This section is from the book "Mrs. Lincoln's Boston Cook Book", by Mary J. Lincoln. Also available from Amazon: Mrs. Lincoln's Boston Cook Book.
Any uncooked fat, such as suet, the fat from chickens, and all superfluous beef fat, should be saved and clarified, or made pure and clear. Cut the fat into small pieces, cover with cold water, and cook over a slow fire until the fat has melted, and the water nearly all evaporated. Then strain and press all the fat from the scraps. When cool, remove the cake of hard fat, or, if soft, draw it to one side and let the water underneath run off. You may put with the new fat any fat from soup stock, corned beef, drippings from roast beef, veal, fresh pork, or chicken; in fact, anything except the fat from mutton, turkey, and smoked meat. If there be any sediment adhering to the fat, add a little very cold water, and, after stirring well, pour the water off, or skim the fat from the water. Place the fat in a pan over the fire, and, when melted, add one small raw potato, cut into thin slices. Let it stand on the top of the stove or in the oven till the fat has stopped bubbling, is still, and the scraps are brown and crisp and rise to the top. Strain through a fine strainer, and keep in a cool place. Fat thus cleared will keep sweet for weeks, if melted occasionally, which should always be done when any new fat is added.
Boiling the fat causes the water in it to evaporate, and the organic matters or impurities to be decomposed, and deposited as sediment; the potato, owing to its porosity and power of absorption (being mostly starch and carbon), absorbs any odors or gases, collects the sediment, and thus cleanses the fat, very much as charcoal purifies water. Clarified fat (or dripping, as it is usually termed) answers for many purposes in cooking, - frying, sautéing, basting roast meat, greasing pans; and as shortening for bread, plain pastry, and gingerbread.