Grape juice concentrate

I read in Mother Earth News a good recipe to make grape juice concentrate. You put 2 cups of grapes in a quart jar, 1 cup sugar, fill the jar with boiling water and process 10 minutes in a BW bath. In six weeks, you have grape juice concentrate. To use, strain the concentrate and add 1 quart water. I was talking to my MIL yesterday and she told me she used to make grape juice that way only she would use 1 cup of grapes and 1/2 cup of sugar without diluting it before serving. I like the idea of making it a concentrate. It would use less jars. I have done 21 quarts. I think I will probably freeze the rest of the grapes to use for jelly or something. My kitchen is very sticky.