Some teapots are made of porous materials like terracotta, sandstone or clay for instance. These teapots will absorbe some tea particles called tannins. They’ll keep a trace of every teas that has been brewed inside them. And thus will reveal deep tea flavors.
Progressively tannin will coat the inside of the teapot. However this process requires lot of patience and know-how to get good results.
It’s said that tea Masters’s teapots have great value.