Do you have copper bottom pans or any copper around the house that needs cleaning? Here's an alternative to buying copper cleaner. Ketchup and salt. I can't think of anyone who wouldn't have these two ingredients in their pantry.
Put a generous glop of ketsup on there. Add a good bit of salt. Smear it around to cover all the copper and let it set for about 5 minutes.
Smear it all over the copper once again and let it set for another 5 minutes. You should see the copper color coming back.
For the stubborn burned on spots you can use baking soda as a scouring agent after the ketsup and salt have done their best. The baking soda will also put a shine on your copper after it's cleaned.
I could have done a bit better but I was in a hurry to clean the pot so I could use it for some cooking.