<論説>三角縁神獣鏡の鋳造法と同笵鏡

Abstract

三角縁神獣鏡は舶載鏡(中国製) も仿製鏡(日本製) も、鏡背と鏡面二枚の鋳型を合せた縁部に湯口をもつ真土型鋳型によって鋳造されたもの、と現在では想定されている。ところが、鏡背文の四方に走る筋傷に着目すると、鏡背鋳型は四部分を組合せてあった可能性が強くなる。さらに検討を進めると、鏡背四分鋳型の証拠が外にも見付かる。それは三角縁神獣鏡に特有の文様などと議論のある傘松形である。傘松形は、皇帝が命令を遂行させるため使者に授けた節という一種の旗の表現であるが、筋の本義である割符からその表現が型の合せ目に用いられたに相違ない。これだけでなく、湯口も鈕の頂部に設ける方が、真土型の鏡背四分鋳型を数回使って同范鏡を鋳造するのには機能的であって、頂部に認められる円形の突起や粗面は湯口の痕跡である。鋳型の構造と鋳造法をこのように復原して、従来の鋳型の想定とかなり違ったものを考えると、三角縁神獣鏡の最も特徴的な、名称の由来でもある三角縁について、その成立状況を推測することが可能になる。当時、魏の鏡工人は小型・中型鏡しか作っていなかった、大型鏡を鋳造する必要が生じたとき、鏡背四分鋳型の周縁部に三角縁を巡らせて鋳型を平らに保持する方法が考案されたと。卑弥呼による大型鏡への懇望は、日本で各地の首長たちが大切に伝承している後漢前半の方格規矩四神鏡や内行花文鏡などの大きな伝世鏡に対して見劣りしない大きさの鏡を求めたことに因るのであろう。

It is widely accepted that bronze mirrors with a triangular-in-section rim were cast with a two-piece mould: one flat image-face and its backside with a centre knob and lots of decoration. An aperture is prepared in any part of the rim, and then molten metal was poured into the mould. Could we believe that there is conclusive evidence for the reconstruction above? Observing in detail the decoration, we find some marks that suggest a four-piece mould for their backside. There are two kinds of marks; one is represented by four small lines in cruciform as casting seams, escaped from being filed and polished off. The other is found on the motif of Kasamatsu-gata 笠松形; the word comes from the resemblance with an umbrella-like pine-tree. This motif, a kind of flag, is the representation of Chieh 節, the symbol of a certain part of the power which the Emperor of the Han Dynasty granted to his messengers. The Chieh derived its origin from a bamboo joint, which proved that the messenger with a split bamboo was the very person to whom the Emperor granted his authenticity; he had to show his identification by joining his piece to another. And this system persisted into the Wei 魏 Dynasty. For casting large bronze mirrors it was also necessary that the four pieces of mould for the backside were joined together in perfect accordance. Then, molten metal was poured into the mould through the aperture set at the centre of the backside, the conjoint part of the four pieces. After the casting jet was knocked off, normally its remainder was to be filed and polished off. Nevertheless, it is possible to see the traces on the top of some knobs of large bronze mirrors with triangular-in-section rims. This was an improved technique that made easy the production of larger mirrors; the circular rim of triangular-in-section made it possible to retain horizontal stability for the joints of the mould backside pieces. It has been said that this kind of mirrors were awarded to the Queen Himiko 卑弥呼 of Yamatai 邪馬台 State by the Emperor of Wei魏. In compliance with her wishes to have larger mirrors, the artisans of the Wei魏 Court established this technological innovation. When was the five-piece-mould technique devised? Now we are faced with the new question awaiting solution, of which I am, at present, of opinion that it was the second half of the Later Han.