These teapots are vitrified clay, and, as such, can be used for tea or other hot liquids. However, as you can read on the disclaimers page, many of them are impractical or even unsafe for actual use.
With few exceptions, these teapots are stoneware clay that was reduction fired at Cone 10. That means that they were fired at temperatures somehwere around 2350 degrees Fahrenheit, and that the atmosphere in the kiln was low in oxygen.