Fill the teapot with enough milk to cover the crack and extra so it doesn't dry out during the sealing process. Keep it in a warm place for a week or more, letting the milk go really really sour. The casein in the milk will fill the crack like glue and will seal it. Incidentally, casein is used in the manufacture of strong glues.
Before use, clean the teapot well.
PSW, November 2009