In case you can't see the articles on ACS or want to avoid looking at pictures of open wounds, here's the section from the article by Mitch Jacoby.
Mitch Jacoby wrote:While some researchers focus on new glass formulations for future applications, others study ancient ones. Weidong Li of Shanghai Institute of Ceramics together with coworkers at China’s Fujian Museum have been working to uncover the origin of the decorative patterns of streaks (“hare’s fur”) and spots found on glazed tea bowls unearthed from the Jian Kiln site in southern China. Now prized collector’s items, the bowls, which were used in traditional tea-tasting ceremonies, hail from the Song Dynasty (A.D. 960–1279) and were manufactured via long-lost techniques.
On the basis of microscopy and spectroscopy analysis, the group finds that the streaks and spots are iron oxide crystallites that precipitate during formation of the K-Ca-Mg-Fe oxide aluminosilicate glaze. The precipitation mechanisms are complex. For example, Li noted that some samples bear evidence that oxygen bubbles generated by pyrolysis of Fe2O3 rose to the glaze surface, dragging bits of anorthite (CaAl2Si2O8) and Fe2O3 with them. The firing treatment caused the low-viscosity, iron-enriched material to flow, form streaks, and eventually cool and precipitate between anorthite crystals. “Based on our study, we have been able to make good replicas of Jian bowls,” Li remarked.