Melt the fat slowly. Mix the lye and water in a bowl or kettle (do not use a tin pan), stirring with a stick until the potash dissolves. Add the borax and allow the mixture to cool. Cool the fat and, when it is lukewarm, add the lye, pouring it in a thin stream and stirring constantly. Stir with a smooth stick until about as thick as honey, and continue stirring for ten minutes. Pour the mixture into a box and allow it to harden. Cut into pieces the desired size and leave in a cool, dry place for ten days, to ripen before using.
When making the soap, be careful not to spill potash or lye on the hands, as it makes a bad burn. If the hands are burned, rub them with grease at once. Do not wet them.