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The importance of the scientific support to the preservation of cultural heritage: 1-Pollutants and organic materials in the Museum Environment Ilaria Bonaduce and Maria Perla Colombini
伊拉瑞·波娜德斯和玛丽亚·贝拉·哥伦比尼(意大利比萨大学化学和工业化学系)
Over the years,the preventive conservation not only deals with climatic considerations but is increas-ingly considering the effects of indoor air pollution in cultural institutions.In this scenario,museums,ar-chives and libraries have a special position because particular climatic requirements have to provide a human well-being and at the same time protect cultural assets against deterioration.In this field,the scientific re-search is aimed to assess the complexity of the physical,chemical and biological risks in situ,both in mu-seums,galleries,churches,and in microenvironments such as showcases and micro-climate frames[1,2].
In view of the extremely wide range of materials that are on display in different types of museums with varying level of visitor interest,it is appropriate to evaluate the extent and possible effects of such exposure on the combinations of materials in art and archival objects.To understand the effect of climate changes,lighting exposure and pollutant concentrations on material properties,laboratory tests on reference materials are performed by using accelerated ageing.This practice helps to understand the main degradation processes undergone by materials.Anyway,considering that reference materials are undamaged,fresh materials aged for relatively short periods,what has been occurred over the lifetime of an artwork may significantly differ from the processes observed in the conditions of accelerated ageing tests.However,this kind of study is the only way to achieve an understanding of most of the degradation processes undergone by artworks stor-ed in micro-environments.Because the experimental results be valid,the qualitative similarity of any ac-celerated cycling treatment used to simulate the degradation effect in the experiments to the actual chemistry that occurs in displayed and stored objects must be ascertained[3].
To understand the conservation problems and to control the indoor environment of Museums,scientists with complementary expertise in microclimate,chemistry,physic,material science and biological con-tamination need to work together.
Generally,most of the problems are related to the use of heating,ventilation and air conditioning devices as well as the presence of mass tourism.Actually,these devices should permit the strict control of temperature,relative humidity and the abatement of the main air pollutants.Paradoxically,the use of air conditionings may generate undesired air motions,heat and vapour variations,an increased deposition rate of gaseous and particulate pollutants,and may influence biodeterioration.Moreover,the increase of mass tourism generates important microclimate perturbations and increases indoor pollution,transporting dust,cloth fibres,human cells and sporae[4]
In Museums the control of relative humidity is fundamental.When the relative humidity increases,hy-groscopic materials adsorb water vapour molecules,increase their size with dimensional changes not ident-ical in the three dimensions and generate internal stress.When the relative humidity level drops,the equi-librium moisture content of the artefacts also decreases causing shrinkage.Cycles in dimensional changes are in the long run dangerous due to their cumulative effect.The amount of consequent damage is directly related to the type of material,the constraints in the material,the amount and number of environmental changes.Water can participate in the processes of material deterioration in a number of ways: physical,photochemical,and hydrolytic.Concerning paintings,water can induce:
chemical hydrolysis of the ester or amide bonds of binders;
physical loss of the bond between the vehicle and a substrate or pigment;
photochemical generation of hydroxyl radicals or other chemical species;
facilitation of ionization and the mobility of ionic entities.
For instance,this last effect is an important aspect in the corrosion of metals; while the hydrolysis of ester groups in cellulose acetate-butyrate polymers yields destructive acidity [5]
Traditional museums located in historic buildings,benefit from the temperature and humidity buffering action of thick walls and interior finishes made of porous hygroscopic materials(e.g.brick,plaster,stucco,wood).Fortunately,historic buildings benefit from long-established knowledge of the response of the structure to external climatic changes; and the building features,to which some mitigative active systems have been adapted,mainly determine the microclimate.
In any case,reducing the span of relative humidity and temperature cycles is beneficial to extending the lifetime of artworks.
In order to obtain a satisfactory microclimate,sensitive works of art are often displayed in tightly sealed showcases[4]
The interior finishes,furnishings and number of visitors influence the particle types and concentrations.Mass tourism is an economic resource,but visitors are responsible for transport of pollutants,emissions of water vapour,CO2,increase of heat and are the principle source of airborne fibre,dust and particles from clothing and shoes.Air conditioning is thus required with a quite high ventilation which may increases the risk of pollution damage to the objects because more pollution from outside is brought into the galleries.Nevertheless,air conditioners for the microclimate control are necessary for the well being of people in Mu-seums,and in order to maintain constancy of temperature and humidity,these devices should operate day and night and not only during museum opening times as often happens.
