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The Role of Physical Science in the Study of Cultural Heritage

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The Role of Physical Science in the Study of Cultural Heritage
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  AAPPS Bulletin June 2004 21 1. CULTURAL HERITAGE PROBLEMS ADDRE- SSED BY PHYSICAL SCIENCE METHODS For thousands of years practitioners of arts and crafts in everypart of the world have gone about their trades using sensoryorgans. In the same vein, students of cultural heritage pursuedtheir study by sights and touch. This humanistic approach ischanging as technology in current physics research have beenfinding their way into the traditional humanity stronghold of archeology and art. Carbon 14 dating of carbon-containingman-made artifacts is probably the most well known exampleof the use of scientific technique in archeology [1]. Similarlyart objects such as Roman paintings have been analyzed byRaman spectroscopy to reveal the organic composition of thepigments [2]. Radiocarbon dating and Raman spectroscopyallow us to probe physical parameters undetectable by humansensory organs. The purpose of radiocarbon dating is to dis-cover the time of manufacture of the artifacts that are oftenpreceded or unaccompanied by written record. Informationabout organic content of the pigments helps the restorationand protection of the paintings much ravaged by old age. Evi-dence gathered by scientific measurements augments and veri-fies written historical record. These are extra dimensions af-forded by modern technology. The advantage of scientific methods is of course the quanti-tative nature of the information rendered by scientific mea-surements that are untainted by human meddling. Beyond aca-demic interests, the art market is also finding scientific meth-ods valuable in the dealing of art objects, by virtue of the sci-entific objectivity. Some financial analysts have ranked artobjects above stock and real estate as the best yieldinginvestment. The authenticity of the art object must however be assured for the investment to have any chance of return.The counterfeiters have been very shrew in taking advantageof the inclination of the art dealers to rely solely on sensationsevoked by an art object’s form, color and touch. Hence therehas been no shortage of look-alike fakes in the art market. Theobjectivity of scientific authentication surely provides themuch-needed peace of mind to the dealers and buyers. Bothscientific dating and elemental/composition analysis have beenapplied to the authenticity determination. We will review thestrength and weaknesses of the techniques. 2. ELEMENTAL/COMPOSITION ANALYSIS OFANCIENT ARTIFACTS In the past decades, increasingly advanced hardware in phys-ics research such as synchrotron and inductively coupledplasma mass spectrometer (ICP-MS) has been put into ser-vice for dating or authenticating ancient artifacts. This trendreflects the advance and demand of science in heritage Articles Professor Dennis LoC&C Authentication Laboratory Limited,201 Photonics Center, Science Park,Shatin, New Territories, Hong KongSAR, ChinaPhysics Department, The Chinese Uni-versity of Hong Kong, Shatin, NewTerritories, Hong Kong SAR, China The Role of Physical Science in the Study of Cultural Heritage Dennis Lo We review the recent development of the use of scientificmethods in the study of cultural heritage, in particular ar-cheology and art. The strength and limitation of the scien-tific approach to problems in cultural heritage are described.Examples that illustrate successful application of physicalscience methods to the fields of archeology and art are presented. Finally, we draw on our experience in dating andauthentication of Chinese ceramics to show that even thecounterfeiters are going high-tech and our counter-measures.  22 AAPPS Bulletin Vol. 14, No. 3 preservation, and in authentication and counterfeiting of artobjects. It has been found that the correct use of tools for-merly available only to physical science may provide defini-tive answers to questions of provenance and authenticity of archeological finds. For archeological finds, one is interestedto know: How old is artifact? Where is it made? Was it madelocally or was it imported? What are materials and technologyused in making it? For works of art, one is likely to ask: Is theart object authentic? Which part of the object is authentic or which part is restored? Who executed the art object? With thehuge array of modern scientific hardware now available for use, the choice of measurements to be made is broad and theresulting data is so complex that sophisticated mathematicalmodels are often required for subsequent analysis. In employing scientific techniques to tackle problems in artand archeology, one must be aware