crushed shell or crushed pottery were often used to make bonfire-fired ceramics because they provided an open-body texture that allowed water and volatile components of the clay to escape freely. The coarser particles in the clay also acted to restrain shrinkage during drying, and hence reduce the risk of cracking. Form: In the main, early bonfire-fired wares were made with rounded bottoms to avoid sharp angles that might be susceptible to cracking. Glazing: the earliest pots were not glazed. The potter's wheel was invented in Mesopotamia sometime between 6,000 and 4,000 BC (Ubaid period), and revolutionised pottery production. Moulds were used to a limited extent as early as the 5th and 6th century BC by the Etruscans and more extensively by the Romans. Slipcasting, a popular method for shaping irregular shaped articles. It was first practised, to a limited extent, in China as early as the Tang dynasty. Transition to kilns: The earliest intentionally constructed were pit-kilns or trench-kilns, holes dug in the ground and covered with fuel. Holes in the ground provided insulation and resulted in better control over firing. Kilns: Pit fire methods were adequate for simple earthenware, but other pottery types needed more sophisticated kilns. History by region Beginnings of pottery Pottery may well have been discovered independently in various places, probably by accidentally creating it at the bottom of fires on a clay soil. The earliest-known ceramic objects are Gravettian figurines such as those discovered at Dolní Věstonice in the modern-day Czech Republic. The Venus of Dolní Věstonice is a Venus figurine, a statuette of a nude female figure dated to 29,000–25,000 BC (Gravettian industry). But there is no evidence of pottery vessels from this period. Weights for looms or fishing-nets are a very common use for the earliest pottery. Sherds have been found in China and Japan from a period between 12,000 and perhaps as long as 18,000 years ago. As of 2012, the earliest pottery vessels found anywhere in the world, dating to 20,000 to 19,000 years before the present, was found at Xianrendong Cave in the Jiangxi province of China. Other early pottery vessels include those excavated from the Yuchanyan Cave in southern China, dated from 16,000 BC, and those found in the Amur River basin in the Russian Far East, dated from 14,000 BC. The Odai Yamamoto I site, belonging to the Jōmon period, currently has the oldest pottery in Japan. Excavations