When I was growing up, we always made soap starting with the fat when we butchered. We used homemade lye from hardwood ashes or used store-bought lye. You can add other stuff if you want it. It's not a super gentle soap, but it works well. Make bars if it turns out firm enough, or use it with some water added to make soft soap for doing laundry with.
You can use any fats to make soap, from used cooking fat, as long as it isn't burned, to the trim from butchering. Animal and vegetable fats together make a superior soap. You can clean cooking fat or used oils by adding an equal amount of water and bring to a boil. Remove from heat, stir, and add cold water. (1 quart to each gallon of hot liquid.) The clean fat should float to the top. You can skim it off or wait until it is firm and carefully peel it off. If it has an "off" odor, add 5 cups water and 1 cup vinegar to 6 cups of fat. Boil for 15 minutes, cool, skim clean fat.
Pure lye purchased from a store makes the most uniform soap, but you can make your own lye by placing a barrel or tub a few feet above ground on a rock or platform. Bore some small holes in the bottom of the tub. Cover the holes with small rocks to slow drainage. Fill the tub with hardwood ashes. (Hardwood trees lose their leaves in winter.) Pour hot water over the ashes, catching the runoff in a bucket. Add more ashes as the ashes settle, and pour the drained water over the ashes a second or even a third time, the longer the water takes to soak through and drain out, the stronger the lye will be. It should be strong enough to float a fresh raw egg.
You should use soft or rain water to make soap with, so no other minerals or contaminants get into your soap. Heat the lye water you have just made. In another container, glass or graniteware, melt the fat you are making into soap. Slowly add lye water until the mixture is completely combined, not too hot, or you will have a mess while doing this.
It is best to set up and make soap outdoors. Use a wood or graniteware spoon to stir slowly while mixing in one direction. The whole mass should be clear, when you have added enough lye water. To test, place a small amount from the center of the kettle on a piece of glass and allow to cool. If soap continues to be clear, it is ready. Add scent if you must. Soap should remain clear and may be too soft to hold bar shape. You may try placing in molds for bars, or store in glass jar crock or plastic container. It will not lather like detergent, but will clean well.
Editor's Note: Lye is very caustic and can cause burns and eye injuries. Whenever using lye, take precautions like using rubber gloves and safety goggles. Other precautions can be found at: http://certified-lye.com/safety.html and http://www.millennium-ark.net/News_Files/Soap/Lye_Safety.html.
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