Fat Liquor

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.

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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

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

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

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.

Big shoes to fill

Ready to Wear (RTW) is a term used to describe shoes manufactured for the general public, using a (somewhat) standardized measurement method, and produced in quantity.   Over 90% of the shoes sold in the world are RTW.

Contrast this with the fact that every human foot in the world is shaped differently than the next, even the feet of an individual person differ slightly in shape from each other.

Typically the foot opposite the dominate hand will be slightly longer, and the other foot will be slightly thicker.

The general rule is to buy your shoes to fit the length of the longer foot, and the width of the thicker foot.   This almost guarantees you will never find a perfect fitting pair of shoes unless you go bespoke (custom made).

The differences in size and shape between a person’s feet vary per person, with some people having feet of almost exactly the same size, to people with a difference of half a size or more.

So how do you make your shoes fit your feet the best they can?

Well, part of this issue is addressed by the flexible nature of feet, along with the malleable nature of the leather upper to form to your foot.

Unfortunately, the shape imbedded into the sole (insole/outsole) of the shoe to fit the bottom of the foot is not so forgiving.  A shoe is designed to fit a foot from the heel of the foot to the ball of the foot (as I have mentioned in other articles).

When stepping forward your heel strikes the ground first, driving the heel of your foot into the heel cup of the shoe (above the shoe heel), and against the internal heel counter (at the back of the shoe).  The heel cup sits lower in the shoe than the arch to accommodate the shape of the heel bone, just as the heel counter of the shoe curves around the back of the foot to accommodate the shape of the heel bone.   This is very important to the fit of the shoe.

The part of the sole where the ball of the foot rests also has a curve to it and sits lower than the arch to accommodate the ball of the foot striking the ground.

When walking, the foot first gets driven into the heel cup, then as you step forward the foot slides forward (ever so slightly – if the shoe fits properly) to place the ball of the foot into the curvature designed for it.


If the shoe is too long the foot will have to slide farther to allow the ball of the foot to press down where it needs to.  This will also cause the heel of the foot to come too far out of the heel cup of the shoe and rub against the heel counter (causing blisters, and possibly other issues).

If a shoe is too small the ball of the foot will be pushed forward into the toe of the shoe (which is not only uncomfortable, but can cause blisters and other issues).  As a side note: this is where women get most of their foot problems from; the shoes are either too small for their feet, or poorly constructed in the toe for proper movement (sometimes by design).

The more common problem in men’s shoes is that they are slightly too big (on one foot, or both), which can be addressed (to some degree) with inserts and pads.

The most common insert is the removable rubber insole that you can find at most department stores.  This is also probably one of the least effective ways of making a shoe fit properly.


The reason that a removable rubber insole is a bad choice for improving shoe fit is because it pushes to foot slightly up and out of the heel cup, and presses the instep and toes into the tongue and/or vamp of the shoe.

While pressing the instep into the tongue or vamp may be a desired effect to make the shoe feel tighter, it caused the heel of the foot to rest higher against the heel counter which can again cause blisters and other issues, as well as impacting the proper motion of the foot.

If you feel you must get a removable rubber insole, try to get the thinnest one you can that has the desired effect.

Another pad that is used on shoes that slip too much in the heel (typically because the shoe is too long) is a heel pad that sticks to the inside of the back of the shoe over the internal heel counter and curves to match the horizontal curvature of the back of the shoe.  While this solution does not necessarily push the foot up, it does push the heel of the foot forward, and therefore pushes it out of the heel cup.  This pad also has the potential to cause damage to the Achilles tendon over time.


One of the better solutions for a shoe that is too big is to use a tongue pad.  A tongue pad is a triangle shaped cotton pad with adhesive on one side to stick to the underside of the tongue (or vamp) of the shoe to add more girth inside the shoe.


This helps the fit of the shoe because it pushes the foot down into the insole and the heel cup.  The downside is that your socks won’t slide quite as well into your shoe with the cotton pad there, and it takes a while to remove the adhesive residue if you remove the pad.

A tongue pad will not solve the problem of a shoe being too long, but it creates a better fit than the other solutions.

Please note that this article does not address the use of orthotics.