This section is from the book "Hints To Housewives On How To Buy, How To Care For Food", by Mayor Mitchel's Food Supply Committee. Also available from Amazon: Hints to Housewives on How to Buy, How to Care for Food.
To Make White Hard Soap. Save every scrap of fat each day; fry out all that has accumulated, however small the quantity. This is done by placing the scraps in a frying-pan on the back of the range. If the heat is low, and the grease is not allowed to get hot enough to smoke or burn, there will be no odor from it. Turn the melted grease into lard pails and keep them covered. When six pounds of fat have been obtained, turn it into a dish-pan; add a generous amount of hot water, and stand it on the range until the grease is entirely melted. Stir it well together; then stand it aside to cool. This is clarifying the grease. The clean grease will rise to the top, and when it has cooled can be taken off in a cake, and such impurities as have not settled in the water, can be scraped off the bottom of the cake of fat.
Put the clean grease into the dish-pan and melt it. Put a can of Babbit's lye in a pail; add to it a quart of cold water, and stir it with a stick or wooden spoon until it is dissolved. It will get hot when the water is added; let it stand until it cools. Remove the melted grease from the fire, and pour in the lye slowly, stirring all the time. Add two tablespoons of ammonia. Stir the mixture constantly for twenty minutes or half an hour, or until the soap begins to set.
Let it stand until perfectly hard; then cut it into square cakes. This makes a very good, white hard soap which will float on water. It is very little trouble to make, and will be found quite an economy in a household. Six pounds of grease make eight and a half pounds of soap.
Save all pieces of soap that are too small to handle. Melt them in a little water over a slow fire and then put it into glass-covered jars. This makes a jelly-like substance which can be used for washing dishes, boiling clothes or any other purpose for which soap is used.