 Bonshō are cast in a single piece using two moulds , a core and a shell , in a process that is largely unchanged since the Nara period ( 710 – 794 ) . The core is constructed from a dome of stacked bricks made from hardened sand , whilst the shell is made using a <unk> board . This is a large , flat , wooden board shaped like a cross @-@ section of the bell , which is rotated around a vertical axis to shape the clay used for the mould . Inscriptions and decorations are then carved or impressed into the clay . The shell fits over the core to create a narrow gap , into which the molten bronze is poured at a temperature of over 1 @,@ 050 ° C ( 1 @,@ 920 ° F ) . The ratio of the alloy is usually around 17 : 3 copper to tin ; the exact admixture ( as well as the speed of the cooling process ) can alter the tone of the end product . After the metal has cooled and solidified , the mould is removed by breaking it , therefore a new one has to be created for each bell . The process has a high failure rate ; only around 50 per cent of castings are successful on the first attempt , without cracks or imperfections . 
