How do you make sure the lid on a newly canned jar of homemade jam is sealed properly?
Fill a large pot with tap water (enough to the water will be 2 inches above the top of the canning jar) and bring it to a boil. Place the sealed jar of jam into the pot and allow it to sit for 10 minutes. After 10 minutes has elapsed, grasp the jar with a pair of tongs and pull it out of the water. Allow it to cool overnight. The next day, press on the center of the jar's lid. If it makes a popping noise the jar is not sealed properly. You will need to repeat the sealing process until the center of the jar's lid does not pop up.