要原因。上述研究结果提示,在石质文物的保护中应慎重使用憎水性表面化学保护剂。     

陶质文物科技保护研究现状

为了提升陶质文物科技保护水平,本研究从陶质文物病害认知、制陶工艺研究、保护机理研究、保护修复案例等方面进行文献综述,回顾了国内外陶质文物科技保护的历史与现状,分析了目前陶质文物科技保护中存在的问题:陶质文物本体形成的物理化学基础研究尚不深入,预防性保护缺乏预测陶质文物病害方面的研究,缺乏科学规范的保护修复效果评价体系,未来应深入地研究陶质文物保护技术基础理论,开发出考古发掘现场成熟的系统的综合保护应用技术,并将陶质文物保护修复材料研究、保护修复技术标准化、规范化研究方面深入下去。     

硫酸盐与氯化物对壁画的破坏性对比研究——硫酸钠超强的穿透、迁移及结晶破坏力证据

莫高窟壁画酥碱病变的可溶盐主要为NaCl和Na2SO4,其中Na2SO4是破坏性最严重的盐。为此,以莫高窟壁画典型盐害为模拟对象进行仿真试验研究,重点研究Na2SO4盐在温度交变、干湿循环作用下对砂砾岩、地帐层的剥蚀破坏和强度的影响。结果表明,Na2SO4具有超强的穿透、迁移能力及结晶破坏能力,其结晶行为机理及壁画损害表现形式与氯化钠有很大不同。Na2SO4引起的试块膨胀和剥蚀破坏随着Na2SO4浓度和干湿循环次数的增加明显增大。气象条件中,32.4℃以下的温度区间的温度变化及40%以上的干湿度循环交变,是造成病害加剧的主要因素。盐涨试块强度测试表明,盐的结晶对壁画基体材质砂砾岩的强度损害是非常大,常见不可逆的粉末状酥碱及粒状剥离。研究结果可为正确保护壁画作理论探索时参考。     

薄荷醇/石膏复合体系用于考古发掘现场临时固型提取的可行性研究

复杂而且脆弱的文物或化石的提取和搬迁是考古或化石发掘现场最常见的技术难题。采用石膏对这些文物或化石进行现场固型再提取是常用的应急性保护手段之一,对此类文物或化石的提取和搬迁具有重要作用。但石膏含有可溶盐,应用时需引入水,并大量放热,对文物或化石有潜在危害,而且石膏难以完全去除。 近年来,薄荷醇、环十二烷等可挥发性临时固型材料受到了极大关注,被越来越多地应用在发掘现场。其最大的优点就是固型材料可以通过升华轻易被去除,无残留,不会影响后续的考古研究和保护工作。但是有机物小分子固化后强度偏低,不适合较大较重文物或化石的固型提取。 针对考古发掘现场复杂脆弱而且较大、重的文物或化石的提取和搬迁需求,提出了新型临时固型材料薄荷醇和石膏复合应用的现场固型提取技术体系。本工作研究了石膏和有机小分子薄荷醇的相容性和匹配性,实验室研究结果显示薄荷醇除了在样品表面实现有效固型,还能快速在模拟样品表面形成一层致密的疏水隔离层,有效地阻止石膏加固过程中水和可溶盐的进入;而且实验数据显示石膏放热不影响薄荷醇疏水隔离层的功能。实验结果显示薄荷醇和石膏二者性能互补,有望满足发掘现场对大、重文物或化石的固型提取需求。 以新疆哈密翼龙化石的现场发掘提取为例,采用薄荷醇/石膏复合材料体系现场固型并提取一块约20 kg重的化石样品,结果证实石膏和薄荷醇复合使用技术可以很好地固型并提取较大较重的样品,同时避免了水和可溶盐对化石的破坏,而由于薄荷醇隔离层的存在,石膏的去除也非常彻底,整体固型提取效果良好。     

土质文物盐害中硫酸钠的研究——从微观到宏观

土遗址和壁画遗址可溶盐主要为NaCl和Na_2SO_4,其中Na_2SO_4是破坏程度最严重的盐。一般认为,硫酸钠只有在温度、湿度变化时才不稳定,才发生得失结晶水引起的晶型及体积变化。为研究土质文物盐害的形成机制及内在原因,调研了全国几处典型土遗址和壁画遗址的盐害状况并取样分析。分析发现:1)土遗址和壁画遗址中硫酸钠的形貌、结构是动态变化的(无论在任何极端干燥环境甚至真空条件下);2)环境因素如温度、相对湿度的变化只是加剧上述变化进程。硫酸钠结晶析出时,其体积膨胀约4倍。这一系列的过程,深刻影响着硫酸钠的相变复杂性和对依存土质文物本体的破坏性。本工作从微观和宏观两个层面探讨了这些发现,并通过不同盐分(不同浓度氯化钠、硫酸钠)侵蚀破坏模拟试验、不同温度条件下模拟试验、不同相对湿度下模拟试验,从宏观层面印证了上述微观机制在土遗址和壁画遗址本体的外在表现。本研究发现土遗址中硫酸钠的形貌、结构是动态变化的,该工作弥补了以往研究和认识上的局限。本研究将对阻止、延缓及预测土质文物盐害的发生、发展提供科学参考。     