The characterisation of air pollutants in the museum environment has been predominantly focused on inorganic compounds such as ozone(O3),nitrogen oxides(NOx),sulphur dioxide(SO2)and hydrogen sulfide(H2S)and more rarely on organic acids(formic acid,acetic acid)and formaldehyde.Only recently,attention has been paid to volatile organic compounds(VOCs),semi-volatile organic compounds(SVOCs)and elements in settled dust as well as their mixtures[6,7]
Actually,VOCs represent a major fraction of indoor and outdoor air pollution.Even low concentrations cause malodorous atmospheres and lead to significant health hazards.Less known is their role in the deterio-ration of art objects which makes VOCs a threat to our cultural heritage.In particular,the emission of carbo-xylic acids by materials used for museum display cases has been observed.Wood products,coatings,silicone-based sealants and polyvinyl acetate adhesives,usually employed in the fabrication of frames or storage containers,emit aldehydes and organic acids that are potentially harmful to the art objects[8].Parti-cularly,it has been evidenced the importance in selecting the interior wood material for storage rooms and frames considering fully the characteristics of VOCs emitted by wood,and not only their total amount[9,10]
Figure 1 gives an overview about influences and interactions between works of art and indoor and out-door pollutants as well as climatic parameters.
Figure 1.Interactions between works of art,climate parameters and pollutants.
Since gaseous pollutants(NOx,SOx,03,VOCs)from outdoors tend to be homogeneously distributed within rooms in Museums,their monitoring can be adequately performed by the use of only a few sampling sites.Generally,the air conditioning systems are fitted with particle filters but there are no facilities to remove gaseous pollution.Mitigation strategies to control these pollutants would mainly concentrate on re-ducing their inlet amount and by adding a chemical filtration system in the air conditioning.
Table 1 summarises some results found in the literature[11].
Table 1.Effect of gaseous pollutants on materials stored in museums and measured pollutant concen-trations.
Generally,the fungal load indoors cannot be considered as harmful,neither for the exhibited works of art nor for the employees and visitors.In contrast,sometimes a high bacterial load has been detected mostly depending on the season.However,to decrease the potential biological damage,exposition of plants in museums should be avoided or kept under strict control.
New conception museums can be designed taking into account the most modem conditioning devices provided by technology.Since the indoor turbulence generated by these systems is responsible for the indoor mixing of air and for increasing the deposition rate of suspended particles,devices for extracting coarse par-ticles during the nighttimes have to be adopted to decrease this negative effect and to maintain a stable indoor climate.
References:
[1] P.Brihlecombe,The effects of air pollution on the built environment,Imperial College Press,2003 UK
[2] D.Camuffo,Microclimate for Cultural Heritage,Elsevier,1998 Amsterdam
[3] T.T.Schaeffer,Effects of light on materials in collections,Getty Publications,2001 Los Angeles,Califimia
[4] Dario Camuffo,ReneH Van Grieken,Hans-JuK rgen Busse,Giovanni Sturaro,Antonio Valentino,Adriana Bernardi,Nigel Blades,David Shooter,Kristin Gysels,Felix Deutsch,Monika Wieser,Oliver Kim,Ursula Ulrych,Atmospheric Environment,Supplement n.1,35,S127-S140,(2001)
[5] L.L.Feller,Accelerated Aging.Photochemical and Thermal Aspects,1994 The J.Paul Getty Trust,USA
[6] A.Schieweck,B.Lohrenget,N.Siwinski,C.Genning,T.Salthammer,Occurrence of organic and inorganic biocides in the museum environment,Atmospheric Environment 39,6098-6108,(2005)
[7] M.P.Colombini,A.Andreotti,I.Bonaduce,A.Ceccarini,“La caratterizzazione dei materiali organic”,Meeting “I1 Camposanto di Pisa: un progetto di restauro integrato”,6-8 Matzo 2008 Pisa,Italy
[8] A.F.L.Godoi,L.Van Vaeck,R.Van Grieken,Use of solid-phase mieroextraction for the detection of acetic acid by ion-trap gas chromatography-mass spectrometry and application to indoor levels in museums,J.Chromatogr.A,1067,331-336,(2005)
[9] T.Oikawa,T.Matsui,Y.Matsuda,T.Takayama,H.Niinuma,Y.Nishida,K.Hoshi,M.Yatagai,J Wood Sci 51,363-369,(2005)
