It went against my grain to throw out the tallow from a side of beef we bought, so I called our County Agent to see if he had any directions for making soap. To my surprise, he did; to my further surprise, they were easy.
I rendered the first batch of tallow by cutting it into chunks, filling the pan about one-third full of water and occasionally stirring the fat while it cooked at moderate heat. The stirring was a bit sloppy and the fat took quite a while to melt, so I ground subsequent batches of tallow in the meat grinder. This sped up the process considerably because I could then get more in the pan, it was easier to stir and the fat melted more rapidly.
The melted tallow and water was sieved and cooled. When the fat had solidified, I lifted it off the top of the water — and it was ready for soap.
While I was heating 6 pounds of the rendered fat (13-1/2 cups) to 120-130 degrees, I stirred together a 13 oz. can of lye and 5 cups of cold water and heated that mixture to 90-95 degrees.
When the fat and the lye were at their respective proper temperatures, I slowly poured the lye solution into the fat and stirred the resulting blend with a wooden spoon. In about 30 minutes the soap was thick enough to hold its shape so I poured it into a heavy cardboard box lined with a towel wrung out in cold water. It is important to NOT use aluminum utensils when making soap as lye reacts rather violently with this metal.
After my soap had aged for a little over a week, 1 grated it and mixed nine parts of the soap to one part borax to make eleven pounds of soap flakes. Since our water is very hard, I use one-third cup of washing soda to 1-1/2 cups of soap flakes in my washer and—despite the implications of some detergent ads—I'll compare my wash with anyone's.
Now I'm saving fat from cooking for the next batch of soap (which will be "all purpose" instead of the thin-lathering saddle soap" made from tallow alone). The fat I'm collecting will have to be washed by bringing it to a boil with an equal amount of water, then removing it from the fire and stirring in one quart of cold water for each gallon of hot liquid. The cold water precipitates cooking debris and the clean fat rises to the top when cool.
Apparently if the above directions are not followed carefully, your ingredients may sometimes separate and not form soap. Don't despair! You can save the mixture by melting it over low heat. Stir the batch during this reheating to keep it from sticking and remember, do not attempt this remelting in an aluminum pan. Add enough water to give the mixture the consistency of thick syrup and boil until soap forms.
For more information on making soap, see Soap Making Recipes and Tips for the Homesteader.