Cut the grapes from the main stem, leaving a small twig on each grape.
Wash the grapes will in salted water. Drain them in a colander, then pack them into a sterilized jar.
In a saucepan over medium heat, boil the vinegar and sugar until the sugar dissolves.
Add the mustard seeds, cinnamon stick, black peppercorns, and bay leaf and return the mixture to a boil.
Boil for 2 minutes, then pour the hot vinegar overhot grapes. Cover and seal the jar.
Store the grapes in a cool, dark place. Leave for 4 weeks before using.