Lew Hodgett wrote:

Get a Revere Ware, S/S teakettle with a copper bottom.

Throw in a handful of glass marbles, pea gravel, etc, which will
prevent calcium deposits from forming when the water boils and
maintain the heat transfer characteristics.

I'll second that, mine's 11 years old. Peeked in it and saw no
deposits, but then I don't use hard water to make coffee!