Leather Conditioning

Section through finished leather

Leather Hides

Raw hides have four main parts - an epidermis, grain, corium and flesh

Two of these layers, the epidermis (which is a thin protective layer of cells during the life of an animal) and the flesh are removed during tanning by a process called liming.

This leaves just the grain and the corium, the parts that are used for automotive leather upholstery .The grain layer is made of collagen and elastin protein fibres and its structure varies quite a bit depending on the age, breed and lifestyle of the animal. The grain carries many distinctive marks such as insect bites, growth marks and wound scars giving the leather a unique appearance.

A simple test – place a drop of moisture on the surface of the leather - if it soaks into the leather you have unfinished leather (sometimes called aniline style) If there is a very light colour or clear coat on top of aniline-dyed leather, it is often referred to as “semi-aniline.” Semi-aniline leather offers modest protection while retaining much of the aesthetic beauty of unfinished aniline-dyed leather but still remain absorbent to moisture. If the moisture sits on the surface and does not soak in and darken the material you have finished leather.

The corium [Latin term for the dermis] or skin layer, is packed with collagen protein fibres, arranged in larger bundles and interwoven to give the structure great strength, excellent elasticity and durability. In the tanning process these fibres and impregnated with collagen's that are designed to hold them together and keep them supple after which they are sealed, these polymers are neither volatile nor migratory, rendering the use oil-based conditioner a mute point

The other critical factor is moisture (re-hydration) any leather is going to lose its moisture in hot ambient environments.  Much of the suppleness of leather comes from its moisture content, which is the reason maintain the moisture content of the corium is so important. When leather tanners talk about leather conditioning they specifically mean replacing its moisture content (re-hydration).  After tanning the skin is protected with a thin pigmented (colour) urethane and then a clear topcoat.

Urethane has micro-pores that allow evaporation and hydration (the passage of water vapour through a membrane or pore) they are not sealed per se. Oils are not compatible with water-based pigmented urethane coatings and their molecules are too large to permeate, although some may enter via stitching, usually it remains on the surface to be removed by clothing

Since the leather hide has a pigmented urethane layer and clear topcoat finish, when treating the leather, so you are in fact dealing with a urethane (to all intents and purposes, a ~150 µ (micron) thick plastic surface coating, about the same thickness as automobile clear coat paint; that simply needs to be kept clean and hydrated…it really is that simple.

Before 1980 most automotive leather upholstery used was high quality top grain full grain leather with a pigmented solvent-based Nitrocellulose lacquer coating and their unstable plasticizers would easily migrate in the heat from sun exposure and again create 'fogging' on the internal surfaces of the windows.

As a result of this plasticizer migration the use of oil-based products that contained solvents to keep them from becoming brittle and cracking, creams, waxes and oils were needed in after-care products to try and bring the suppleness back into the leather by applying an oil-based conditioner; a term that became known as ‘feeding ‘

Automotive leather has a pigmented urethane clear coat to add colour and a clear top coat to provide abrasion resistance and protection from the harsh environment of the vehicles interior 

Two stage paint; base (colour) coat and clear coat were adopted (around 1982) as an automotive industry standard to replace these older lacquer paints. There is a minor difference between vehicle and leather clear coats, clear coat paint or resin has no pigments and hence imparts no colour to the vehicle. It’s simply a layer of clear resin that is applied over coloured resin. 

Whereas finished leather’s clear coat has pigmentation added to provide colour. Almost 95 percent of all vehicles manufactured today have a clear coat finish, the new clear coat urethane paint did not require oils but the old method of feeding paint took many years to fade out.

This same resistance to change also applies to finished leather even though urethane’s only requirement is to be kept clean and hydrated, it doesn't require conditioning

It’s possible that there is an association with old world quality (i.e. European automobiles with unfinished leather upholstery and real burl wood interiors) with these types of products, despite the fact that the automotive industry has been using aqueous (water- based) urethane covered pigmented leather since 1980.

Something else that always surprises me is that many detailers aren't old enough to remember Nitrocellulose lacquer coating, yet they are adamant that conditioning is a prerequisite for finished leather

I hope you've found this article both informative and helpful.

TOGWT® Autopia Detailing Wiki Articles

1.  “Unfinished leather” - http://www.autopia.org/forum/autopia-detailing-wiki/145381-unfinished-eather.html#post1543547
2. "Proper  Finished Leather Cleaning and Care” - http://www.autopia.org/forum/guide-detailing/136421-proper-finished-leather-cleaning-care.html
3.  “Leather Upholstery Type Surface Identification” - http://www.autopia.org/forum/autopia-detailing-wiki/136895-leather-upholstery;-surface-identification.html#
4. “Leather Articles Hyperlinks” -http://www.autopia.org/forum/autopia-detailing-wiki/141973-leather-articles-hyperlinks.html