If you bought a Japanese cast iron teapot -- or tetsubin -- and you're concerned about orangish rust that has developed inside it, you could follow the way of connoisseurs and revel in the subtle way the rust enhances the flavor of tea. Or you can dissolve the rust with acid. Don't be alarmed by the idea of putting acid in your teapot; tea itself is full of tannic acid, which is strong enough to handle rust. Vinegar, which contains acetic acid; cola, which contains phosphoric acid; and lemon juice, which contains citric acid, can also dissolve rust from your cast iron or enamel teapot.
Cast Iron Teapots
Most cast iron teapots have an enamel coating on the inside, but even with that coating, rust sometimes develops. One strategy to prevent it is to boil several batches of water, allowing the minerals in the water to form a whitish layer that prevents rust from forming. Before doing this, you may want to remove the rust that has already formed:
Pour out anything in the pot, rinse it out with clean water and dry the inside thoroughly with a paper towel.
Brew a strong pot of green or black tea in a separate pot, bring the tea to a boil and pour it in the teapot. Let it cool for 20 minutes.
Discard the tea, but save the leaves in case you want to repeat the procedure. The tannic acid actually combines with the rust to form a protective coating on the inside of the pot.
Maintain the pot by rinsing it with hot or warm water after every use, drying the inside with a paper towel and turning it upside down on your countertop to allow it to air-dry.
Using Vinegar, Cola or Lemon Juice
The acids in vinegar, cola and lemon juice also dissolve