The simple, linear shape was conceived to fit in any space. The shallow and wide shape allows tea leaves to spread slowly and is suitable for green tea. The advantages of this shape are that the hot water flows easily, it is easy to wash, and it is stable.
The potter's clay is rich in iron and has long been blended for teapots. It is fired without glaze in a way that the iron content turns it black. In particular, by not glazing the inside of the pot, the fine irregularities on the surface moderately reduce the astringency of the tea, making it mellow and tasty.
How a teapot is made (JP)