[10] European research project PROPAINT,within the EU 6th framework program
[11] P.H.Hatchfield,Pollutants in the Museum Environment,Archetype Publication Ltd.,2002 London
The importance of the scientific support to the preservation of cultural heritage:2-The characterisation of paint binders
Ilaria Bonaduce and Maria Perla Colombini
Introduction
Proteins,glycerolipids,plant and animal resins,natural waxes,and polysaccharides are natural organic substances commonly used as binders,adhesives and varnishes for the realisation of paintings [1].These materials are normally subjected to phisico-chemical modification in the course time,as an effect light and oxygen exposure,daily and seasonal variations of temperature and humidity.As a result,organic ma-terials“age”and their identification in a paint samples becomes a challenge for the chemist.In addition the paint sample is extremely small,for obvious reasons,and its chemical characterisation requires extremely sensitive analytical techniques,able to give as much information as possible.Analytical investigation of paint materials is very important,not only to choose the best conservation practice,and conservation con-ditions.The knowledge of the paint techniques used by the artists can give fundamental information on tech-nical skills of a period,technologies available,and help reconstruct the original overall impression of the work of art.
As follows,as an example will be discussed:a sample collected from the polychromy of the Qin Shihuang's Terracotta Army has been characterised,in order to understand the painting technique.
The analytical procedure used allows to determine glycerolipids,waxes,resins and proteinaceous ma-terials in the same sample[2].The procedure requires a sample pre-treatment able to separate various organic components in two different fractions(a lipid-resinous one,and a proteinaceous one),which are separately analysed by gas chromatography-mass spectrometry(GC-MS).By this way egg,animal glue,milk,and garlic(proteinaeeous materials),linseed,walnut and poppyseed oil(glycerolipids),beeswax and Carnauba wax(natural waxes),Pinaceae resin,sandarac,mastic and dammar resins(plant terpenoid resins)and shellac(animal terpenoid resin),are simultaneously determined on the same paint micro sample(1-5mg)
Analytical procedure for the determination of the paint binder
Sample description.
The results of the analyses of a paint sample from the polychromy of the Terracotta Army will be shown.The priming layer of qi-lacquer,waterlogged by the long time in humid soil,shrinks when the relative hum-idity decreases below 92%.As a consequence,it flakes off the terracotta shortly after excavation.Thus the paint layers are often separated from the qi-lacquer and tend to adhere to the soil.Though this process leads to the irreversible loss of the paint layers from the terracotta,the flakes and layers in the soil can still be used for scientific purposes.
Determination of the paint binder
The sample is subjected to an ammonia extraction.In this way proteins are separated from glycerolipids,waxes,and terpenoid resins.The proteinaceous material contained in the extracted solution is subjected to a purification step,followed by acidic hydrolysis assisted by microwaves,and freed amino acids are analysed by GC-MS.The residue of the extraction,containing glycerolipids,waxes and terpenoid resins,is thus subjected to a salification/saponification assisted by microwaves.This permits to saponify wax esters and to complete the Cannizzaro type reaction for shellac sesquiterpenoid acids,in conditions that are suited to the saponification of glycerides.Neutral and acidic compounds are then extracted from the reaction mixture after acidification,and analysed by GC-MS.The analytical procedure is schematised in Figure 1 and ex-perimental details are published in the literature [2].
Figure 1.Analytical procedure for the characterisation of proteinaceous,lipid and resinous materials in paint samples.
Discussion and results
The sample didn't show the occurrence of any glycerolipid,waxy and resinous material.The analysis of the proteinaceous frac-tion revealed the occur-rence of amino acids and the chromatogram is re-ported in Figure 2.
Figure 2: Chromato-gram(TIC mode)of the proteinaceous fraction of the paint sample from the Terracotta Army.
