Not all casings for meat products are natural casings. The alternative is to use cellulosic, collagen, fibrous, muslin, or synthetic casings. Cellulosic, which is made from solubilized cotton linters, is a strong casing that is commonly used for some of the various varieties of frankfurters. Collagen casings that are produced from animal parts (bones, cartilage and connective tissues), are common casings for use in sausage making. It is a uniform in texture, soft, and yet very durable for stuffing meats. Fibrous casings are very durable for use in stuffing coarse ground meats that are tightly packed into the casing. Semi-dry and dry sausages are most often made with a fibrous casing. Muslin casings are made with a fine weave cotton fabric that covers blood sausage, some bologna, liverwurst, and salami. Synthetic casings are generally those that are made from polymers formulated for use in the food industry. They are often used to encase some sausages in colored casings, such as white for liverwurst, clear for salami and red for bologna.