Bamboo Charcoal Pottery is based on an older tradition of making cups from simple fired charcoal bamboo end pieces. Bamboo Charcoal Pottery (竹炭陶) is a achieved through mixing of clays made from hyper fine ground bamboo charcoal and Hong Ni (red clay) mined in Yixing and other areas deep underground. Both the bamboo charcoal and clay used are free of impurities. In fact, the whole purpose of using Bamboo Charcoal Pottery is to remove impurities and hardness in the water you use. The molecular structure of the charcoal absorbs heavy metals as well as other impurities in water.
When you get the cup, make sure to boil it in water or tea for atleast 15 minutes before use. This will remove the very subtle "kiln" taste that the charcoal has.
Notice how quickly the cup will dry off after being doused with water, or how quickly it absorbs the water or tea after the cup is mostly empty.
When I am stuck just drinking tap water I fill several of these cups with water and drink the next day. The improvement in taste is more than noticeable.
Plastic cozy is removable, but can be handy when the cup is very hot.