Put fat in enamel pan or stone jar. Pour water into another stone jar. Add lye to water, then add ammonia, borax and sugar. With a wooden stick, stir mixture until it cools to warm. When cool, pour into fat, stirring constantly while pouring slowly. Add sassafras and stir until thick and creamy, about 15 minutes. Place in cold place. Let stand 36 hours. Cut into squares. Let stand 1 week before using.