When making Russell Moore's sausages, we used hog and sheep casings. Hog casings, a sturdy option for larger sausages, like Moore's boudin blanc, come in a variety of diameters: 1 1/8–1 1/4-inch for frankfurters, 1 3/8–1 1/2-inch for bratwurst, and 2 1/4-inch if you want to make traditional black pudding. Sheep casings, which are more delicate, are best for smaller sausages like breakfast varieties, or Moore's garlic and herb sausage. These natural casings, if packed in salt, should be rinsed and soaked in lukewarm water for 24 hours before using. If submerged in brine, 15–30 minutes of soaking will do. Synthetic varieties are less expensive and ready for immediate use, no cleaning necessary, but you'll want to steer clear of those made from cellulose—they are not safe to eat.