Baking Soda and White vinegar
Lay the canning jars sidewise in a large tub. Pour a healthy dose of concentrated white vinegar over the jars so that they are completely immersed in it. Let the jars soak in vinegar for almost half an hour before cleaning them gently with a non-abrasive scrubbing pad. For hard stains, sprinkle some baking soda on the pad and scrub the jar with added pressure. Finally, wash the jars with a soapy solution and rinse thoroughly with tap water before drying.
Cream of Tartar and Hydrogen Peroxide
Make a thick paste using tartar and hydrogen peroxide in a medium size mixing bowl. Apply this mixture generously to the stains on your canning jars and let it stay for five to ten minutes. Wash it properly in a soapy solution, rinse and pat dry.
Cream of Tartar, Scouring Powder and Hydrogen Peroxide
You can also add non-abrasive scouring powder to the above hydrogen peroxide and cream of tartar mixture to enhance its stain removing power. Mix cream of tartar and scouring powder in equal amounts in a medium size mixing bowl made up of glass. Add hydrogen peroxide to the mixture spoon by spoon, until you get a thick paste. Apply this paste to the stains so that they are completely covered and leave the jars aside for half an hour. Rinse thoroughly and dry with a towel before using.
Place the jars in a tub or any other large container and fill it with warm water until the jars are completely covered. Add two to three tablespoons of liquid Calgon to the tub and swish it around with your hands to mix it well. Allow the canning jars to soak in this solution for an hour (30 minutes if the stains are not hard). Remove the jars from the Calgon solution and rinse well.