Surface treatment techniques of ceramic molds for iron casting from the late Warring States period at the ancient capital city site of Zhu state

A collection of ceramic molds for iron casting from the late Warring States period has been unearthed at the ancient capital city site of Zhu (邾) state in Zoucheng City, Shandong Province. An array of analytical techniques was employed to examine their superficial layers and coatings, including wavelength-dispersive X-ray fluorescence spectrometry, X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy with energy-dispersive spectrometry, and polarized light microscopy. The results revealed that the coatings used talc, bone ash (hydroxyapatite), lime plaster (calcite), and analcime as release agents to facilitate the removal of castings from the molds, with analcime being the earliest known used for this purpose. These high-performance, reusable coatings significantly improved the casting efficiency and surface quality of the products. Comprehensive evidence from the compositional, surface, and cross-sectional structural features indicates that the fabrication technique of the surface layer is similar to that of modern immersion coating methods. Using hydrated lime as a binder can efficiently create a smooth surface layer on ceramic molds. Collectively, these technologies supported the iron casting industry in the production of thin-walled, ductile cast iron farm tools with low iron consumption. They embody technological choices and systems that sought to optimize efficiency, quality, and cost-effectiveness.