上代鋳造の型技法

Shaping implements used for casting can be classified, in view of their purposes, into three groups : those for producing the final shape in negative, those for intermedia serving to transmit the shape from the positive to the negative, and those for reproducing the shape. The first group, called sōjata (“full mould”), is the mould made from scrap without the use of the original. Ordinarily, moulds have to be made of plastic material because they are negatives, but the sōjata alone among the.n, because of the method employed for them, can be made either of plastic or of hard material such as stone (ishi-gata) or metal (kana-gata). Among sōgata made of plastic material there is the type consisting of a mixture of clay and sand (suna-gata). A suna-gata is used in the raw state, that is, not fired for hardening, and is produced by pressing with a “wood model” (ki-gata). The “wood model” is a model for producing a mould. It is simply a shape in the positive, and is to be discriminated in its function from the “original.” A sōjata being a negative made without the instrumentality of a positive, the imppression on the mould and the cast work obtained from it are reversed in effect : raised portions become depressed, and right becomes left. The reversal is in notable evidence especially in the delicate raised lines on bronze mirrors with zigzag patterns or “bronze bells” (dōtaku) of the Æneolithic age, and the reversed Chinese characters found on bronze mirrors of the Han style. The second group in the present-day method of casting is repesented by komi-gata (“embedded model”), in which the original is wrapped in a mixture of clay and paper fibre to make the negative mould for it. This method has the merit of retaining intact the original used for the model, but it is questionable if the method was employed in the “early historic periods” (Asuka and Nara Periods) in which the models were not retained after casting. However, the group of three bronze statues of Shaka (Śākyamuni) Triad in the Main Hall at the Hōryū-ji in Nara, representative of the Tori style of Buddhist sculpture, is considered to be an example in the komi-gata method : after the moulds had been made the models were scraped on the surfaces, and melted bronze was poured into the space between the moulds and the reduced models. A radioactive light experiment on the group of some fifty gilt bronze statuettes, originally in the Hōryū-ji and now kept by the Tokyo National Museum, proved that the hollows inside the Tori style ones among them were large and in shapes conforming the shapes of the statues, and that their thickness of the bronze was thin and even. These facts probably tell that the komi-gata method was in use in the Asuka Period among Tori style sculptors. The flattened, relief-like effect universally noticed on Tori style statues was due to the imitation of Chinese relief carvings in rock-cave temples of the Six Dynasties, but obviously the effect was enhanced by the “split mould” (wari-gata) technique of the komi-gata method in which the mould consisted of two negative blocks for the front and the back. The method widely used for the second group in ancient times was the “wax model” (rō-gata). In this method a core is prepared roughly in the desired shape and covered thickly with beeswax. The wax covering, carved precisely into the shape, becomes the model (positive), which is subsequently wrapped in clay. The whole thing is then heated, and the wax runs out of the holes made through the clay mould, leaving the space for the bronze between the core and the mould. The model in this case is no longer in existence after the casting is done. Objects made in the “wax-model” method are characterized by the irregular shapes of the hollows inside them, due to the manner of making the model. The third group, in fact, does not represent a method, but it refers to the functions of the above-mentioned methods. In the komi-gata method using the fragile mould, the mould is made from the model one for each casting. In the sōgata method, objects are made directly from the same hard mould without the use of the model. In the suna-gata method the clay mould is broken at each casting, but the easiness of making similar moulds with the same wood model causes it to be convenient for producing numerous replicas. In the rō-gata only one piece can be cast, but the wax model is very helpful as a secondary positive for the sōgata negative to be used in making many reproductions of an object with relief designs, such as an ancient bronze mirror, for it enables the reproduction without causing any damage to the precious original.