Teaparker has an article about this question. He gives these 3 rules to choose a teapot for a particular tea:
1. For teas that have flat dry leaves (like WuYi Mountain Shui Xian oolong), a rather flat teapot fits best so that tea and water are close together,
2. For teas that are knitted like small round balls (like Taiwan oolong), a round teapot is best to let the leaves open up,
3. For heavy roasted teas, the teapot should be made of softer clay (like zisha or duan ni), fired at a little bit lower temperature. (My picture, for example, shows a small round duan ni teapot that would be best with old Taiwan oolong or with roasted Tie Guan Yin).
A reader also asked me to write about this subject. So let's add the other 'rules' to help you make the best pairing of tea and teapot.
You will choose a gaiwan instead of a teapot to drink:
- (white, yellow and) green teas, or any tea with light and fine fragrance,
- any tea that you want to test, evaluate, compare, (for instance before you purchase it),
- and start learning gongfu cha. It lets you better see and smell the tea while you're doing it.
- tea in a hurry and have little time to clean the teapot.
Other principles to guide your choice of a teapot:
- Thick walls will retain temperature longer and are better fit for teas with long aftertaste, like roasted teas,
- Thin walls fit very fresh and fragrant teas.
- The yixing clay employed should be pure, or even come from a stone that has been grinded into powder so that it contains natural minerals and pores,
- The temperature at which the teapot is fired should be over 1150 degrees in order to reduce the water content of the clay below 2%. Such teapots have a high pitched sound when you open the top. This ensures that the zisha pot fulfills all its functions. Zisha clay will filter the tea: reduce its astringent and sour taste, and make it more round and mellow. It will also keep the tea and the leaves warm and it woun't burn your hands.
In conclusion, to know which tea will be best brewed in which of your teapots you should test it yourself. With a good teapot your tea will have a better fragrance as well as a better yun (finish) than with a gaiwan.