What is Soap?

A little chemistry is in order. Soap molecules consist of a hydrocarbon chain (made up of hydrogen and carbon atoms) with a "head" of oxygen atoms (as well as the stray sodium or potassium atom; detergents have more atoms in the head of the molecule to prevent ions in hard water from binding to the head and producing a precipitate). Animal fats and oils are made up of a molecule which consists of three of these molecules joined at the oxygen end by a glycerol molecule. In soap-making, fat is heated along with an alkali (generally sodium or potassium hydroxide), which breaks the bond between the oxygen ends and the glycerol, producing soap plus glycerin (which possesses moisturizing properties). Some soap manufacturers then remove the glycerin (using a salt to separate the soap from the glycerin).

Soap molecules have two parts: the hydrocarbon chain, which is hydrophobic (not attracted to water) and lipophilic (attracted to oil), and the oxygen head, which is hydrophilic (attracted to water) and lipophobic (not attracted to oil). When oil and water are mixed, they do not combine. However, when soap is added to the mixture, the lipophilic end connects to the oil and the hydrophilic end connects to the water. Enough soap, and the oil is completely covered with soap molecules and mixes right into the water. This is called emulsification. At that point, anything that removes the water (or dilutes it) does the same to the oil (thus washing the dirt away). Additionally, soap acts as a surfactant, reducing the surface tension of the water, and allowing it to penetrate whatever is being washed (particularly important when washing fabric, thus the use of additional sufactants in laundry detergent).

Soap is made with a careful eye toward balancing the fat or oil (also referred to as fatty acids) and the alkali to produce the right amount of soap and free acid or free alkali. Free acids (that is, fatty acids that have not been converted) provide some level of moisturization, although too much and the soap can go rancid quickly. Free alkali will tend to dry skin (and leather) and may irritate skin as well, although alkali soaps are still used in industrial laundry operations and carpet cleaning, as they clean fabric better than balanced soap.

Saddle soap is a specialized formulation of soap. Saddle soap doesn't have any free alkali. Alkali tends to be very harsh on leather, drying it out. Second, certain oils which are particularly effective at protecting leather are included. The oils penetrate the leather, moisturizing it. Saddle soap isn't particularly useful for hand-washing, largely because the additional oils tend to leave skin greasy.