If you were to look at leather under a microscope you would see that the tangle of connective tissue fibers leather is made up of resembles the structure of a scouring pad (on a much smaller scale). These fibers are held together with collagen protein bonds.
To create leather, hides are soaked in chemicals in the tanning process, to prevent the fibers and their bonds from decomposing. And then, through a process called fatliquoring, oils are soaked into the hides to keep their collagen protein bonds from drying out.
The fatliquoring agents are roughly classified in an emulsion type and a non-emulsion type. The emulsion type fatliquoring agents include anionic type ones such as sulfated oil, sulfited oil, sulfonated oil, soap, phosphated oil and aliphatic acid condensate base oil; cationic ones such as aliphatic acid amine base oil; amphoteric ones such as aminocarboxylic acid base oil and betaine compounds; and nonionic ones such as polyethylene oxide derivatives. The non-emulsion type fatliquoring agents include natural oils such as fish oil, beef tallow oil, vegetable oil (olive oil), animal oil (beef tallow, lard and mutton tallow), wool grease, mineral oil, wax, paraffin wax and the like.
Keeping oil around those collagen protein bonds is necessary to protect them and give longer life to the leather. If those collagen protein bonds dry out entirely they will shrink, become stiff and break. The broken bonds weaken the leather permanently and damage the shoe in general. Introducing oil back into the leather at this point is useless, as the damage is already done, and irreversible.
When water gets into leather from perspiration or external weather it bonds with the oils to some degree and floats some of the oil to the surface as the water evaporates. Adding oil to the leather introduces it back into the fibers where it can replace the lost oils and continue to protect the collagen protein bonds.
Putting oils back into shoe leather is sometimes referred to as “nourishing” the leather, but that is a misnomer since the leather is truly dead. The proper term is “conditioning” the leather, because you are trying to keep the leather in supple condition by lubricating the protein bonds in the leather, nothing more.
One of the main concerns with shoe leather is keeping it conditioned with oils, and not damaging the leather with oil that has become rancid within the shoe over time. To address this we have to look at what causes rancidity:
Only three types of rancidity exist:
Hydrolytic rancidity occurs when water splits fatty acid chains away from the glycerol backbone in triglycerides (fats). The chemical term is ester hydrolysis. Usually this hydrolysis process goes unnoticed, since most fatty acids are odorless. When, however, the triglyceride is derived from short chain fatty acids, the released carboxylic acid can confer strong odors. This happens when leather gets too wet repeatedly, or is wet for long periods of time.
Oxidative rancidity (how oils normally go rancid) is associated with the degradation by oxygen in the air. Via a free radical process, the carbon double bonds of an unsaturated fatty acid can undergo cleavage, releasing volatile aldehydes and ketones, and creating hydroperoxides. This process can be suppressed by the addition of antioxidants (orange oil is an antioxidant). Oxidation primarily occurs with unsaturated fats, but can and will happen in all oils as the double carbon bonds are broken down.
Iodine Value (IV) measures the reactive carbon double bonds present in oil (the weaker molecular bonds in oil – common in unsaturated fats). The lower the IV number the better the oxidative stability. Coconut oil has an IV value of 10, the lowest of any plant/animal oil. Lanolin has an IV of about 20, Neatsfoot oil (bovine tallow) has an IV between 50 to 70, Olive oil has an IV of 80, Castor oil has an IV of 85, and Mink oil has an IV between 80 to 95.
Microbial rancidity refers to a process in which microorganisms, such as bacteria, use their enzymes such as lipases to break down fat. Using oil with anti-bacterial properties is beneficial in this case.
Cleaning and conditioning your leather shoes on a regular basis will give you shoes you can enjoy for longer. Leather shoes should be cleaned (with leather cleaner or saddle soap) every six months to a year on average. Leather shoes should be lightly conditioned every three to six months, and conditioned after each time they are exposed to rain or external water in general.