Myoshinji Temple, the head temple of the Rinzai sect of Zen Buddhism, is located in the city of Kyoto. Housed within this temple is the Ohjikicho temple bell, Japan’s oldest temple bell and a designated national treasure, which resonates with a sound considered the epitome of the ideal Bonshou bell tone. In 1974, for the first time in approximately 1300 years, the Iwasawa Bell Company undertook the reproduction of the Ohjikicho bell. To do so successfully, it would be necessary to ensure that the bell rang in the ohjikicho pitch, a tone in the gagaku scale of Japanese imperial court music. The quality of the material that the original Ohjikicho bell was cast in was unknown, and with its designation as a national treasure it was impossible to perform a close analysis of the bell, making this an undertaking of the utmost difficulty. However, by conducting many tests using past bell casting data, and aided by research conducted jointly with a university, the Iwasawa Bell Company was able to accurately calculate the thickness of the bell walls and faithfully reproduce the bell.
The craft of casting a Bonshou bell consists of fitting together a two-part mold consisting of the inner mold (the “core”) and the outer shell, and pouring a copper-tin alloy into the space between. The creation of this mold is the single most important factor in determining the outcome of the bell. All Bonshou bells are cast as a single piece, with no subsequent additions. Each painstakingly handcrafted bell mold will be broken away after the casting, and the completed Bonshou bell will emerge from within.
The climax of the Bonshou production process is the lighting of the casting furnace. After the copper has been heated to a temperature in excess of 1000°C, tin is melted into the copper and the molten metal is poured, in a single stream, into the mold. Instead of an electric furnace, the Iwasawa Bell Company uses a traditional heavy oil furnace. As Buddhist priests or other officials chant sutras in the background, the casting is carried out with the utmost seriousness, for a single mistake may cause irreversible damage. Bonshou bells are cast in a mixture of at least 85% copper to 12-15% tin, but factors relating to how well the metals mix together and how fast the cooling takes place may affect the sound of the bell.
After a careful consideration of the mixing ratio of copper to tin, the first Ohjikicho bell was completed. However, the sound of this bell, which had a fundamental frequency of 130Hz, was deemed unsatisfactory, and this necessitated a second attempt. Another mold was made, the casting took place once again, and a new bell was extracted from the mold. After the unwanted “casting fin” generated at the seam of the mold had been removed, the metal surface was polished, and the second Ohjikicho bell was complete. This time, the tone of the bell, with a fundamental frequency of 129Hz, was judged to be acceptable. The patient, laborious efforts of a group of dedicated individuals had gone into achieving this subtle difference of a single Hz.
An outer shell and a core are the two parts of the mold required to produce a Bonshou bell, and the core is created by piling hardened blocks of natural sand up like bricks. The outer mold is created by rotating a “strickle board,” a wooden template shaped like the cross section of the bell to be cast, and materials ranging from coarse sand to clay are used to complete the mold. The rows of protruding rounded “bosses” that encircle the upper portion of the bell are carved into the mold, and wooden characters are used to make impressions for inscriptions. Finally, the mold is left to dry thoroughly before the casting.