THE OLD POTTER’S ALMANACK Volume Eighteen, Number One: June 2013. ISSN 0965-7479 CRACK PATTERNS MORPHOLOGY OF ANCIENT CHINESE WARES Sophia Lahlil, Weidong Li and Ji Ming Xu Shanghai Institute of Ceramics of the Chinese Academy of Sciences, Key Scientific Research Base of Ancient Ceramics, 1295 Dingxi Road, Shanghai 200050, P.R. China. Email: [email protected] Introduction This paper presents the first results obtained within the project “Scientific Research on the famous crackled glazed ceramics of the Song Dynasty” launched in October 2012. This research study intends to provide information on the manufacturing technology, the function and the degradation of the so-called Guan and Ge Chinese crackled wares through the study of the formation and morphology of crack domains using image processing on archaeological Guan and imitation Guan wares. We show that this new methodology can be a very suitable non-destructive tool to study the morphology of ceramic crack domains. The contribution of this project to authentication and conservation aims to be of interdisciplinary interest. The Old Potter’s Almanack Page 2 Figure 1. Photographs of the three crackle wares from the Song dynasty (960-1279 AD) and detail of their crackle glaze. a) Ru bowl stand, V&A Museum; b) Guan vase, V&A Museum; c) Ge plate, Shanghai Museum. Archaeological background 1368 AD), Ming (1368-1644 AD) and Qing (1644- 1911 AD) dynasties, the same level of quality was Glaze crackling is a frequently occurring never achieved afterwards (Krahl and Harrison-Hall phenomenon in ancient Chinese wares. It is observed 2009). as early as the Shang (1600-1046 BC) and Zhou dynasties (1046-256 BC) on Deqing primitive Guan ware often has relatively large cracks. For celadons and later wares such as Yue or Yaozhou decoration purposes, cracks were sometimes stained ceramics dating from the Eastern Han (25-220 AD) with iron-oxide or charcoal, most commonly for and Tang (618-907 AD) dynasties. However, it is only wares after the Song dynasty (Wood 1999). Ge ware during the Song Dynasty (960-1279 AD) - which is is often characterized by scholars by its crackling the period that marks the summit of China’s effect called “gold threads and iron lines”. This is a intellectual and technical achievements - that people double crackle network with lines varying in width especially the nobles and the royal family began to and colour: the major crackle is deliberately stained pay more attention to glaze crackling and valued it to black and the minor crackle appears gold-brown an aesthetic level. Chinese potters of this period have (Vainker 1993; Scott 1993). empirically developed processes to manufacture crackle glazes evoking natural phenomena such as The shape, size, colour and decoration of Guan and ice-crackles or fish scales. These ceramics were made Ge wares have been well studied and classified in specific kilns and were exclusively dedicated to the according to the period and site of production court. They hold a very special position in Chinese (Vainker 1993; Scott 1993; Li 1998). The global ceramic history because of their aesthetical qualities chemical composition of their glazes and bodies is and scarcity (Kerr and Thomas 2004). also well known (Wood 1999; Li et. al. 2001; Li et al. 2009; Miao et al. 2012). However, many questions During the Northern Song dynasty (960-1127 AD), remain on the origin and evolution of manufacturing Ru ware was celebrated for its sky-blue luminous techniques of these wares. crackled glaze attesting its technical superiority over preceding wares. Although modelled on Ru ware, the Aims of the study most sophisticated achievements of crackle glazes were obtained later on with Guan and Ge wares To manufacture glazed ceramics, a three step process produced during the Southern Song dynasty (1127- is commonly used: 1) the glaze raw materials are 1279 AD) (Figure 1). These ceramics had their own applied on the ceramic body; 2) after drying, the character, usually with a prominent crackle as their whole is fired at a temperature high enough for the only decorative feature. Although they were glaze to vitrify and cover the ceramic body; 3) the last extensively copied under the following Yuan (1279- step is the cooling, during which the glaze solidifies The Old Potter’s Almanack Page 3 Figure 2. Succession on cracks in hierarchical patterns in a test glaze-body sample. From left to right, first (red), second (blue), third (green), fourth (yellow) and higher orders (orange, pink and light-pink). and binds to the ceramic body. Cracks form during used image processing in order to