Changes in the Casting Technique of Bronze Mirrors in the Ancient East Asia

点击翻译按钮获取中文摘要

Fumikaeshi mirrors are a series of mirrors cast by the use of molds repeatedly made out of a single master mirror. It has generally been indicated that these molds may shrink 20/1000, or 7 to 15/1000 smaller than the master mirror. However, in our experiments the contractibility was only between 0.5/1000 and 4/1000.Thermoluminescence technique was applied to judge the authenticity of the molds of the Warring States Mirrors stored in institutions in Japan. This study has helped uncover more hands-on aspects of the ancient mold-making technique. The three pieces of the molds of the Former Han Mirrors available in Japan are all thick but very lightweight. Specific gravity of the two is 0.9 and 0.95 respectively, making it possible for these molds to float in water. The specific gravity of conventional mane ceramic clay molds is approximately 1.6, which suggests that the materials used for the Former Han mirror molds are unique. According to the recent report published in China, the metallographic study of the mirrors has shown that many of their surfaces were heat-treated (tempered). We have set up a hypothesis that a time lag occurs between the flat side and the projected decorative side of a mirror during contraction as it rapidly cools down, which makes the flat side contract towards the projected side resulting in a convex profile. In our experiments, forty flat mirrors were cast and tempered. An acceptable condition was achieved to reproduce convex mirrors with warp similar to the excavated mirror, however, several issues remain outstanding such as over-warping and unevenness. Other findings in property changes include that depending on how it is tempered, high-tin content bronze becomes harder to break but easier to scrape, and its color turns silver-white.