of the many pitfalls inconverting data to answers in words. Seeking answers to thequestions in the paragraph above from the vast amount of datafrom measurements is no easy task. The idea of making physi-cal measurements and subsequent data analysis of an artifactcan be likened to the careful cataloging of human dimensionsfor the purpose of identification. Micron scale high-precisionmeasurements of a person’s overall height, weight, fingerprintsand footprints, as well as DNA data can be readily achieved.Such information though precise and detailed, when used inthe absence of supporting evidence such as skin color, hair color, and sex, may not afford much help to someone whowishes to locate a visitor in a crowded airport. The task of thepractitioners of scientific study in art and archeology who arerequired to make judgment on unfamiliar artifacts by viewinga wide array of data is no less difficult. The time-proven ap-proach that has consistently yielded sensible result is the cor-rect choice of technology and the proper interpretation of thescientific data. Only the correct choice of technology can pro-duce useful data, and through the appropriate use of data inthe defined context and in accordance with established mea-surement accuracy could sensible conclusion be drawn. Although radiocarbon dating may have been the best-knownexample of the application of physical science technique incultural heritage study, elemental/composition analysis of ob- jects of antiquity is in fact dated to 1930s. European Museumsoften grouped metal artifacts of similar impurity patterns of minor elements. It was (and has been) hoped that the patternsof the minor and trace elements revealed in elemental analysiscould reflect common centers of production and possibly thesrcin of the raw materials. Such assumption may have beenan oversimplified one, since the raw materials used in the arti-fact are not the only contributing factors. The impurity pat-terns were much affected by the thermodynamics during manu-facturing and storage, and the same bronze materials were of-ten recycled by the later generations to make contemporaryartifacts. The production, the use, the storage and the recov-ery of the artifact comprise the chain of events, each of whichcontribute to the constituent elements of the artifact. Exchangeof materials in and out of the artifact in each of the event iscommon. Each part of the chain could significantly alter theelemental analysis result. For the data obtained in elementalanalysis to be of any value, one must have good knowledge of the chain of events leading up to the physical measurements.In spite of such obvious shortcomings, elemental/compositionanalysis of works of art and archeological finds stays very muchin vogue, perhaps on the mistaken basis that the sheer pres-ence of scientific measurements could enhance the value of humanity study, and that the more expensive the equipment,the more credible the result. The use of elemental/composition analysis has been ex-tended from metals to ceramics and paintings. Technology for elemental/composition analysis has matured to the stage thatsurface or bulk analytical techniques that are mini-or micro-destructive are readily available for the archeologist to pur-chase [3]. For someone who is worried about very minute dam-age to the artifacts, surface techniques such as X-ray fluores-cence (XRF) or particle-induced X-ray emission may be thechoice. Either technique leaves no visual damage except for the X-ray irradiation. If one is to avoid the surface heteroge-neity caused by corrosion, bulk analytical techniques such aslaser-ablated ICP-MS in fact afford highly reliable data. Ei-ther a material sample must be removed from the artifact or the artifact must be small enough to be fit inside the sampleholder. In the later case, only a crater a few tens of microns indepth remains after the application of laser ablation. The tech-nique of elemental analysis has had success in well-recordedarcheological finds. An example is the painted sarcophagusfound in Crete, of late Minoan period. Information obtainedfrom elemental analysis sheds light on the pigments used inthe frescoes and the degree of technological know-how of theancient Minoans. Elemental analysis has been less successfulin authenticity study however. A well-known case is the pair of blue and white vases purportedly of Yuan Dynasty. Basedon the similarity of impurity patterns obtained from elementalanalysis using PIXE and XRF techniques, the pair of vaseswas declared authentic. Obviously the practitioners of PIXEand XRF in this case were not heeding the warning of Harbottlewho professed that unequivocal sourcing based on impuritypattern is impossible [4]. The authenticity of the pair of vaseswas refuted later by thermoluminescence dating and by his-torical arguments [5]. Therefore one must view authenticitystudy of artifacts based on elemental analysis data with greatcaution. 