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Primitive Environment Control for Preservation of Pit Relics in Archeology Museums of China ZhaoLin Gu,*,† Xilian Luo,† Xiangzhao Meng,† Zanshe Wang,† Tao Ma,‡ Chuck Yu,†,§ Bo Rong,∥ Ku Li,⊥ Wenwu Li,† and Ying Tan† †

Xi’an Jiaotong University, Xi’an 710049, China Xi’an Center for the Conservation and Restoration of Cultural Heritage, Xi’an 710075, China § International Society of the Built Environment (ISBE), Milton Keynes, United Kingdom ∥ Emperor Qin’s Terra-cotta Warriors and Horses Museum, Xi’an 710600, China ⊥ Hanyangling Museum, Xi’an 712038, China ‡

S Supporting Information *

ABSTRACT: Immovable historical relics in some archeology museums of China suffer deterioration due to their improper preservation environment. The existing environmental control systems used in archeology museums are often designed for the amenities of visitors, and these manipulated environments are often inappropriate for the conservation of abiotic relics. This paper points out that the large open space of the existing archeology museum could be a cause of deterioration of the relics from the point of view of indoor air convective flow. The paper illustrates the need to introduce a local pit environmental control, which could reintegrate a pit primitive environment for the preservation of the historical relics by using an air curtain system, orientated to isolate the unearthed relics, semiexposed in pits to the large gallery open space of the exhibition hall.

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INTRODUCTION As the heritage of human culture and civilization, relics are nonrenewable cultural resources that have significant historical, artistic, and scientific value. Most of the unearthed cultural relics are preserved and exhibited in various museums. The International Council of Museums reported that there were more than 55 000 museums in 202 countries in 2010.1 The museums for cultural relics are mainly categorized into two types of indoor-display museum and archeology museum according to the various exhibition methods for the cultural relics. For an indoor-display museum, the collection is preserved in an enclosed cabin or showcase such that a small space is provided to separate the relics from the visitors’ environment. An archaeology museum is constructed to protect immovable historical relics on site. For an ancient civilized country, a large proportion of the existing museums in China are archeology museums. Among most of the archeology museums, the excavation sites are presented in pits. An investigation organized by the China’s State Administration of Cultural Heritage reveals that there are more than 3000 museums in China, among which about 51% of the collections are suffering deterioration or even ruin due to the improper preservation environment.2,3 There has been an increasing awareness throughout the world about the environ© 2013 American Chemical Society

mental control of museum, since the specific environmental condition would be a key to realize a desirable preservation for cultural relics.4−9 Researchers have studied the specific environmental specifications4,5,7,8 to establish the required steady temperature, humidity, and air quality environments in the cabins or showcases for exhibition of the relics. However, due to the unique exhibition requirements and building characteristics, the environmental control of archeology museums is still a challenge to environmental scientists and technologists, to provide environmental management to control air pollution, heat and mass transfer, ventilation, and air conditioning environments for the preservation of historical relics being exhibited at the excavated site. The Emperor Qin’s Terracotta Warriors and Horses Museum, China, was opened in 1979 and provides an exhibition of Qin Shihuang’s mausoleum. Qin Shihuang was the first emperor (259−210 B.C.) in China’s history. This is a world-famous historical site, designated by UNESCO as one of the world’s cultural legacies, which has attracted more than 5 Received: Revised: Accepted: Published: 1504

June 19, 2012 December 30, 2012 January 2, 2013 January 2, 2013 dx.doi.org/10.1021/es303981m | Environ. Sci. Technol. 2013, 47, 1504−1509

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about 2.5 cm height above the pit soil surface and the data acquisition were at 10 min intervals. The gaseous pollutants: sulfur dioxide (SO2), nitrogen dioxide (NO2), nitrogen oxides (NOx), and ozone (O3) were monitored inside the museum, and these were carried out using air sampling and off-line analyses. Model for Predicting Air Flow. Some previous numerical studies were carried out on the ventilation of museums to access an alternative heating method and thus to decrease the temperature gradient,14 or to obtain more details of the micro environment.15 Here, we developed a computational fluid dynamic method (CFD) to predict the air flow pattern of the natural convection in the enclosure of the exhibition hall of Hanyangling Museum and the performance of air curtain system. All the variants (velocities, temperature, turbulent energy and dissipation energy) to be solved are denoted by ϕ. The general transport equation for ϕ can be written as

million visitors a year. However, many of the relics in the exposed pits had changed appearance when unearthed and during exhibition due to the inappropriate environmental management approach10,11 being used by the museum. The creation of a suitable environment for the long-term preservation of relics in archeology museum is, therefore, an important urgent issue. To upgrade the environment of the relics in archeology museums, great efforts have been made in recently years, and these include implementation of space division within the archeology museum where the relics are exhibited in enclosed space free from visitor gallery and two environmental control systems are being operated for visitors’ gallery and relic gallery, respectively. The underground exhibition hall of Hanyangling Museum, China, opened in 2006, is such a modern archeology museum. The museum exhibits partially exposed pits containing the pottery figurines in the coburial grave of the fourth Emperor (188−141 B.C.), and Queen of Han Dynasty, China. Many technical measures including lighting, radiated heat, ventilation, and air conditioning devices have been incorporated. However, the relics in this new type of archeology museum are not preserved as well as had been expected. Slight surface cracking and weathering were found on the pottery figurines. Air pollutants, such as SO2, NOx, were found to have penetrated into the enclosed hall environment.12 In this study, we present the argument that the layout of a large open space for archeology museum is not appropriate from the preservation of relics and that the reintegration of the pit primitive environment for the relics is essential to mitigate the heat and mass transfer occurring in air−relic−soil system. Some technological measures are also presented for consideration.

∂ρϕ + div(ρ Uϕ) = div(Γϕgradϕ) + Sϕ ∂t

(1)

where ρ is the density of the fluid, U = (u,v,w) is the velocity vector, Γϕ is the generalized diffusion coefficient, and Sϕ is the source term. If Γϕ, Sϕ, and ϕ are adequately prescribed, eq 1 can provide the continuity, momentum, energy, and other scalar equations (Table S1, S2 (Supporting Information)). To develop a solution for the problem, geometric models were first built up based on the actual dimensions of the Exhibition Hall. The Cartesian mesh method16 was employed to decompose the computational domain into small elements. The boundary meshes were subdivided using Octree method to ensure an accurate solution. On each cell, the finite volume method was used to replace the above continuous differential equations by a system of simultaneous algebraic difference equations. The boundary condition for the temperature and velocity were determined by an on-site measurement (Table S3, S5 (Supporting Information)). The standard k−ε models with wall-function method were employed to simulate the turbulent flow.15,17

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MATERIALS AND METHODS Environmental Issues in Archaeology Museum. The architectural design of existing archeology museums is mainly orientated toward accommodating the panorama of unmovable ancient sites and cultural relics in a single space, and most of the existing archeology museum buildings have followed this architectural layout, where the relics are featured in an open and large-space layout. The No. 1 exhibition hall of the Emperor Qin’s Terracotta Warriors and Horses Museum, China, is a representative of the traditional archeology museum. The relics in this archeology museum are presented to the public as still being partly connected to their primitive environment (i.e., in the unearthed soil environment). The relics are usually also exposed in a large open space due to the ongoing archeological and preservation work and also to allow the exhibition of the relics to the coming and going of visitors. The exposure of the relics to the large open space with the possibility of outdoor air penetration could always impose changes in the indoor air quality of the museum.10−13 The indoor environments of archeology museums for unearthed cultural relics would therefore be different from the earth environment before excavation of the cultural relics. Collection of Environment Data. The environment data, including microenvironments and pollutants the Emperor Qin’s Terracotta Warriors and Horses Museum were monitored. The temperature and relative humidity (RH) of the outdoor environment were measured by an automatic meteorological station that was located at the green belt around the museum buildings. The pit temperature and relative humidity were measured by using Testo 175H1. The sensors were placed at

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RESULTS AND DISCUSSION Gaseous Pollutants. In a humid environment, the air pollutants can play a fundamental role in the deterioration of the relics; for example, the formation of gypsum on the surface of relics could be related to the interaction between SO2 or SO42− in the aerosol in the air and CaCO3 in the soil and on the surfaces of the relics.18 The gaseous pollutants analyzed using the samples collected in the No. 1 pit of the Emperor Qin’s Terracotta Warriors and Horses Museum are shown in Figure 1. The average concentrations of the pollutants were higher than the suggested values of atmospheric pollutants in the museums reported in literature5,8 (e.g., SO2 < 10 μg/m3, NOx < 10 μg/m3, and O3 < 2 μg/m3). The very high levels of SO2 could be attributed to the emissions released from the nearby thermal power plant. Enhancement of Pollution Diffusion in Large Space Exhibition Hall. For the an exhibition hall with large air volume, it is difficult to maintain steady and uniform indoor environmental parameters for environmental control since the flow pattern of indoor air movement is related to the characteristic dimension, L (usually the largest height or length of the exhibition hall) for the convection movement of air or L3 for the natural convection movement of air.19 Larger indoor dimensions would means more intense air convection, and as a 1505

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the end of 2011, as examples, we monitored the air temperature and RH at the pit soil surface in these two different exhibition halls of the Emperor Qin’s Terracotta Warriors and Horses Museum, as shown in Figure 2c. The pit primitive environment of the No. 1 exhibition hall was completely destroyed, such that the present pit temperature and RH have become sensitive to the changes of outdoor air temperature and RH. The marked fluctuations of the temperature and RH could be the main cause of thermal stress rupture occurred on the relics surfaces. The steady pit temperature and RH measured and monitored in the new exhibition hall was due to the high soil water content and high specific heat capacity of the underground soil. The excavation had not completely destroyed the primitive environment to destroy the unearthed relics. The exposure of the relics to the exhibition hall environment was only a short time to have any serious effect worthy of consideration. In Hanyangling Museum, an enclosed, glazed structure was used to separate the funerary pits domain from the visitor gallery to allow the visitors to view the relics at different angles within a short distance in the mysterious exhibition hall, as shown in Figure 3a. This latest design of archeology museum has provided an improvement to the preservation environment since the enclosure structure would negate the negative influences from visitors and atmospheric pollution. However, the exhibition side of the funerary pits domain is still a large scale open plan hall, which can lead to a significant natural convection in the room. The results of the environmental monitoring have shown that the relics in the pits were not preserved as adequately as expected. Slight surface cracking and weathering were found on the pottery figurines. Moreover, a small air-exchange between the exhibition side of funerary pits containing the relics and the outdoor environment could still

Figure 1. Annual average concentrations of gaseous pollutants in the No. 1 exhibition hall.

result, the indoor air would always be in an unstable state to enhance the heat and mass transfer between the relics and their exposed surroundings, especially considering the rapid development of industries, agriculture and transports in nearby urban centers; the pollution generated could lead to a serious deterioration in the air quality of the museum’s outdoor environment and could have caused an increasingly negative impact on the relics, for example, the sediment of hazardous pollutants, dust, and also molds, which could deteriorate the relics’ primitive form and material properties.9,13,20 The consensus related to the environmental specification of funerary pit relics is that the temperature and relative humidity (RH) of the atmospheric environment are the decisive factors that could cause an impact on the longevity of the relics’ preservation. Taking the No. 1 exhibition hall, opened in 1979 (Figure 2a), and the new exhibition hall (Figure 2b), opened by

Figure 2. Large open plan exhibition hall of the Emperor Qin’s Terracotta Warriors and Horses Museum, China (Courtesy of the Emperor Qin’s Terracotta Warriors and Horses Museum, China). (a) The No. 1 exhibition hall, excavated many years ago. (b) The new exhibition hall, excavated several month ago. (c) The pit temperature and RH in the No.1 exhibition hall and the new exhibition hall. 1506

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Figure 3. Evaluation of the microenvironment (without and with an air curtain system) for the pits of Hanyangling Museum, China. (a) The large enclosed exhibition hall. (Courtesy of Hanyangling Museum, China). (b) The air movement, pollutant dispersion, heat transfer, and soil moisture diffusion between the terracotta figures in the pit and the air layer overhead. (c) The pit space divided by the air curtain system.

take place through the joints of the airtight glass walls and air ventilation. Air pollutants, such as SO2 and NOx, were found to have penetrated into the enclosed hall environment.12 The nonuniform temperature distribution that is causing a natural convection movement was measured in the exhibition hall of funerary pits containing the relics. We use a computational fluid dynamics (CFD) method to simulate the flow field and transport path of air pollutants, soil moisture and heat in the No. 21 pit, as depicted in Figure 3b (see Figure S4, Supporting Information). A natural convection was being induced by the heating glass wall, and the penetrated air pollutants and heat were brought into the exhibition domain by the sinking flow. The inevitable temperature gradient surrounding the pottery figurines could be the direct cause of

the thermal stress cracking of pottery surface. The one-way transport of moisture from the soil to the air could cause the carbonate from the micropores or ultramicropores of the pottery wall being deposited on the relics’ surfaces and some carbonates could react with the penetrating SO2 to form sulphates. Space Division for Exhibition Side of the Funerary Relics. The above enclosed large space environmental control model shows the air convection movement within the exhibition hall would always exist due to the inevitable nonuniform temperature distribution between the air and the pits containing the relics. However, the impacts could be further minimized by dividing the enclosed large space into 1507

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Terracotta Warriors and Horses Museum and Hanyangling Museum would, at the moment, only exhibit a very small proportion of the excavated sites; more exhibition halls are being planned. However, as mentioned, the large open space overhead above the pits would still be used in the new exhibition hall of the Emperor Qin’s Terracotta Museum. Reintegration of the primitive environment to establish the equilibrium of heat and mass transfer in pits would be a necessary choice for the unearthed relics in the development of new exhibition halls. In conclusion, archeology museums have the responsibility of preserving and exhibiting the cultural inheritance of our ancient civilization. The challenge for the archeology museums is to produce an appropriate environmental control to ensure longterm preservation of relics within the premise that could also maintain the panorama view of the excavation sites. The introduction of an air-curtaining system would allow reintegration of the relics to their primitive environment. However, although the space division by an air curtain system could provide a high efficiency (theoretic isolation efficiency up to 92%) and low energy consumption (The refrigeration capacity is only 0.72 kW for a pit) method to separate the relics’ environment from the main exhibition hall overhead and to the visitor’s viewing gallery; it is difficult to obtain a detailed specification of the primitive environment, since it is determined by the underground soil environment before excavation. Further understanding of the primitive environment of different underground soil and water content in the pits containing the relics is necessary for the preservation of the historical relics.

smaller ones, while the panorama of the excavation sites, meanwhile, can still be preserved. Fortunately, China is rich in earthen relics and many of them are excavated and found in pits, unlike the ancient sites in the European countries, most of them are stone cultural relics or historical buildings. The pits could be reintegrated into small spaces by using an air curtain system, as illustrated in Figure 3c. The small space would be composed of an air filtering module and an air curtain system; the former is designed to provide the filtered air which is conditioned with appropriate temperature and humidity as demanded, while the air curtain sheer (see Figure S3, Supporting Information) is devised to diverge the air pollutants and heat from penetrating into the pits (i.e., the relics’ domain). The performance of the air curtain can be estimated by using the isolation efficiency,21,22 With the local air curtain system, the pit is separated out from the large space hall. Taking the No. 15 pit as an example, we predicted the performance of the air curtain system and demonstrated that this system could isolate the relics’ domains from the display area, with the theoretic isolation efficiency of up to 92%. The temperature distribution of the relics’ domain was steady and uniform, the temperature difference between the air and soil was less than 0.2 °C, as shown in Figure 3c (see Figure S5). More ventilation measures could be used to enhance the isolation efficiency. The Primitive Environment of Relics and Its Reintegration. We propose the following to further understanding of the pit environment to develop a control strategy for preservation of the funerary relics: The two parameters of above temperature and RH cannot be realized to adjust the open air-pottery-soil multiphase system in the local pit, to maintain the primitive environment required for the preservation of the relics, even when incorporating the above air curtain sheer system. The carbonate deposition on the relics’ surfaces have shown the existence of a nonequilibrium partial vapor pressure occurring between the carbonate solution in the micropores or ultramicropores of the pottery wall and the moisture in the air within the local pits, due to Gibbs phase rule. The natural convection movement in the enclosed large space would enhance this nonequilibrium of partial pressure of vapor. The strategy of the air curtain based environmental control for the air−pottery−soil multiphase system in local pits has suggested that an inversion temperature layer at the bottom of pits could prevent the induced convection of the air curtain sheer, thus forming a stagnant air layer over the pottery. Although there is a balance of moisture transfer between the earthed pottery and its surrounding soil environment in the exposed pits of the archeology museum, the former balance of moisture transfer could be broken down and two paths of vapor transfer (i.e., the soil-pit air and the soil-pottery-pit air could occur). Therefore, the moisture and temperature generated in the pits due to the air curtain overhead, and also, the temperature of the pottery-soil system should be adjusted to obtain a balance of vapor partial pressure. This adjustment should prevent the vapor dispersion from the microporous surface of pottery. Note that more than 50 archeology museums are in progress of being constructed in China. Besides, the Emperor Qin’s

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ASSOCIATED CONTENT

S Supporting Information *

S1: Description of the numerical model. S2: Numerical simulation of the conservation environment of the Exhibition Hall. S3: Numerical simulation of an air curtain system. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-29-83395110. Fax: 86-29-83395110. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Shaopeng Huang (Xi’an Jiaotong University) for his constructive suggestions in the manuscript preparation and Chengwei Wang, Xi Gao, Xianglei Li, and Cheng Qiu (Xi’an Jiaotong University) for their help with the environmental monitoring. This study was supported by National Science and Technology Ministry of China (2012BAK14 B01).

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