Old-Fashioned Concord Grape Jelly

Grape jelly is tricky—or, at any rate, it has been for me. You really need to have at least some partly unripe grapes in the mix so that there’s enough pectin in the juice to set it into jelly and also so that the resulting spread is not too cloying. However, I am such a fan of plain old grape jelly (and few things pair so nicely with natural peanut butter—one of the many pleasures I’ve come to appreciate again as my daughter discovers them) that if I come across Concords that are tasty but