The science of making your own medicine is very relevant, especially if you're building your knowledge of self-sufficient means of survival. Eventually, we all get sick, so learning to prepare your own remedies is crucial.
"Soaps are naturally divided into hard soaps, which are made with soda alkali, and soft soaps, which are made with Potassa alkali. As different fats vary in proportion of their fatty acids, and the caustic soda and potassa of commerce vary in caustic strength, definite formulas for the Soap bases cannot well be given, but the following general formulas and processes given by W. J. Menzies in the Manufacturers Review for November 15, 1880, will be satisfactory. The first can be used either for making Castile Soap or Curd Soap"
This home pharmacy book guides you through the second half of Part 3 of Fenner's Complete Formulary. Written in the late 19th century, the information and processes described within are still useful today.