Grandma’s grapes were from the Lake Erie Islands. She also used to make grape juice with them. There did seem to be something different about that “Ohio product” from the Concord grapes more easily available around the country. You must stir this constantly while cooking. Cool until no liquid separates around rim of butter when a small amount is dropped onto a cold plate. It should mound when dropped from a spoon when done. Butter must be sealed in sterilized jars and paraffin, if used, must be ladled on immediately after pouring jam while still very hot.
Take blue grapes (Ohio product) and pick them off the stem as you wash them. Put in kettle with skins on and cook until juicy and done, stirring often so they won’t burn. When done, strain thru sieve into another kettle. Measure how many cups of juice you have, and add that many cups of sugar. Cook for about ½ hour and put in jelly glasses with paraffin on top.