Test for leakage by filling with water.
If no cracks or risk of leakage then.....
Fill with 100% Heinz white vinegar, set on burner on stove and begin to boil...
Lower heat slightly and maintain a slow.. roiling boil.....
If you do not test first... your stove will be a huge mess for sure...
Sometimes I have had to dump and refill with fresh distilled vinegar, several times... all depends on how thick the calcification is on inside to the Tea Kettle.
If it's really bad, and still has calcified mineral deposits on inside after several boils, then I get out my Pick Set, and very gently go to work....
Signed,
Glutton for Punishment.... ::) ::) ;D