3. RADIOCARBON AND THERMOLUMINESCEN- CE DATING Compared to elemental/composition analysis, dating of arti-  AAPPS Bulletin June 2004 23 facts by radiocarbon or thermoluminescence generate relativelysimple data sets. There is no question of similarity and dis-similarity of the impurity patterns with the established groups.The time of decease (for radiocarbon) or the time of manufac-ture (thermoluminescence) of the artifact is arrived at by mea-suring the level of radioactivity in either case. Since the levelof radioactivity of the artifact is relatively immune to externalevents (except for the case of intentional irradiation to pro-duce fake; see the section to follow), the mode of manufacture,use and storage would not alter the dating result. For a care-fully calibrated and well-executed measurement, scientificdating can provide an accuracy of  10% of the actual time of manufacture. The sparseness of the dating data may not offer fertile evidence to the archeologist who wants to probe deepinto the ancient world, but may prove succinct enough for au-thenticity determination, which is often decisive in any deal-ing of art objects. 4. THERMOLUMINESCENCE DATING OF CHI- NESE CERAMICS We will draw on our experience on dating and authenticationof ancient Chinese ceramics to illustrate the conflicting roleof physical science in art dealing, in part because of the rela-tive importance of Chinese ceramics in the antique market.Presently Chinese ceramics are estimated to take up more than50% of the trading volume of all Chinese antiques. AntiqueChinese ceramic is routinely sold for hundreds of thousandsof US dollars. Such high price proves to be strong stimulusfor fabricating fakes. Fortunately scientists and collectors alikeare equally determined to devise counter measures to defeatthe fakes by scientific authentication. Since the first observation of luminescence emission froma ground-up pottery upon heating by Kennedy and Knopff in1960 [6], thermoluminescence (TL) testing has found wideapplications in the dating of pottery of archeological interestand the authentication of work of art [7]. Samples must betaken from the object to be tested. The required weight is aslittle as a few milligrams to preserve the physical integrity of the artifact. The sample is then processed into fine grains.Luminescence is emitted from the fine grain sample on a hotplate when temperature is raised from room temperature toabout 500 °C. The plot of luminescence intensity versus tem-perature (in degree Celsius) is called a TL glow curve. In prin-ciple the last date of firing of the ceramics and hence the ar-cheological age can be determined from the TL glow curve.However it was reported [8] that the TL glow curve could befaked by irradiating a copy with x-rays. In addition, the mar-gin of error given by several TL service providers has been far too wide to be of value in determining the true archeologicalage ( e.g.  a pottery figurine was given an estimated age be-tween 1,000 to 2,000 years before present by a TL serviceprovider. The span of 1,000 years in fact covers more than 10dynasties from Western Han to Northern Song in Chinesehistory.) We will give a brief review the principle of TL test-ing in art authentication. Later we will discuss methods toimprove the accuracy and uncovering forgeries exposed to ex-ternal radiation. Estimate of the archeological age of the sample is obtainedfrom the following formulaAge (years)=where Dose accumulated  is the radiation dosage absorbed by thesample over the years and Dose annual  is the annual absorbeddosage. The accumulated dosage is determined from the in-tensity of the luminescence emitted from the sample whenheated (thermoluminescence). This is because the accumulatedradiation is stored in the sample as trapped electrons that canbe released as luminescence when activated by heat. There-fore there is a conversion of radiation into luminescence (xamount of radiation for y amount of luminescence). The ratio  y ¯   x   is sometimes called the TL sensitivity and can be deducedfrom the TL response of the sample to a calibrated dose of radiation. Dividing the peak intensity by the TL sensitivitygives the accumulated dosage. Generally the accuracy of themeasured accumulated dosage improves with higher sensitivity. The annual absorbed dosage requires measurements sepa-rate from that of the accumulated dose. For ceramic artifactsin archeological sites, the contribution to the absorbed radia-tion derives from the environment and from the artifact itself (clay minerals that are weakly radioactive). For work of art, itis mainly the radioactive minerals in the ceramics artifacts thatproduce the radiation. Because of the short penetration depthsand hence