There is no literature on traditional techniques for painting sculptures in China.Animal glue or plant gums,if necessary mixed with alum to reduce the water solubility,are mentioned for “paintings”-a term that probably refers to water colour-paintings on paper or silk.Tung oil and qi-lacquer were used in archi-tecture.It is not clear whether they knew about casein.Proteinaceous materials that are known to have been historically used in Europe and the Mediterranean basin as paint binders are:
Animal glue,obtained boiling bones of mammalians,hide or other cartilaginoid parts of animals.It is made of collagen;
Egg,which can be used whole,or using only one of its components: the yolk or the glair.Dry whole hen's egg contains 45% or proteins,41% of fats,and 2% of cholesterol.Ovoalbumin(54%),conalbumin(12%),ovomucoid(11%),e lysozyme(3.4%)are the most abundant proteins;
Milk and casein.Milk is a water emulsion of proteins and lipids.Dry cow milk contains 26% of proteins(casein,lactalbumin,lactoglobulin),26% lipids,and sugars.Casein,the main protein of milk,is obtained from the enzymatic treatment of milk
A protein is a high molecular weight polymer,whose constituting building-bloks are amino acids.Twenty are the natural amino acids,which are biosynthesised in cells.The sequence of these amino acids(relative abundances and order)determine the characteristics of each protein.For this reason,in some cases,a way of distinguishing a protein from the other is to compare relative amino acidic compositions.In Figure 3 histograms relative to the average amino acidic composition of animal glue,casein and egg ob-tained from reference paint samples with the analytical procedure described,are reported together with that observed in the Terracotta Army paint sample(the quantified amino acids with this analytical procedure are 14).
Figure 3: histograms relative to the average percentage amino acidic content of egg,animal glue,and casein compared to that obtained from the analysis of the Terracotta Army paint sample.
The histograms reveals that the average composition of the reference samples are quite different one from the other.The comparison of the profile of reference samples with the paint sample,highlight the dif-ficulties arising in this kind of analyses: the attribution of the composition of the paint sample is not straight-forward.A useful approach to support data interpretation in analytical chemistry is based on statistical treat-ments of the data.The Principal Component Analysis is a multivariate method for the statistical treatment of the data.It allows to compare a large number of data that are characterised by a high number of variables.In our case we have eighty reference samples of reference paints realised with egg,animal glue and casein[3].The reference samples contained different pigments,and were subjected to artificially ageing.Each sample is characterised by its amino acidic percentage content(variables).The PCA permits to compare data,highlighting similarities and differences between samples.A bi-dimensional graph(which is called loading plot)is produced,where similar samples appear close,and different ones are far one from the other.In the case of our eighty reference samples of egg,animal glue and casein,the loading plot obtained shows three clusters,one corresponding to egg,the other to casein and the other to animal glue(see Figure 4).This simply means that all egg samples are similar one to the other,casein samples are similar one to the other,and animal glue samples are similar one to the other.The similarity refers to the amino aci-dic composition.
Figure 4: PCA score plot,relative to the eighty reference samples of egg,animal glue and casein
When there is a sample of un-known composition,its amino ac-idic percentage content is subjec-ted to the PCA together with the reference database,which means that we compare, by means of this statistical treatment of the data, the amino acidic composition of the paint sample with the amino acidic composition of all eighty reference samples.The resulting score plot is reported in Figure 5.The position of the paint sample in the score plot(the paint sample is represented by a star),close to the egg cluster, indicates that egg was used as paint binder for the terracotta army.
Figure5: PCA score plot,rela-tive to the eighty reference samples of egg(□),animal glue(◇)and casein(○),together with the paint sample from the Terracotta Army(star).
Conclusions
These results highlight how im-portant is to perform chemical analy-ses on paintings.The identification of egg,in fact,as paint binder,re-presents a first step to understand the painting technique used on ancient sculptures in China of which-despite their fame-nothing is known.The artisans'choice of material on the burial goods of Qin Shihuang,in many cases would seem to prove that they were aiming for long-term stability,and it could also have influenced the choice of the binding medium.It suggests that egg tempera had probably been traditionally used by the artisans of that period-because of its long-lasting properties.
References:
[1] J.S.Mills and R.White,The Organic Chemistry of Museum Objects,Butterworth Heinemann,2nd Edition,Oxford,1994.
[2] A.Andreotti,I.Bonaduce,M.P.Colombini,G.Gautier,Fr.Modugno,and E.Ribechini,Combined GC/MS Analytical Procedure for the Characterization of Glycerolipid,Waxy,Resinous,and Proteinaceous Materials in a Unique Paint Microsample,Anal.Chem.,78,4490-4500(2006).
[3] M.P.Colombini,F.Modugno,E.Menicagli,R.Fuoco,A.Giacomelli,GC-MS characterisation of proteinaceous and lipidic binders in UV aged polychrome artifacts,Microchem.1J..67 291-300(2000).
