Step 1 In a medium saucepan, cook the oysters in their liquor over high heat until plump, 5 to 7 minutes, stirring occasionally.
Step 2 Strain 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.