The gong used for Chanoyu came to Japan during the Muromachi from Korea and, more popularly, Namban dora from Southeast Asia were often taken from gamellon sets and other sources. They were pierced and strung on thick cords, sometimes being suspended inside a wooden frame if not hung from the ceiling of a tokonoma's side space (tokowaki) or somewhere in the mizuya. The baton used to strike the dora, called a bachi, is usually a carved rod with a head covered in soft leather and strung on a cord, from which it may be suspended from a nearby wall or from the side of the aforementioned frame.
Dora are made of copper, tin and trace elements to make a resonant alloy which most often is hammered into a shape, although cast pieces and other alloys are known. The sound depends on the exact composition of the alloy and thinness of the piece. Some dora have deep rims with thich edges, others are shallower. Some have a boss for striking while others are gently convex. The larger the dora, the deeper the tone.
By its long-lasting and wavering reverberation, the dora functions as a musical instrument, as a way of communicating with troops or workers at a distance, as well as a way of communicating with the otherwise hard-of-hearing gods and Buddhas. In the Buddhist frame, the sound takes us to the "Other Shore " of the Pure Land. I've seen and heard one about 140cm (about 4 foot) across that would take one across the Paradise River and back! The dora's long reverberation also functions to manifest the impermance of life, the once-in-a-lifetime Thusness of the Here and Now, to the waiting Guests.
When using the dora, it is usually brought into the tearoom and either held by the Host's assistant or the frame is positioned so that when struck, the reverberations reach the Guests where they are sitting outside in the roji. The pattern for striking the dora is also fixedÅF five hits in the order of loud, soft, medium, medium, loud when there are fewer than five Guests and seven hits with a repeat of loud, soft at the beginning then the set five, for more than five Guests.
The characters for dora mean "copper" plus a character used for transliterating, 銅羅.