Remove the top of the ham can and heat the ham in the oven until the gelatin softens. Invert the ham on a rack in a pan, punch holes in the bottom of the can and lift it off. Pour off the can liquid.
Cover the raisins with warm water.
Score the ham in diamonds and stud with cloves. Mix the mustard and molasses and spread on the ham. Bake 1 hour, basting with the beer.
Transfer the ham to a platter and boil the drippings until reduced to 1-1/2 cups. Drain the water from the raisins into a measuring cup and add more water to make 1 cup. Add to the drippings and bring to a boil.
Mix the cornstarch with 1/3 cup water, add to the boiling broth and boil, stirring, 1 minute. Add the vinegar and raisins and serve the sauce with the ham.