Yixing clay was first used to make teaware in Song Dynasty, wasn't until Ming Dynasty it became popular and well spread in tea culture. To understand why Yixing teapot is superior compare to teapots made with other materials, follow topics are to be acknowledged but not limited to them:
1st of all what is Yixing clay?
2, how does a clay teapot impact tea?
3, how tea impact the clay hence improves tea itself - why raise a clay teapot?
4, how to raise a clay pot?
1st topic: what is yixing clay?
Soft clay discovered between rocky layers of mountains in Yixing region. 3 main types of clay: purple, green/yellow, red. All have high content of iron. Soft clays are left in open fields for months and years to be dried and corroded natural. Corrosion is to break up large chunks of clay into small pieces for further break down. Grinding pebble size clays into small grain size sandy clay, then soak in water for a days or even years to "ferment", the longer the better. Depending on the need and finishing texture of teaware, various size of sandy clay are used for wanted results. Sifter is used to sift therefore group similar size sandy clays before using. Number of "eyes" is number of holes of the sifter per square inch, ie 40, 80, 120 eyes, the larger the number, the finer grains they are. Pot makers mix large grains of actual sands for texture and color contrast in appearance. By theory, large grains of clay and/or mixed with sands produce more porous teapots.
Purple is most common, medium porous, green/yellow is most porous, red is the least. Purple clay varies in color by firing temperature, "fermentation" and contents of clay from different rock layers. 2 pieces of exact fermentation from the exact same batch, 1 fired at 1120 degrees is more brownish purple, the other fired at 1150 is darker brown purple, at 1200 degrees the color is a lustrous dark chocolate purple with higher pitch in sound. Purple is a general concept only. Green clay is earthy green in color before firing, becomes earthy yellow after high fire. "Little Red Clay" is similar to Zhu Ni but from 2 different mountains, Zhu Ni is no longer available for mining. Both red clays make beautiful pot and tea, even though Zhu Ni is the most sought after due to rarity for 2 main reasons. It's no longer available on earth and pots made with such clay has low success rate in finishing products. 25% shrinkage after high fire made it difficult to have perfect shaped teapots, not to mention cracked defects or imcomplete products. Pot makers were and are reluctant to make them. This does not mean Zhu Ni must be better than Xiao Hong Ni in brewing tea.