几种商用保护材料对模拟秦俑陶胎的加固和防盐风化性能研究

目前对古代陶胎的研究多集中在加固材料的筛选及其保护效果评估上,普遍缺乏深入的保护机理的研究,尤其保护材料对陶胎盐风化的影响及其机理研究在国内更是罕见。因此,本实验选用TEOS(正硅酸乙酯)、Paraloid B72和WD10对模拟古代秦俑陶胎进行了加固和防盐风化保护研究。重点观察了不同材料处理后陶胎样品表面盐结晶的形貌,并对其形成机理进行了探讨。最后对Paraloid B72在耐盐实验中出现的保护性破坏现象给予关注。结果显示,TEOS处理后陶胎的抗压强度得到提高,但对陶胎的耐盐性能没有改善;Paraloid B72处理后,陶胎颜色加深,耐盐实验中处理陶胎出现了严重的盐风化。WD10处理后陶胎的耐盐性能显著增强。对盐结晶的微观形貌研究初步揭示,保护材料能够对陶胎表面进行改性,不同的憎水性和渗透性会显著影响盐结晶的微观形貌,进而会带来不同类型的盐风化。本研究结果启示文物保护工作者必须深入了解各种保护材料的优缺点,根据被保护文物的具体实际需求,慎重开发和选择保护材料,以避免保护性破坏现象的出现。     

Detection of chloride from pottery as a marker for salt: A new analytical method validated using simulated salt-making pottery and applied to Japanese ceramics

Salt is an essential mineral in the human diet, and ancient peoples obtained salt either directly from rock salt, from salt lakes, or by concentrating saline waters from salt springs or seawater in pottery vessels. However, because sodium chloride, the major component of salt, is soluble in water, it has been thought unlikely that any trace of salt would remain in the pottery after a long period of time. A new methodology for retrieval of water-insoluble (retainable) chloride ion trapped within a pottery matrix is presented as a method for detecting previous use of the pottery for salt making. Simulated salt-making pottery was used to make salt by repeatedly boiling seawater over a fire. After chloride had been extracted with distilled water, to mimic the removal of chloride by natural waters such as rainwater and groundwater, an aqueous ammonium fluoride solution was shown to be capable of extracting chloride ions remaining in the pottery. A chloride-selective electrode was used to quantify the amount of extracted chloride in the presence of fluoride. This method was then successfully applied to excavated Japanese pottery vessels suspected of having been used for making salt. Identification of retainable chloride in pottery can offer insights into salt trading networks, which reflect the growth and affluence of an ancient society.     

The Origin of Soluble Salts in Rocks of the Thebes Mountains, Egypt: The Damage Potential to Ancient Egyptian Wall Art

Soluble salts are indigenous to the marine sediments of the Esna Formation and Thebes Formation in central Egypt. Natural weathering processes and human impact cause salt efflorescence in tombs and on walls of ancient Egyptian monuments in the Theban Necropolis, resulting in the disintegration of wall paintings and carved hieroglyphs. Determination and quantification of soluble salts and measurements of the porosity were done on tomb marls in order to understand the origin and damage potential of the salts in the royal tombs of the Valley of the Kings. The study shows that sodium chloride (NaCl) is the predominant salt species. Sulphates, anhydrite (CaSO₄) and gypsum (CaSO₄ . 2 H₂O) also occur in the limestone deposits of the Thebes Formation and the underlying shale deposits of the Esna Formation. Member I of the Thebes Formation, into which most of the ancient tombs were hewn, shows the highest amount of soluble salts (up to 6·2 wt-% dry). In addition, the porosity and permeability of the marls in Member I are high, enhancing water molecules and brine migration. Wetting and drying of the tomb surfaces in the past has affected solution, transportation and recrystallization of water soluble minerals, and has led to major destruction of wall paintings and ornamentations. This study shows that large amounts of salt minerals still remain in the wall rocks. Future humidity changes due to flash floods and tourists may favour salt crystallization in the ancient Egyptian monuments. The authors urge the authorities to undertake steps to prevent further destruction to save the magnificent cultural heritage found in the Thebes area.     

