Research background
Heritage is our legacy from the past, what we live with today, and what we pass on to future generations (UNESCO World Heritage Centre 2005). In addition, as the Japan Consortium for International Cooperation in Cultural Heritage (2013) underlines, cultural heritage is a testament that has been created over humankind’s long history. The protection and management of cultural heritage are means of ensuring their maximum possible vitality, values, and functions to the benefit of current and future generations (Szmelter 2013). Conservation of cultural heritage is often compared with human healthcare (Smith and Přikryl 2007). In such an analogy, degraded cultural heritage is compared with a sick person. To prevent cultural heritage from further degradation, diagnosis of the present heritage condition is given priority to perform any conservation treatment or restoration work. It is similar to the case that no medical treatments are provided to a patient before a precise medical check. The causes of the degradation should further be revealed based on the present condition like identification of the disease. Finally, countermeasures can be discussed by considering how to remove the revealed causes in a manner similar to the extraction of the cause of a disease. As basic medical studies are indispensable for human healthcare, studies on precise description and weathering are needed for conservation of cultural heritage.
Stone weathering has been investigated on several stone monuments from such a viewpoint (Doehne and Price 2010). Among them, studies on the on-going weathering rate are especially effective to evaluate the present condition of stone monuments (Winkler 1986), for example, to ascertain whether the target monument requires an immediate measure to stop weathering or continuous prudent monitoring for a while. To understand the present weathering condition of target monuments, the first step would be to measure the precise shape of the stone surfaces.
Contact and non-contact methods have been practiced to measure the surface change rate of stones (Moses et al. 2014). Although contact methods, such as with micro-erosion meter, have been practiced on quite a few objects (Stephenson and Finlayson 2009), a non-contact and non-destructive method is generally preferred for avoiding value reduction of the cultural heritage. The non-destructive laser scanning measurement technique is often applied to describe the present surface condition of cultural heritage (Ikeuchi et al. 2007; Jordá et al. 2011). Finding a difference between the results of two time measurements of the same target with a certain period leads to detecting the weathering during that period (Gomez-Heras et al. 2008). On the contrary, three-dimensional measurement based on photogrammetry has also been applied to measure cultural heritage because the technique is simple and quick compared with the laser scanning method (Westoby et al. 2012; McCarthy 2014; Fujii et al. 2018). Therefore, it is expected that the degradation speed of cultural heritage may be easily estimated without any special equipment, by measuring the same target twice with an interval using such a method and comparing the data.
However, it is necessary to understand long-term weathering for long-term conservation of cultural heritage. To detect a long-term degradation, an adequate length of interval is required. To solve this problem, it is expected to be effective to measure an old replica of a target made in the past together with the original. If the replica is close enough to the original, the difference can basically be regarded as the progress in the degradation of a target since the replica was made. In this study, the degradation of the Wareishi Rock Cliff Sculpture in the previous 30 years is discussed by comparing it with its replica made 30 years ago.
Wareishi Rock Cliff Sculpture
The target of this study is the Wareishi Rock Cliff Sculpture (Hiroshima Prefecture 2008) (Fig. 1a), designated as an important cultural property of the Hiroshima Prefecture, in the Sagi Island, Mihara City, Japan (Fig. 2). An inscription besides the sculpture shows that it was carved in A.D. 1300, directly on a core stone (Holmes 1978) of Cretaceous Hiroshima Granitic Rocks, located near a shoreline. No salt efflorescence has ever been reported on the core stone. The sculpture is a bas-relief of an approximately 1 m tall sitting Buddha. The ~ 4 m globular granitic core stone appears above the sea level during low tides (Fig. 1a), whereas a part or all of it sinks below the sea level during high tides (Fig. 1b). The average sea level during the high tide of 2016 was estimated to be around the chin or neck of the sculpture based on the tide table in Itozaki (Japan Meteorological Agency 2017), the nearest point of the Meteorological Agency (Fig. 2). The lower mass of the core stone below the chest of the sculpture is covered with dark-colored materials that seem to be algae (Fig. 1a).
About the degradation state of the sculpture, the original surfaces are supposed to be still retained on the upper body, including the face, because of the sharpness of the edges (Fig. 3). On the contrary, the edges of the lower body, including the garment, are not sharp, indicating that the lower part of the sculpture is degraded more than the upper part (Fig. 3). Local people emotionally insist that the degradation of the sculpture is progressing rapidly because of the impact of sea waves; however, this notion does not have any scientific data. Because an investigation must be conducted only during low tide, quantitative data about the sculpture’s degradation state have not been obtained to date.
The replica
There is a replica of the Wareishi Rock Cliff Sculpture made of fiber-reinforced plastics (FRP) in 1986 (Fig. 4), which is located in the Hiroshima Prefectural History Museum (Hiroshima Prefectural History Museum 1998). It is said that the replica was made not by imitating with human eyes but by directly copying the sculpture using a mold; first, the mold was formed by using silicon on the original sculpture through thin vinyl sheets, and then, the replica was made with FRP styling in the mold. Because the replica has been conserved inside the Museum since then, the difference between the shapes of the present Wareishi Rock Cliff Sculpture and of the replica can be basically regarded as the degradation during the previous 30 years.
The lower part of the replica is also dark-colored, but the upper boundary of the darkened part is around the stomach of the sculpture (Fig. 4), i.e., lower than that of the present Wareishi Rock Cliff Sculpture, around the chest (Fig. 1a).