多年来,文物保护既要考虑外部气候因素,又要顾及文物机构内部的空气污染的影响。其中,博物馆、档案馆和图书馆占有特殊的地位,这里所需求的特殊气候要对人类的健康有益,同时又要保护文化遗产免其退化。在这一领域,科研的目的在于评估博物馆、美术馆、教堂以及陈列柜等微气候环境中存在复杂的物理、化学和生物现象。[1,2]。
鉴于参观者的兴趣不同,而且不同类型的博物馆,所使用的展品材料不尽相同,因而评估艺术品和档案材料的内容和裸露状态对其所造成的影响是很有必要的。为了了解气候变化、光照和污染物浓度对物质特性的影响,我们通过加速老化法对参考物质进行实验室测试。这种做法有助于了解物质所经历的主要退化过程。然而,考虑到参考物质要完好无损,新鲜的物质要在较短时间内老化,一件艺术品一生中的变化情况可能会与我们观察到的加速老化试验过程大不相同。然而,这种研究是了解保存在微环境中的艺术品所经历的大部分退化过程的唯一办法。为了获取有效的实验结果,我们有必要证明加速循环处理是否会产生退化效应,这一点是很重要的。我们还要将加速循环处理所产生的退化效应与实际发生在文物对象上的退化效应进行比较[3]。
为了了解文物保护存在的问题,控制博物馆的室内环境,需要微气候、化学、医学、材料学和生物污染学领域的专家们取长补短共同努力。
一般来说,大多数问题都与使用暖气、通风和空调设备以及大量团队旅游有密切关系。实际上,这些设备应该严格控制温度、相对湿度并尽力减少主要空气污染物。然而矛盾的是,使用空调设备可能会产生不良的气流、热量和蒸汽变化,增加气态和颗粒污染物的沉积速率,并且可能影响生物腐烛。此外,大众旅游业的增长会严重扰动微气候,增加室内污染、促进运输粉尘,布纤维、人体细胞和孢子的产生[4]。
对博物馆里相对湿度的控制至关重要。当相对湿度增加时,吸湿性材料吸附水蒸气分子就会变大,其尺寸变化在三维空间里并不完全一致,而且还会产生内部压力。当相对湿度下降时,文物的平衡含水量也下降,这就导致其收缩。尺寸的周期变化从长远来看,会因其累积效应而隐藏重大危险。所造成危害后果的程度与材料的类型、材料中的限制性物质、环境变化的规模直接相关。水可以通过多种方式(物理的、光化学的、水解的)参与到物体的退化过程中来。对于绘画作品,水可以引发;
酯或氨基化合物粘结剂的化学水解;
显色剂和基质或者颜料之间粘合的物理损失;
氢氧自由基或者化学物种的光化反应的产生;
对电离的促进;离子实体的移动
比如说最后一条效应是金属腐蚀的一个重要方面,而在醋酸丁酸纤维素聚合物中酯基的水解会产生破坏性酸度[4]。
坐落于历史建筑中的传统博物馆,得益于其厚墙壁和多孔吸湿材料(如:砖、石膏、灰泥、木材)对室内温度和湿度的缓冲作用。幸运的是,历史性建筑因为为人熟知的建筑材料和外部气候变化的关系而从中受益,这些建筑通常采用积极的抗衰退系统,也就是说建筑的特点决定了微气候。在任何情况下,减少相对湿度和温度循环的跨度都有利于延长艺术品的寿命。为了获得令人满意的微气候,敏感的艺术品往往被置于密封的陈列柜里展览[5]。
内部装饰、家具和游客人数都影响粒子类型及浓度。大量的团队旅游是重要的经济来源,但游客是污染物的运载体,这是由于水蒸汽和二氧化碳的排放及热量增加的原因,加上来自衣服和鞋子的空中纤维、灰尘及粒子的影响。鉴于以上原因,空调设备需要有较高的通风量,这就可能降低文物破坏的风险。因为来自外界的更多的污染会被带进博物馆、画廊,因此,使用空调控制微气候是有必要时,它有益于营造健康的博物馆环境。为了保持温度和湿度的恒定,这些设备应该昼夜运作,而不应该像在现实中经常发生的那样,仅仅在博物馆开放的时间才运作。
对博物馆环境中空气污染物的特性研究主要集中在无机化合物方面,如:臭氧(O3)、氮氧化物(NOx)、二氧化硫(SO2)和硫化氢(H2S),却很少集中在有机酸(甲酸,乙酸)和甲醛上。直到最近,挥发性有机化合物(VOCs)、半挥发性有机化合物(SVOCs)、稳固灰尘及其混合物里的原素[6,7]才开始受到了重视。
