Although, by definition, every soap is made by the saponification (a type of chemical reaction) of lye, water, and fat, one soap will differentiate from the next depending on the kind of fat, the kind of lye, and how much of each is used. Lye made from wood ash, for example, produces soft soap, so called because of its jellylike consistency, In contrast, soap made from commercial lye (sodium hydroxide) will be hard. Soaps containing coconut oil tend to lather well in cold water but may have a drying action on the skin. Superfatted soaps, such as castile, that contain excess amounts of unsaponified fat are particularly gentle and make excellent “toilet soap”.

For the sake of convenience or for some special use, soap can be altered in consistency and appearance. Jellied soap for doing the dishes is obtained by slicing off shavings of hard soap and boiling them in water until they dissolve; about 1 pound of shavings per gallon of water should be used. To produce soap flakes for laundry use, simply grate any ordinary hard soap, then add a few tablespoons of borax to improve water softening ability and quicken sudsing action. The preparation of liquid soap is somewhat more complicated. It is generally based on vegetable oils rather than animal fats and requires the addition of glycerine and alcohol during the soap making process, followed by filtering.