Microclimate and Salt Crystallization in the Crypt of Lecce’s Duomo

Ancient masonry structures are often damaged by soluble salt crystallization, which is activated by even small microclimatic variations. Unsuitable environmental conditions can accelerate this process, affecting the type and the quantity of salts and the consequent damage to the masonry. Therefore the importance of monitoring salt diffusion to control salt crystallization and microclimate over time is widely recognized. This study proposes an integrated monitoring methodology to obtain information on the relationship between salt efflorescence and microclimate in the Crypt of the Duomo of Lecce (South Italy). By combining ion chromatography, powder X-ray diffraction analysis, and Raman microscopy with environmental monitoring and deterioration maps, salt components were identified and efflorescence diffusion on masonry was monitored over time. Due to this approach, a possible explanation for the process is finally given.     

Freezing and melting behaviors of H2O‐NaCl‐CaCl2 solutions in fused silica capillaries and glass‐sandwiched films: implications for fluid inclusion studies

Fluid inclusions of the H₂O‐NaCl‐CaCl₂ system are notorious for their metastable behavior during cooling and heating processes, which can render microthermometric measurement impossible or difficult and interpretation of the results ambiguous. This study addresses these problems through detailed microscopic examination of synthetic solutions during cooling and warming runs, development of methods to enhance nucleation of hydrates, and comparison of microthermometric results with different degrees of metastability with values predicted for stable conditions. Synthetic H₂O‐NaCl‐CaCl₂ solutions with different NaCl/(NaCl + CaCl₂) ratios were prepared and loaded in fused silica capillaries and glass‐sandwiched films for microthermometric studies; pure solutions were used with the capillaries to simulate fluid inclusions, whereas alumina powder was added in the solutions to facilitate ice and hydrate crystallization in the sandwiched samples. The phase changes observed and the microthermometric data obtained in this study have led to the following conclusions that have important implications for fluid inclusion studies: (i) most H₂O‐NaCl‐CaCl₂ inclusions that appear to be completely frozen in the first cooling run to ?185°C actually contain large amounts of residual solution, as also reported in some previous studies; (ii) inability of H₂O‐NaCl‐CaCl₂ inclusions to freeze completely may be related to their composition (low NaCl/(NaCl + CaCl₂) ratios) and lack of solid particles; (iii) crystallization of hydrates, which is important for cryogenic Raman spectroscopic studies of fluid inclusion composition, can be greatly enhanced by finding an optimum combination of cooling and warming rates and temperatures; and (iv) even if an inclusion is not completely frozen, the melting temperatures of hydrohalite and ice are still valid for estimating the fluid composition.     

Soluble Salts Amplify the Effect of Small Heterogeneities in the Structure of Porous Building Materials During Drying

Salt crystallization is a major cause of degradation in old buildings. One of the issues that stills need clarification is regarding the influence of the salts on the capillary absorption and subsequent drying of porous building materials. This article presents an experimental study that included capillary absorption and evaporative drying tests on two types of material (lime mortar and ceramic brick) using pure water or saturated solutions of six salts (sodium chloride, sodium sulfate, sodium nitrate, sodium carbonate, potassium nitrate, or potassium carbonate). The results of capillary absorption agree only roughly with the linear relationship, predicted by theory, between sorptivity and the square root of the ratio between viscosity η and surface tension σ of the solution (σ/η)^1/2. This poor agreement is probably due to material heterogeneity. The drying dynamics was regular and showed little dispersion between specimens, but only for the uncontaminated materials. Indeed, the drying dynamics of the salt contaminated materials was often irregular or diverged among similar specimens, and the same happened with their salt decay patterns. The main conclusion is that soluble salts can amplify the effects on drying of the small structural heterogeneities that porous building materials normally depict.     

典型硫酸盐环境中砖质文物的脱盐与加固材料

中国保存有众多砖质文物建筑,目前这些文物多处于户外环境中,面临严重盐害导致的掏蚀损毁问题。因而,研究兼具脱盐功能的加固材料成为砖质文物等保护的一种创新思路,加固材料的研制和工艺技术的开发是其重点和难点,具有重大的社会需求和急迫性。为此,本工作从可溶性盐的破坏机制出发,研究出一种双组分无机加固材料,加固方法是Ba(OH)_2甲醇溶液与H_2C_2O_4、H_3PO_4乙醇溶液交替滴加,并模拟可溶性盐(NaCl、KCl、Na_2SO_4)分布的典型环境,进行加固、除盐,借助X射线衍射仪(XRD)、X射线荧光光谱仪(XRF)和无线阻尼抗钻仪等现代科学仪器,追踪盐的流出与转化过程。结果表明,本材料不仅能够加固模拟样品,同时能对最具破坏性的盐Na_2SO_4实现不可逆转化,生成重晶石的主要成分BaSO_4,加固与脱盐一次完成,是一种具有实际应用潜力的新材料。     