事实上,挥发性有机化合物(VOCs)是室内和室外空气污染的一个主要因素。即使是低浓度的挥发性有机化合物也会产成恶臭气体,也会对健康构成严重的危害。然而很少有人知道挥发性有机化合物在艺术品退化中起的作用以及对我们的文化遗产构成的威胁。尤其需要提及的是:已经检测到的博物馆陈列柜所使用的材料排放出的羧酸。木制品、涂料、硅酮密封剂和聚醋酸乙烯酯胶粘剂通常被用来制作框架或储存容器,它们排放出对艺术品有潜在危害的醛和有机酸[8]。需要特别指出的是,选择储藏室和框架的木材时,不仅要考虑数量,还需考虑其释放出的挥发性有机化合物的特点[9,10]。
图1表示出关于艺术品和室内、室外污染物以及气候参数之间相互作用的概观。
图1艺术品和污染物及气候参数之间的相互作用
由于来自室外的气体污染物(氮氧化物、硫氧化物、臭氧、挥发性有机化合物)往往均匀分布在博物馆的展室内,我们仅选择少数几个采样地点就能充分进行监测。一般来说,空调系统都配有微粒过滤器,但并不配备消除这些气体污染的设备。控制这些污染物的办法主要集中在减少其进入量和在空调系统里添加化学过滤系统两个方面。
表1总结了在文献中发现的一些结果[11]。
表l气体污染物对展品的影响以及测量到的污染物的浓度
一般来说,室内真菌负荷无论对展出的艺术品来说还是对博物馆工作人员及参观者来说,都不能算是有害的。相比之下,高细菌负荷的出现很大程度上取决于季节。然而,为减少潜在的生物性破坏,应该避免或严格控制在博物馆里摆放植物。
新型博物馆在设计上可以将现代高科技调节装置纳入考虑中。由于这些系统产生的室内湍流是室内空气混合以及悬浮颗粒的沉积速率升高的原因,为了减少这种负面影响并保持一个稳定的室内气候,必须配备夜间提取粗颗粒的装置。
参考资料
[1] P.Briblecombe,The effects of air pollution on the built environment,Imperial College Press,2003 UK
[2] D.Camuffo,Microclimate for Cultural Heritage,Elsevier,1998 Amsterdam
[3] T T.Schaeffer,Effects of light on materials in collections,Getty Publications,2001 Los Angeles,Califirnia
[4] Dario Camuffo,ReneH Van Grieken, Hans-JuK rgen Busse,Giovanni Sturaro,Antonio Valentino,Adriana Bemardi,Nigel Blades,David Shooter,Kristin Gysels,Felix Deutsch,Monika Wieser,Oliver Kim,Ursula Ulrych,Atmospherie Environment,Supplementn.1,35,S127-S140,(2001)
[5] L.L.Feller,Accelerated Aging.Photochemical and Thermal Aspects,1994 The J.Paul Getty Trust,USA
[6] A.Schieweck,B.Lohrengel,N.Siwinski,C.Genning,T.Salthammer,Occurrence of organic and inorganic biocides in the museum environment,Atmospheric Environment 39,6098-6108,(2005)
[7] M,P.Colombini,A.Andreotti,I.Bonaduce,A.Ceccarini,“La caratterizzazione dei materiali organic”,Meeting “I1 Camposanto di Pisa: un progetto di restauro integrato”,6-8 Matzo 2008 Pisa,Italy
[8] A.F.L.Godoi,L.Van Vaeck,R.Van Grieken,Use of solid-phase microextraction for the detection of acetic acid by ion-trap gas chromatography-mass spectrometry and application to indoor levels in museums,J.Chromatogr.A,1067,331-336,(2005)
