These crackles, which resemble spider webs or icebergs, were once considered firing defects of porcelains, resulting from quickly cooling glaze. Until the Song Dynasty (960-1279), masters at the famous Ru Kiln, one of China's Five Great Kilns, learned to control the density of color of crackling and began to create the effect on purpose.
Video: Center for Language Education and Cooperation, Chinese Ministry of Education