Split the garlic head in half and rub it cut side down all over the ham. Sprinkle on the salt, cayenne, and thyme and rub it into the ham. Put the ham fat side up into a roasting pan and cook it for about 25 minutes per pound or until the internal temperature registers 155 degrees F on an instant-read thermometer. Remove the ham, cover it with foil, and let it rest for about 15 minutes. The temperature will continue to rise to 160 degrees F. Slice and serve.