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[10] European research project PROPAINT,within the EU 6th framework program
[11] P.H.Hatchfield,Pollutants in the Museum Environment,Archetype Publication Ltd.,2002 London
文化遗产科学保护的重要性:2-漆粘合剂的特性
伊拉瑞·波娜德斯和玛丽亚·贝拉·哥伦比尼(意大利比萨大学化学和工业化学系)
简介
蛋白质、甘油酯、植物树脂、动物树脂、天然蜡和多糖这些天然有机物质被普遍用于绘画作品所需的粘合剂、胶粘剂和清漆[1]。由于受到光线、氧气、温度和湿度、日夜和季节性变化等因素的影响,这些物质通常会发生物理和化学变化。结果,有机物的年龄和所用涂料的种类的鉴定就成为化学家的挑战。此外,由于涂料的样本尺寸极小,其化学特性决定只有采用极为敏感的分析技术,才能提供最多的信息。涂料的分析调查,对于我们选择最佳的保护方式和保护条件来说都是非常重要的。了解画家所采用的绘画技能,可以使我们了解当时的绘画技巧,以及当时的科技工艺。这些重要信息,有助于我们重现原艺术作品的原貌。
比如:我们对秦始皇兵马俑彩漆层上获得的一个样本进行特征鉴定,旨在了解当时的绘画技术。
我们所采取的分析程序有助于我们在同一样本中鉴定出甘油酯、蜡、树脂和蛋白质材料[2]。该程序需要对样品进行预先处理,以便将各种有机组成分成两种不同的小部分(一种是树脂质部分,另一种是蛋白质部分)。我们将通过气相色谱—质谱分析法(GC-MS)对这两小部分分别进行分析。通过这种方式,在同一个涂料微型样本(1-5毫克)中同时鉴定出了鸡蛋、动物胶、牛奶、大蒜(蛋白质材料)、亚麻籽、胡桃、罂粟油(甘油脂)、蜂蜡和巴西棕榈蜡(天然蜡)、松科树脂、山达脂、乳香脂、达马树脂(植物萜类树脂)和虫胶(动物萜类树脂)。
鉴定漆粘合剂的分析步骤
样本描述
该样本从秦始皇兵马俑的彩漆层中获得,其底层是一种天然漆,由于长时间被浸在潮湿的土壤中,在相对湿度低于92%的时候会产生收缩,导致陶俑出土后,漆皮会很快从陶体上剥落下来,漆层往往会脱离底层天然漆并附着在土壤上。尽管在此过程中漆层不可避免地从陶体上脱落,但脱落下来的薄片和附着在土壤里的漆层仍可用做科学研究。
漆粘合剂的鉴定
我们把该样本放入玻璃器皿中,然后注入氨水让两者发生反应,这样以来,蛋白质就与甘油酯、蜡和萜类树脂分离开来。提取液中所含的蛋白物质经过净化后再借助微波被酸性水解,我们用气相色谱-质谱分析法(GC-MS)把释放出的氨基酸做了分析。提取液的残余包含有甘油酯、蜡和萜类树脂,它们借助微波的作用发生盐化/皂化。这使得蜡酯皂化,并在适合甘油酯皂化的条件下完成虫胶倍半萜烯酸的坎尼扎罗反应。然后,用气相色谱-质谱联用仪分析,中性化合物和酸性化合物在酸化作用后从反应混合物中被提取出来。我们通过图表1扼要表示出该分析程序,具体的实验细节刊登于文献资料里[2]。
图1:涂料样本中的蛋白质、油脂和树脂材料的特性的分析步骤
讨论和结果
该样未发现包含有甘油酯、蜡质和或树脂。我们对蛋白质部分的分析显示出其中含有氨基酸,色谱图见图表2。
图2:从兵马俑中取得的涂料样本中蛋白质部分的色谱图(TIC模式)
关于中国彩绘陶俑这一传统技术方面的文献记载不足,而有记载的绘画方法是,有时会将动物胶或植物胶与明矾混合来减少水溶性,这里所说的“绘画”大概指的是绘于纸上或者丝绸上的水彩画。桐油和漆一般用于建筑。中国古代人是否了解酪蛋白目前无从知晓。在欧洲和地中海盆地地区,蛋白质材料自古以来就被用来做漆粘合剂,这些蛋白质材料是:
由哺乳动物的骨头熬制成的动物胶,兽皮或者其它动物的软骨部分。它由胶原制成;
蛋,可以全部被使用,也可以只取其中的一部分,即蛋黄或蛋白。鸡蛋含有45%的蛋白质,41%的脂肪和2%的胆固醇。卵白蛋白(54%),伴白蛋白(12%),卵类粘蛋白(11%),溶菌酶(3.4%)是最丰富的蛋白质;
