Tanning is the process that turns a raw animal skin into leather – making it more flexible and less susceptible to bacterial. attack and thus rot. Without attempting a very academic treatment of the chemistry of tanning, this section simply tries to explain what is interesting from the perspective of shoemaking and shoes.
Let’s at least have a look at what makes up an animal skin and which bits might finish up covering your feet! Here it is on the right. Staring inside the animal there is meat – which you may or may not have eaten. Then there is a thick (several mm, depending on the animal) layer of coarse, tangled fibres – 90% protein – that make the strongest section of the skin, that stops the animal’s insides from falling out. This is called the corium.
Close to the outside surface, there is a thinner layer – about one third of the thickness of the corium – that is formed of smaller, more densely-spaced fibres – the grain layer.
Finally, there is the skin surface – the epidermis – which is around 50 times thinner than the corium. Its main function is to keep out unwanted substances and light (!) and to provide some strength, although most of that comes from the corium. There are, of course hairs on animal hides and things like sweat glands. There may also, of course, be insect bites and scars – all of which make the hide less desirable for making nice shoes.
Tanning has several stages:-
Preparation for Tanning
1) Soak the hide to soften it up, remove dirt and make the hide soggy – ready to accept tanning chemicals.
2) Scrape off both any residual meat stuff from the inside and the epidermis from the outside, leaving only the corium and grain layer. This is a pretty unpleasant business, as you can imagine.
3) Soften the skin still further with a “bating” agent. The bating agents used in the past have included dog poo and chicken poo, so tanning gained a reputation for being a smelly business – best placed well away from any houses! Happily, tanners now use carefully-concocted enzymes to alter the protein structure in the hide, so tanning isn’t quite such a scatological and anti-social activity.
4) Now the skin is tanned …
The tanning process replaces the water in the hide with tanning agent, which makes the leather more supple, more robust, less inclined to go squelchy when warm and most importantly, far less susceptible to rot. In a loose sense, tanning “rot-proofs” the fibres that make the leather and links them together in a more pliable way that makes up for the loss of the regular supply of nutrients that the skin enjoyed when it was on the animal. In the past, Vegetable Tanning was used exclusively. This needed a ready supply of tannin, which came from the bark of trees like the oak, chestnut and spruce. This provides a nice excuse for a dash of etymology. The German name for the oak or spruce – “Tannenbaum” – gave rise to the use of the word “tanning”. It’s not because it gives the leather a tan colour – “tan” was first used to describe a colour in 1590, to describe the colour of vegetable-tanned leather! So – the colour tan is named after the tanning process, not the other way around. The abundance of trees in the Northampton area is also cited as one of several reasons for the growth of shoemaking there. There were probably lots of cows there as well. Isn’t this educational?
Nowadays, Chrome Tanning is used for uppers and vegetable tanning for soles. Chrome tanning is faster (weeks, not months). The leather produced is lighter, feels nicer to the touch, is more heat-resistant and easier to dye. Vegetable-tanned leather is stiffer, harder and thus far better for soles. It stands up to the stress of being walked on far better than would chrome-tanned leather.
The tanned leather is then either sprayed or dyed to the desired colour and dried – generally stretched on a frame. Finally is ironed! In some cases, an artificial grain is embossed on to the leather. Types of leather as used for shoes are described in another section.