Material Matters: Leather Part 1
Date: October 20 2016
By: Adam Jane
The basic ingredient for leather is animal skin – anything from fish to birds, big animals like elephants to tiny reptiles and lizards. Every kind of skin has three layers; the outside layer called the epidermis, a thick middle layer called the corium and a connective layer of tissue called the flesh. The corium is what we use to make leather. It has a solid structure with short, tightly packed fibres on the outer side and longer, looser fibres on the inside.
As far back the Stone Age people have used animal hides to make crude footwear but many of the early techniques for preserving skins weren’t all that successful. Skins could be dried in the sun, but would become brittle and would rot in the wet. Some societies would pound the skins with animal faeces, others would chew on them and then rub in animal fat. Much more effective ways of processing skins began a few thousand years ago with the use of ash, quicklime and smoking, most likely discovered completely by accident.
The technique we use to convert skin into leather is known as tanning, but before we get to the tanning there are a few preparatory steps. First the skin soaks in a tub of water and then in lime, this removes dirt and oil and restores the fluid content. Next, the top layer and the bottom layer are removed so all that’s left is the fibrous corium. Traditionally this would be done by scraping the flesh off with a big curved blade called a fleshing iron but these days it can be fed through a machine, which uses a rotating blade to separate the layers, and can be soaked in calcium and sulphur to remove the hair. The skin is then treated with enzymes in a process called bating, to loosen up the fibres so that they’ll soak up the tanning agents better. In the olden days this was done by rubbing animal dung, usually dog or chicken, into the skin – it’s no wonder tanneries were relegated to the outskirts of towns and generally known to be undesirable places. Once the skin has been checked over to ensure there aren’t any defects, like mosquito bites or scars, it’s just about ready for tanning.
Tanning can be done in a few different ways, but each one does much the same thing on a cellular level. The proteins that make up the skin are mostly collagen, which will bind to the tanning agents to change their chemical structure, stabilising them and leaving them unable to putrefy. Once leather has been tanned it doesn’t become fragile and stiff, if it gets wet it will dry off, rather than rot and it won’t become glutinous if exposed to heat. These days the two main methods used for tanning shoe leather are vegetable tanning and chrome tanning.
Vegetable tanned leather uses plant tissues, such as leaves the bark of trees, to convert proteins – originally this process is attributed to the ancient Greeks. The bark contains something called a tannin, which binds to the proteins and coats them. Vegetable tanned leather has to be immersed in liquid for as long as six months, which is the main reason it tends to be more expensive. Vegetable tanned leather will often be used in its natural shade, which is a very pale beige colour but can even look a bit white or pink. These days the main reason for using veg tanned leather on an upper is aesthetic, as it tends to darken and soften with use. One of the main brands you’d associate with veg tanned leather in the market today would be Hender Scheme. Most commonly you’ll find veg tanned leather used as a lining material, to make inner soles and even to make thick leather outsoles.
Chrome tanning comes from a traditional process, which used a kind of salt called alum, to change the chemical structure of the proteins. The oldest kind of chrome tanning we know comes from ancient Egyptian wall paintings, which show early depictions of tanning as an industry. In 1858 the German chemist Friedrich Knapp came up with an alternative of using chrome salts in much the same way and it became the go-to production method – fuelled by the increased demand for leather with the rise of industrialisation. The majority of sneakers available today use chrome tanned leather, as it tends to be softer, more flexible and easier to dye. From start to finish it only takes around six weeks to tan this way, which makes it a much more conducive to consumer production. Once the chrome tanned leather is removed from its liquid bath it has a characteristic blue colour, so it usually goes through a few other processes afterward.
These days the majority of leather comes as a by-product of the meat industry, roughly half of which is used to make shoes. The birth of the sneaker came from the combination of cotton canvas and rubber, but it wasn’t long before leather made it’s way back to athletic shoes. In the early 20th century track athletes were wearing leather spikes, most famously Jesse Ownens’ 1936 spike crafted by Adi Dassler, while early basketball shoe brands had began to offer leather as an alternative material. There have been all kinds of synthetic fibres and materials used over the past 50 years but it always goes back to leather. These days its still used on top-shelf athletic shoes, plenty of modern Jordan models for example, as well as a premium option for casual styles – something you’ll often see on high-end lines like New Balance’s Made in USA.
There’s no doubt that leather is the most successful material used to make sneakers and we don’t see it going anywhere any time soon. This is only part one of the story of leather, next week we’ll look at the different kinds of leather and suede, different grades of leather and finishing techniques.