牛奶和酪蛋白:牛奶是蛋白质和脂类的水乳液。干牛奶中含有26%的蛋白质(酪蛋白、乳白蛋白和乳球蛋白),26%的脂质和糖。酪蛋白是牛奶中的主要蛋白质,通过对牛奶的酶处理可以获得酪蛋白。
蛋白质是一种高分子量聚合物,它的构成要素是氨基酸。有二十种天然氨基酸,天然氨基酸在细胞里生物合成。这些氨基酸的序列(相对丰度和次序)决定了每个蛋白质的特性。出于这个原因,在某些情况下,将蛋白质区与其他成分区分的方法是对比相对的氨基酸性组成。图表3将上述分析过程中从彩漆样本中获得的动物胶、酪蛋白和氨基平均酸性组成成分,与从兵马俑的彩漆样本中观测到的结果一同列举出来(参与此分析程序的量化氨基酸为14)。
图3:关于蛋、动物胶和酪蛋白的氨基酸性组成的平均百分比统计图与从兵马俑彩漆样本中所得分析结果相比较
统计图显示,每个参考漆样本在平均组成成分上有较大差异。该参考漆样本与兵马俑漆样本轮廓之间比较分析困难重重:兵马俑漆样本组成成分的属性并不明确。分析化学最有效的诠释数据的方法要建立在对统计数据处理的基础之上。主成分分析法是一种用于统计数据处理的多变量法,可以用于大量多变数据额的对比分析。就我们的研究而言,我们有80件从蛋、动物胶和酪蛋白中获得的参考漆样本[3]。这些参考样本中含有不同的颜料,并被人工老化。每个样本的特点是其氨基的酸性百分含量(变量)。主成分分析法可以进行数据比较,也可以突出样本之间的异同。该分析法会生成一个二维图(即所谓载荷图),图中相同的样本非常类似,不同的样本差别较大。由80件蛋、动物胶和酪蛋白的参考样本生成的载荷图显示出三个组群:一组与蛋相对应,一组与酪蛋白相对应,还有一组与动物胶相对应(见图4)。这就意味着所有蛋样本的氨基的酸性组成成分基本类似,酪蛋白和动物胶样品也是类似阶情况。
图4:蛋、动物胶和酪蛋白的参考样本的主成分分析图
当漆样本的主成分不详时,我们就用主成分分析法和参考数据库来研究其氨基的酸性百分含量,这就意味着要通过对数据的统计处理来对比漆样本的氨基的酸性组成成分和所有八十件参考样品的氨基的酸性组成成分(见图5)。兵马俑漆样本在图中的位置(星号表示兵马俑涂料样本)靠近蛋集群,这喻示出蛋在当时被用作了制造兵马俑的漆粘合剂。
图5:蛋、动物胶和酪蛋白的参考样本和兵马俑漆样本的主成分分析图(星号表示兵马俑漆样本)
结论
这些研究结果强调了对绘画作品进行化学分析的重要性。事实证明蛋在当时被用来制作漆粘合剂(即:制作兵马俑所需的漆粘合剂)。这一结论的确定标志着人们对于中国古代雕塑品的彩绘技术的了解迈出了可喜的第一步。秦代的工匠们在选择秦始皇随葬品的材料时力图确保随葬品的耐久性,这种做法也影响了他们对颜料粘合剂的选择。这也证明,蛋彩颜料因其耐久性是工匠们当时使用的传统漆粘合剂。
参考资料:
[1] J.S.Mills and R.White,The Organic Chemistry of Museum Objects,Butterworth Heinemann,2nd Edition,Oxford,1994.
[2] A.Andreotti,I.Bonaduce,M.P.Colombini,G.Gautier,Fr.Modugno,and E.Ribechini,Combined GC/MS Analytical Procedure for the Characterization of Glycerolipid,Waxy,Resinous,and Proteinaceous Materials in a Unique Paint Microsample,Anal.Chem.,78,4490-4500(2006).
[3] M.P. Colombini,F.Modugno,E.Menicagli,R.Fuoco,A.Giacomelli,GC-MS characterisation of proteinaceous and lipidic binders in UV aged polychrome artifacts,Microchem.1 J..67 291-300(2000).
汉阳陵·比萨:文化遗产的原址保护与考古博物馆