Any tea kettle that has seen good use is bound to develop some mineral buildup, which doesn't sound like the easiest thing to clean out. But it's a lot easier than you might think to get even electric kettles as clean as new. Calcium and lime buildups can be dissolved with mild acids such as white vinegar and lemon juice.
Mix 1 cup of white vinegar with 1 tbsp. of lemon juice into enough water to fill the tea kettle.
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Boil the mixture well, then rinse out the kettle.
Treat remaining mineral buildup with undiluted white vinegar, letting it sit overnight, then boiling again.
Rinse out the tea kettle thoroughly.