enhance the the cooling, when a mechanical stress occurs between visibility of crack patterns and to obtain statistical the glaze and the body due to differences of thermal data on the number of domains, their size, shapes, expansion (Eppler and Eppler 2000). distributions, etc. Whereas cracks in varnishes and paint have been the In this paper, we present results obtained using interest of many researchers in the field of Cultural imaging processing on archaeological Guan and Heritage these last decades, cracks in glazed ceramics imitation Guan wares. We show that this new have been almost totally neglected (Hodges 1988; methodology can be a very suitable non-destructive Eggert 2006). However, in vitreous materials the tool to study the morphology of crack domains. morphology of crack patterns is a direct marker of the object history since it depends on the glaze-body Method and results properties, which comprises the chemical composition, the microstructure, the thickness etc. Twenty-three samples of crackled glazed ceramics Furthermore, when the crack pattern is hierarchical - dating from the Song and Yuan dynasties have been the cracks are formed successively (Figure 2) - the studied. These shards belong to the collection of the observed morphology can give access to the crack Key Scientific Research Base of Ancient Ceramics. formation process (Bohn et al. 2005a; 2005b; 2005c). They all come from archaeological excavations in the Therefore, we aim to provide information on the sites of Jiaotanxia, Laohudong and Fenghuang hill. We manufacturing of ancient Chinese crackled wares have selected these samples according to their various through the study of the formation and morphology crackle patterns. They have different shapes and of crack domains. sizes, as well as body and glaze colourations and thicknesses (Table 1). It implies that various raw One of the difficulties of this study is that in materials, manufacturing and firing conditions would archaeological samples, the observation of crack have been used. The inner and the outer glazes can patterns is not always obvious. Indeed, cracks are also differ notably within one sample, so they are often translucent and more or less fine so the considered as independent glazes, ranging the total boundaries of crack domains are not always directly number of studied glazes to forty-six. visible. Besides, the crack network can be very intricate and the surface of the samples altered which As mentioned above, it is not always easy to clearly entails a detailed study of the crack patterns observe the crack patterns. Therefore, we have used morphology. One of the techniques to improve the D-Stretch, which is a plugin to ImageJ software, visibility of cracks is to use raking light photography. developed by J. Harman1. Usually, this program is a However, since cracks are oriented in very different tool for rock art researchers. In this paper, we show directions, this method requires various raking that D-Stretch can also be a powerful tool for illuminations, which can be demanding when it researchers working on crackled ceramics since it comes to non-professional photographers. To enhances crackle networks and brings out fine cracks improve our knowledge of the crackling process of that are invisible to the naked eye. such complex artefacts as Chinese ceramics, we have The Old Potter’s Almanack Page 4 Group Group L (cm) L (cm) # Period Site Glaze colours I glaze O glaze I glaze O glaze 1 SS Jx opaque light blue-green A A 0.47 0.67 2 SS Jx translucent, light buff-grey A A 0.25 0.24 3 SS Jx translucent to opaque, light buff-grey B A 0.17 0.22 4 SS Jx opaque greenish-grey A A 0.32 1.41 20 SS Jx opaque blue-grey A A 0.48 0.45 21 SS Jx opaque greenish-blue A A 0.81 0.42 22 SS Jx opaque buff-grey A A 0.29 0.33 23 SS Jx opaque yellowish u A 0.95 0.18 7 SS Lh opaque greenish-blue A A 1.41 0.50 8 SS Lh opaque greenish-blue A u 0.53 2.00 13 SS Lh opaque greenish-grey B B 0.23 0.34 14 SS Lh opaque greenish-blue A A 0.37 0.38 inner: opaque blue-grey 5 SS or Y Fh A A 0.35 0.63 outer: opaque light blue-green 6 SS or Y Fh opaque greenish-blue A A 0.44 0.45 15 SS or Y Fh opaque brownish A A 0.21 0.18 16 SS or Y Fh opaque blue greenish-grey A A 0.26 0.42 9 Y Lh opaque greenish-grey B A 0.28 0.35 10 Y Lh translucent colorless A A 0.17 0.21 inner: opaque dark greenish-blue grey 11 Y Lh A A 0.34 0.38 outer: opaque greenish-grey inner: opaque greenish-grey 12 Y Lh A A 0.34 1.41 outer: opaque greenish-blue 17 Y Lh opaque blue grey A A 0.30 0.27 18 Y Lh opaque blue-grey B B 0.45 0.51 19 Y Lh opaque greenish-blue A A 0.65 0.45 Table 1.