Step 1In a medium saucepan, cook the oysters in their liquor over high heat until plump, 5 to 7 minutes, stirring occasionally.
Step 2Strain and reserve the oysters, keeping the liquor in the pan.
Step 3Stir the vinegar, salt, mace, cloves, peppercorns, allspice berries and cayenne into the liquor and bring to a boil over high heat. Boil the mixture for 5 minutes, then remove from heat.
Step 4Pour the mixture over the oysters and refrigerate, uncovered, until cold. Seal and refrigerate. The oysters will keep, refrigerated, for up to 2 weeks.