Material Characteristics and Manufacturing Techniques Interpretation for the Casting Molds from the Silla Urban Site of Hwangnyongsa in Gyeongju, Korea

Abstract

This study investigates the material characteristics of casting molds found at the Silla urban site of Hwangnyongsa in Gyeongju to interpret manufacturing techniques of casting mold for bronze used in the Unified Silla period. The casting molds appear to be earthen molds for bronze-making, with a thick ceramic wall and are classified into inner and outer mold types. The molds show a layered structure, with a silt layer thinly applied to the earthen mold surface. This layer comes into contact with melted metal during bronze manufacturing and is located on the outside of inner molds and inside of outer molds. The paste layer exhibits fine-grained texture with a bimodal structure. Thin section analysis reveals planar voids in the paste, which were made by the carbonization of rice husks used as a temper during the firing process. The silt layer is mainly composed of very fine-grained quartz and feldspars without clay and plant tempers. It is estimated that the casting molds were made by mixing soil with tempers of minerals and rice husks, applying a silt layer to the surface of the molds, and experiencing temperatures below 1,000℃. Overall, the selection of different raw materials for each layer of the molds is connected with efficiency to ensure the permeability and thermal resistance during the casting process and easy release of the bronze object from the molds.