Fill the pot with water, add about half a cup of apple cider vinegar, and put on the stove. Bring to a boil and let boil for about ten minutes. Pour the water out and rinse immediately. You should see most of the calcium deposits pour out of the pan with the water. If they are really bad, it might take two or three more times of doing this to get them all out. We used to do this regularly with aluminum pots when I was a kid. That was the days when we used an old wood burning cookstove and kept several large teakettles of water on the stove all the time to have our hot water. The calcium would build up in them and we'd have to boil it out every couple of months.