A few days ago I started a discussion about the following teapot on Steepster's discussion board. I invited people interested in Chinese teapots to look at the picture and say what they think is wrong with this teapot and what causes it. I got quite a few responses. Before I write further, let me first sincerely thank those who participate in the discussion. You guys are great.
I appoligize for the picture being a bit dark. This is my dad Teochew Hong Ni (Red Clay) teapot. It is the teapot he leaves on the tea table for our relatives to use. Teochew Hong Ni teapot is similar to Yixing zisha teapot. Their main similarity is the clays used to make each type of teapots are slightly air and liquid permeable. This permeable property is what makes Teochew Hong Ni teapot and Yixing zisha teapot so good for steeping tea.
Knowing the Hong Ni clay is slightly permeable lays the ground for the answer to the original question. The original question has two parts, what and why. Quite a few people were quick to point out the lid and the body have different texture. That's the right answer to the first part of the question. But why? What caused the difference in texture?
Two reasons. The permeable property of the clay and how the teapot is "nurtured".
Let's first look at the the permeable property of the clay. Because the clay is slightly permeable, a trace amount of the tea liquor is absorbed into the clay every time the teapot is used. (The cycle of heating, when very hot water is poured into the teapot, and cooling that occurs inbetween steeps helps the liquor absorption). Over a long period of time of regular use, something from the tea liquor accumulates in the clay. It, along with proper nurturing technique from the owner, give the teapot a subtle glow. (I say "something" because I don't know what exactly is accumulated in the clay. I have heard many things, like tea oil, tanin compound, tea scum, and many more. Anyone knows?). This subtle glow will appear as long as the teapot is used regularly. But in order for the glow to be nice and even, a lot of patience and dedication from the owner is a must. Since this teapot is used by many people and nobody cares to nurture it, it gradually acquires an uneven glow. In other words, it's the lack of nurturing that cause the uneven glow. Among all the abuses done to this teapot, I think the biggest factor that contribute to this uneven glow is leaving tea steeping in the teapot overnight. This leaves the pot in contact with tea much much longer than the lid, causing the pot to acquire the glow much faster than the lid.
(Nurturing the pot, some people say seasoning the pot, is a translation of 养壶 in Chinese. 养(Yǎng, to raise, care for, help to develop) 壶(Hú, pot). YǎngHú is a slow process that require patience and the right technique. It is just like raising a child. A good Yǎng-ed teapot looks and feels better. Most importantly, it enhances the tea it steeps. It is the general concensus in the Chinese clay teapot community that a well nurtured teapot is more valuable than when it was new. I'll write more about that in the future)
Below is an example of comparison between new version and nurtured version of the same teapot.