There seems to be some mystery around how to care for cordovan shell shoes, and how they differ from leather shoes.
My original research led me to believe that cordovan shell was produced from a part of the subcutaneous muscle layer in horses called the panniculus carnosus. This is a thin fibrous twitch muscle lining the abdomen and hind quarter of the horse.
In reality cordovan shell is made from a collagen protein based tissue structure that is a cross between flesh and cartilage called the hyaline layer that exists (specifically on the rump of a horse) between the epidermis and the corium. This layer does not exist in cow skin.
Because this layer is a dense smooth layer of tissue it is also referred to as the “glassy” layer. In a book titled “The Chemistry of Leather Manufacture,” published in 1923 by the Chemical Catalogue Company the following paragraph appears:
“The dense mass of fibers, often called the glassy layer, can be seen running horizontally across the middle of the picture and appearing much darker than the remaining fibers. The portion of the hide containing the glassy layer is known as the shell and is used to make leather sold under the name of cordovan.”
Once the horse leather is cut down to the ovals that contain the hyaline tissue layer the epidermis is shaved off of the hyaline layer leaving no grain side. This is one of the reasons that it is hot stuffed rather than fat liquored. Hot stuffing also allows the tannery to add a lot more fats and waxes than fat liquoring.
Beside the additional fats and waxes, another aspect plays an even bigger role: The difference between the structure of skin tissue and the hyaline layer tissue.
Both skin and the hyaline layer are made up of protein filament bundles (fibrils), but the shape and size of the filaments differ quite a bit. A collagen protein filament in skin/leather is about 80nm in diameter, while the filaments in the hyaline layer have a bulbous head that is about 12nm in diameter and a filament that tapers to much smaller. These tapered filaments fold over onto themselves causing the denser tissue.
The filament size in the hyaline layer is similar in relative size to an actin protein filament found in muscle, which is about 8nm in diameter. This perpetuates some of the confusion around cordovan shell being a muscle rather than a skin tissue.
Because the fibrils in the hyaline layer are so much more compressed (ten filaments for every one of cow hide, and folded over as well) it tends to retain oil and wax much better.
This compressed fibrous tissue has no real grain side, and so it acts like flesh out leather, similar to waxed leather, but with a much more compressed surface. This is also why a smooth deer bone can rub out a scuff.
Since cordovan shell retains waxes and oils so well it does not need to be conditioned very often (some would say the cordovan shell never needs conditioning). However, all oils oxidize over time and should be replenished as needed.
I have found the best way to do this is to apply a little cream polish every now and then (about every 15 to 20 times the shoes are worn). The rest of the time I just brush them very well, and smooth out any scuffs with the round side of a tablespoon.
Brushing cordovan shell with a horsehair brush (ironic I know) vigorously is about the best thing you can do for it. The heat from the friction helps soften the waxes and oils in the cordovan shell and basically allows you to polish your shoes with the ingredients already in your shoes.
The video below will go through a number of steps that horse hide goes through to produce cordovan shell at the Horween tannery: