Desiccation cracking of earthen sites in archaeology museum – A viewpoint of chemical potential difference of water content

Abstract

Unearthed relics and earthen sites are historical evidence of our cultural heritage. Many of them are referred to as immobile cultural relics since both the relics-in-itself and their surroundings are our historic heritage of past emperor’s burial with significant historical importance such that they cannot be moved into indoor-display museums. Researchers have studied the specific environmental specifications to establish the desired steady temperature, humidity and air quality for collections and visitors in indoor-display museums. However, due to the unique exhibition characteristics, the conservation of earthen sites is still a challenge to environmental scientists and technologists, and how to effectively protect these ancient sites has been a major concern globally. Archaeology museum is developing advanced techniques to prevent weathering of relics caused by the sun, wind and rain. The in-situ relics in archaeology museum are far from being well preserved because of the lack of understanding of site conservation and the impact of indoor micro-climatic conditions. Many of them are suffering deteriorations or even ruins due to improper preservation environment adopted for maintenance of relics. Among the hazards that unearthed relics are facing with, desiccation cracking is the most common and problematic. The drying layer of the earthen site in the Banpo museum (Xi’an, China) is thicker than 0.4m (see Figure 1(a)). The same phenomena are also found in the Emperor Qin’s Terra-Cotta Warriors and Horses Museum (see Figure 1(b)), the water content of the earthen site decreased from 15% to 22% in 1994 when it was excavated to less than 5% now, such that serous shrinkage and cracking occurred on the surface of the site. To upgrade the conservation of the relics in archaeology museums, great efforts have been made in recent years, e.g. maintenance of a high relative humidity (RH) and space division in the exhibiting hall have been implemented in archaeology museum as a part of the preservation regime. The underground exhibiting hall of Hanyangling Museum (Xi’an, China) is such a modern archaeology museum (see Figure 1(c)). It exhibits partially exposed pits containing pottery figurines in the co-burial grave of the fourth Emperor (188–141 B.C.) and Queen of Han Dynasty, China. The enclosed space for relics’ domain is being maintained at about 24 C in summer and 16 C in the winter. Moreover, the RH has been maintained at a level higher than 95% to prevent the drying of unearthed relics and sites. However, the water content of the earthen site in the exhibiting hall still decreased from over 15% when it was excavated in 2006 to around 4% in 2009. In addition to the desiccation cracking, gypsum was found to deposit on the surface of relics due to the transport mechanism by which the carbonates were diffused and migrated with moisture across the air-relic-topsoil complex interfaces. The carbonates would react with the penetrating SO2 from the ambient atmosphere to form sulphates (see Figure 1(d)). The reason why desiccation cracking occurs in high humidity environment is not entirely clear. The conservation protocol, i.e. space division and maintaining a high RH as a preservation regime, has been considered by archaeologists as an approved practice and therefore the protocol was applied to new archaeology museums; however, cracking still persists in the terracotta relics and earthen sites. The desiccation cracking of unearthed relics and excavated sites in these archaeology museums seems to be a spontaneous process even though the RH of the atmospheric environment was at approximately the ceiling value (100%). In order to clarify the mechanism of moisture migration behind the desiccation cracking phenomenon and to verify whether a high RH is adequate to prevent the desiccation cracking of unearthed in-situ relics, the chemical potential analysis