History of Japanese Teapots

The colors of a teapot depended on how red clay was baked. A Red teapot was usually Oxidized when baked. While a Black teapot had reduced baking.  Nowadays clay is made from artificially mixed clay. But Before it was made with natural clay. There isn't much of a difference between artificially mixed clay and natural. But artificially mixed clay is the most used.  Red Clay is used to make teapots by hand. While Yellow clay is used to make Teapots on a potter's wheel, since yellow clay is more elastic then red clay.

http://hojotea.com/item_e/mumyoi.htm