high-energy deposition of   and  radiation(penetration depths of tens of microns for   and about 1 mmfor   in typical Chinese ceramics), these two types of radia-tion from the ceramic itself are responsible all the radiationabsorbed by the sample. Determination of   radio-activityrequires the measurement of  -active element (primarily po-tassium-40) in the ceramic sample, whilst  measurementneeds a sensitive scintillation counter. Typically for Chineseceramics, the    activity is weaker. Since the scintillator counter often registers less than 100 counts per day, accurate  count-ing requires several days. In general, both  and  measure-ments are needed, but the TL sensitivity is different for either radiation. It is not unusual for a ceramic sample to respond toeither  or  radiation only. We have tested thousands of ceramic samples of Chinesesrcin, provided by antique dealers, collectors in Hong Kongand from Far East and by research institutes in China. Allsamples show very weak  and   activities as expected of   Dose accumulated   Dose annual  24 AAPPS Bulletin Vol. 14, No. 3 typical clay materials that contain trace amount of radioactiveelements such as K-40, U-238, Th-232. For example the an-nual dose from Chinese blue and white porcelains of Mingand Qing Dynasties vary from 1 mGy to 7 mGy with a scat-tered distribution. That is to say: one blue and white shard canhave an annual dose of 1 mGy and it is just as likely for an-other shard to have annual dose of 7 mGy. Our statistics doesnot indicate a higher probability for the mean dose at 3.6 mGy.The substitute use of actual radioactivity measurement by someso-called typical annual dose will lead to wide-error margin inthe estimate of the archeological age. A case in point is theauthentication of Ge ware with crackling glaze. Since its firstappearance in Southern Song dynasty, copies of Ge ware weremade for official or civilian use in Ming, Qing Dynasties. If one were to use the accumulated dose determined from the TLglow curve and some presumed typical annual dose withoutactually measuring the radio-activity, a copy in late Qing canbe mistakenly determined to be that of Song dynasty or viceversa. Such authentication process would certainly leave thecustomer in distress. 5. HIGH TECH CHINESE CERAMICS FORGERY AND DETECTION It was reported in Wall Street Journal in September 1, 2000[8] that modern copies of expensive Chinese ceramics of Yuanor Ming dynasties were exposed to X-rays to simulate the TLresults. We have in fact come across samples submitted for TLtesting which have been exposed to external radiation inten-tionally or unintentionally. We will show three cases in thesections to follow. In each case, we have been able to revealthe true archeological age of the items by TL testing. Our TLresults also yield conclusive evidence that the items have beensubject to external radiation. Case 1 is an under-glaze-red flask claimed to be of earlyMing-dynasty [Fig. 1]. Similar flasks can fetch a price of US1 million in an art auction. The TL glow curve shows a verycomplex structure of multiple peaks starting at temperature aslow as 100 °C. For comparison, we used a blue and whitelidded jar from Ming Dynasty during the reign of Hongzhiwhich is also in the earl-middle Ming era [Fig. 2]. Both theflask and the jar should have been made in Jingdezheng, theporcelain hub in Southern China. The difference becomes ob-vious when we compared the two glow curves. A single peakat 320 °C is observed in glow curve of the Hongzhi jar. Fromthe annual dosage measurements, we then deduced that theflask has been subject to saturating radiation that could onlybe artificial in nature. Case 2 is a Longquan zong in celadon glaze purportedly of Southern Song Dynasty [Fig. 3]. The customer claimed thatthe item has been subject to examination by PIXE and XRFfor trace elements analysis. The PIXE results indicated thatthe elemental patterns of the glaze and of the clay materials isconsistent with that of genuine Longquan ware in Songdynasty. Small samples have been taken from the foot-rim for TL testing by a TL service provider [see the two circular cutsat the foot-rim in Fig. 3]. The TL provider claimed that theitem might have had secondary heating, rendering the TL re-sult inconclusive. This item was then submitted to us for fur-ther TL testing. A triangular cut was made at the foot-rim toextract some sample [see the triangular cut at the foot rim,Fig. 3]. Routine TL test using the pre-dose technique ( a TLtechnique often used in testing stoneware or porcelain) pro-duced inconclusive result as all curves coalesced. We how-  Fig. 1: Under-glaze-red flask and TL glow curve. Fig. 2: Blue and white lidded jar and TL glow curve.

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Jul 25, 2017
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