THE EFFECT OF SALTS ON THERMAL AND HYDRIC DILATATION OF POROUS BUILDING STONE*

Fifteen desalinated sandstone drill core samples from Umm Ishrin Sandstone Formation in Petra (Cambrian age) were used for this study. The samples were mineralogically analysed using X‐ray diffraction and their physical properties were also determined. Samples with similar physical properties and mineralogical composition were taken for further experimental work. After desalination, thermal and hydric dilatation coefficients were measured, then three types of salts (NaCl, KCl and Na₂SO₄·10H₂O), which have high solubility and consequently are the most dangerous to building stone (and are also detected in the sandstone monuments in Petra), were introduced into the samples and their contents were calculated. The results show that salt crystallization in the pores of building stones can increase their thermal dilatation and decrease their hydric dilatation to varying extents, depending on the nature of the salt. The average increase in the thermal dilatation coefficient per unit mass of salt is the lowest for the Na₂SO₄·10H₂O‐salted samples with a value of 5.3%, while the NaCl‐salted samples have the highest value with 7.8% per salt mass. The average percentage of the decrease of the hydric dilatation coefficient is 1061% for Na₂SO₄·10H₂O‐salted samples per mass of salt content; the NaCl‐salted samples have a value of 1510% per mass of salt content, and the KCl‐salted samples almost the same value. For the salt‐free samples, it was found that in climatic conditions with a high temperature range, the deterioration of sandstone due to temperature fluctuation is more effective than that caused by change in the moisture content, while samples with high salt content suffer more from hydric dilatation.     

Solubility of quartz in crustal fluids: experiments and general equations for salt solutions and H2O–CO2 mixtures at 400–800°C and 0.1–0.9 GPa

The solubility of quartz has been measured in a wide range of salt solutions at 800°C and 0.5 GPa, and in NaCl, CaCl₂ and CsCl solutions and H₂O–CO₂ fluids at six additional P–T conditions ranging from 400°C at 0.1 GPa to 800°C at 0.9 GPa. The experiments cover a wide range of compositions along each binary. At P–T conditions where the density of pure water is low (0.43 g cm^?3), addition of most salts produces an enhancement of quartz solubility at low to moderate salt concentrations (salt‐in effect), although quartz solubility falls with further decrease in X_H2O. At higher fluid densities (0.7 g cm^?3 and greater), the salt‐in effect is generally absent, although this depends on both the cation present and the actual P–T conditions. The salt‐in effect is most readily produced by chloride salts of large monovalent cations, while CaCl₂ only produced a salt‐in effect at the most extreme conditions of high‐T and low‐P investigated (800°C at 0.2 GPa). Under most crustal conditions, the addition of common salts to aqueous fluids results in a lowering of quartz solubility relative to that in pure water (salt‐out effect). Comparing quartz solubility in different fluids by calculating X_H2O on the basis that all salts are fully associated under all conditions yields higher quartz solubility in solutions of monovalent salts than in solutions of divalent salts, absolute values are also influenced by cation radius. Quartz solubility measurements have been fitted to a Setchenow‐type equation, modified to take account of the separate effects of both the lowering of X_H2O and the specific effects of different salts, which are treated as arising through distinct patterns of non‐ideal behaviour, rather than the explicit formation of additional silica complexes with salt components. Quartz solubility in H₂O–CO₂ fluids can be treated as ideal, if the solvation number of aqueous silica is taken as 3.5. For this system the solubility (molality) of quartz in the binary fluid, S is related to its solubility in pure water at the same P–T conditions, S^o, by: Quartz solubility in binary salt systems (H₂O–RCl_n) can be fitted to the relationship: where salt concentration mRCl_n is expressed as molality and the exponent b has a value of 1 except under conditions where salting‐in is observed at low salt concentrations, in which case it is <1. Under most crustal conditions, the solubility of quartz in NaCl solutions is given to a good approximation by: We propose that quartz solubility in multicomponent fluids can be estimated from an extended expression, calculating X_H2O based on the total fluid composition (including dissolved gasses), and adding terms for each major salt present. Our experimental results on H₂O–NaCl–CO₂ fluids are satisfactorily predicted on this basis. An important implication of the results presented here is that there are circumstances where the migration of a fluid from one quartz‐bearing host into another, if it is accompanied by re‐equilibration through cation exchange, may lead to dissolution or precipitation of quartz even at constant P and T, with concomitant modification of the permeability structure of the deep crust.