Even a teapot that has been burnt dry on the inside can be saved. This same solution works for any pot that you forgot about on the stove and ended up boiling it dry. For the exterior, a specific type of cleanser can remove the burn stains and shine up the teapot again.
Vinegar and Baking Soda
The vinegar and baking soda method begins with boiling vinegar and water in the teapot to loosen burnt debris.
Add about 1 to 2 inches of water to the teapot and pour in the vinegar. Bring the vinegar and water solution to a boil. Set the kitchen timer for 10 minutes.
Let the vinegar and water solution boil until the timer goes off.
Remove the teapot from the stove and add the 3 tablespoons of baking soda. When the baking soda hits the vinegar and water, it should begin to fizz.
Empty the teapot of the liquids after the fizzing action has quit.
Add additional baking soda as needed for more scouring.
Wash the teapot in a solution of mild dish soap and water. Rinse and wipe dry with a clean towel.
Oxalic Acid Cleanser
This method requires creating a paste with the oxalic acid based cleanser that you can leave on the teapot to avoid extra scrubbing.
Sprinkle oxalic acid-based cleanser on the inside of the pot with a bit of water to create a paste. Scrub badly scarred areas with the scrubber, and then leave the solution to sit on the blackened areas for 10 minutes.
Scrub again to remove the last bit of the burnt residue. Add a few drops of dish soap to the teapot to clean it. Rinse multiple times until no soapy residue remains.
Make the outside of the teapot or any pot for that matter -- even copper-clad ones -- shine by creating a paste of the oxalic acid-based cleanser on its surface. Scrub the outside of the teapot; allow the paste to sit on the pot for up to 10 minutes. Continue scrubbing until all burned stains are gone. Rinse clean and dry with clean dish towel.
Fabric Softener Method
Fill the teapot with water, adding at least 1 tablespoon of fabric softener to the liquid. Let the teapot soak for 1 hour or longer, as needed, until the burnt-on residue easily comes off. Wash the teapot in mild dish soap, rinsing in cool water and towel dry.
Wet the bottom inside of the teapot with water. Pour salt directly into the teapot so it covers its inside bottom. Let the salt do its work on the inside of the teapot for up to 10 minutes. Scour the the teapot with a scrubber, rinse and wash with a mild dish soap. Rinse and dry with a towel.
Depending on how long you left the teapot on the stove, you might find that it still has stains after cleaning. Add 1 to 2 inches of undiluted white vinegar to the inside of the teapot. Let it sit overnight. You can put it on the stove, but don't turn the burner on. Put a small note on it that indicates it is out of use so that others don't accidentally try to use it.
In the morning, wash the teapot with mild dish soap and water. If the teapot still has stains, repeat the cleaning steps, boiling water and vinegar in the teapot, and complete a final cleansing with an oxalic acid-based cleaner.
If you don't have white distilled vinegar in the house, simply fill the teapot with hot water and boil it on the stove for up to 20 minutes. Pour out the brackish water. Scrub the inside of the pot with a baking soda paste. Rinse clean. Repeat as necessary until any burnt stains are gone.