This is such an easy jam to make and you can use any grapes, not just Concord. For this jam I used a blue-black variety called Moon Drop grapes, shaped somewhat unusually like a torpedo. They are seedless, sweet, and I did not peel them. I used sugar in a 1:2 ratio or half as much sugar as the weight of the grapes so you can adjust up or down as you need to. And instead of using pectin I put in one whole chopped green apple seeded with the peel.
makes 3 cups jam
800 grams grapes
400 grams granulated sugar
1 medium green apple, chopped, seeded, with peel
Juice of 3 small limes
1 cup water
Boil a kettle of water. Wash and rinse jars and fill to the brim with the boiling water. Set aside. Put lids in a bowl and cover with boiling water. Set aside. Put 2 small saucers in the freezer for testing the jam.
In a large pot, put the grapes, water, sugar, apple, and lime juice. You can throw in the lime halves too. Bring to a boil over medium low heat and stir occasionally. Mash the fruit as it softens. Boil until the jam reaches a temperature between 217 degrees F and 220 degrees F, about 10 to 25 minutes. Take a saucer out of the freezer and drop a spoonful of jam liquid in it. Return to the freezer for 2 minutes. After 2 minutes take out the saucer and drag your finger through the jam. If it separates, the jam has set. If the jam runs back together it is not yet set. Boil the jam for a few minutes more and test again.
Fish the lime halves out of the jam and discard. Tip the water out of the clean jam jars and fill with jam, leaving 1/4 inch headspace at the top. Screw on the covers and let cool to room temperature. Refrigerate 12-24 hours to allow the jam to set before eating. After that it can even be frozen.