If you have been boiling only water inside your kettle, the hard deposit layer is mainly alkali compounds and minerals. Acid can dissolve such deposits. Fill the teakettle with a mixture of one part vinegar to six parts water, and let it stand overnight. The next day thoroughly wash out the kettle. If necessary, you can repeat the process.
In contrast, stains caused by acids like tea or coffee are best removed with an alkali such as baking soda.
** Making Coffee