A Treatise on Tanning

Most people are probably aware that leather comes from animals, usually cattle.  I would imagine that most people also know that the hides then undergo a process called tanning that turns them into leather.  So I might be alone in wanting to know how animal hides are given the properties that make them this awesomely useful material we call leather.  I'd wager though, that I'm not alone so just in case that's the case, here's this week's SCIENCE! post; all about how leather becomes leather.

It seems to be a bit tricky to figure out who the very first tanners would have been but we know it was pretty early on.  The Egyptians wrote out their process for tanning roughly 5,000 years ago and there's data to suggest that tanning was commonplace in southern Asia 2,000 years before that.  I couldn't find anything to say that this is when the technology was developed (as opposed to just being our earliest records in those areas) but it stands to reason that we may never know who the original tanners were.  I would hypothesize that tanning developed concurrently with settlements and agriculture as these three aspects of civilization are pretty intertwined.

Early tanning varied everywhere you went but the one universal trait was that it was messy.  Different processes used dung, urine, and/or decaying flesh (I assume for the ammonia that would be present in each) so tanning cities were usually set apart from where most people lived (or at least the people who could afford clean air).  The basic process was pretty similar to vegetable tanning (aka veg tan) today in the basic steps used, but the materials and the details differed dramatically over times and locations.  In one area the tanners may have simply cured the hides and then worked animal fats and oils back into the leather to soften them.  In another they may have soaked the hides in various solutions to produce a result similar to veg tan leather.

Regardless of the details, some process of tanning is necessary to keep the hide from rotting away.  Conventionally tanning is broken down into a handful of stages:

• Skinning - Removing the hide from the carcass of the animal.
• Curing - There's a few different ways to go about curing though most of them involve the judicious application of salt. Curing removes the majority of water from the hide and prevents it from rotting. 
• Beaming - Beaming can involve a number of other processes but it turns the cured hide into rawhide.  The hides are cleaned of any leftover salt, treated with lime to loosen the fibers, and cleaned of any left over hair or bits.  Afterwards the hide might undergo either bating or pickling, which are two chemical processes for softening the hide and preparing it for different tanning processes (veg tan or chrome tanning respectively).  In the old days these operations were performed by laying the hide over an angled beam (hence the name) though these days they tend to be done in drums.

 

 Once all this is finished we have a material we usually call rawhide.  You're probably most familiar with rawhide for its use in doggie bones and drum skins, but it's also the starting stage for most leathers.  From here there are a handful of techniques that can be used that will determine what kind of leather is made from the rawhide.  Vegetable tanning is the process you're most likely to have heard of though chrome tanning is the most commonly used process in the modern day.

Vegetable Tanning

 Some thousands of years ago people started figuring out that if they soaked rawhide in a solution of water and oak bark, they created a material that was flexible, supple, and after the process was refined in the 14th century it could even be tooled.  I've tried finding good explanations of what is chemically happening during veg tanning but what I found was that A) it's really chemically complicated and B) though we've been doing it as a species for longer than we've been writing we still don't understand exactly how it works.  The general notion seems to be that the tannins stabilize the collagen (the chemical "stuff" that makes up the fibers that make up the leather) so that they retain a state similar to that of living skin.

Veg tanning takes a good amount of time, sometimes up to a year, but is relatively harmless to the environment provided the oak and hemlock (the most common sources of tannins for tanning) are sustainably collected.  The unique thing about vegetable tanned leathers is that they can be tooled and molded, making it a highly versatile material.

Chrome tanning
Chrome tanning was first invented in 1858 and now 90% of the world's leather is made using chrome tanning.  It produces leather much, much faster than vegetable tanning allowing leather production to take on a truly industrial scale.  While a typical hide might take 8-16 months to be vegetable tanned, it could be chrome tanned in under a week. 

Chrome tanning relies on a solution of chromium sulfate and produces a leather that tends to be softer, more supple, and more heat resistant than vegetable tanned leather.  Aside from environmental concerns from the collection of the raw chromium, chrome tan leather can give off toxic fumes when burned and if left exposed to the elements it can create a toxic runoff as it decomposes.  This runoff enters the water table and is source for concern in India.

Other methods
There's almost a dozen other methods of tanning leather but they are seldom used today.  Perhaps the most well known is Brain Tanning, well known for being used by Native Americans to create very supple buckskin using oils derived from animal brain tissue.  These days I imagine most tanneries use a different source for the necessary emulsified oils but the buckskin is still flexible and washable making it perfect for garments.

Alum tanning was relatively common place in the ancient world though it doesn't seem to have been a true tanning process.  Alum tanned leather would revert to rawhide if left in water (because the aluminum salts would dissolve into the water) so it doesn't seem like it was chemically bonding with the leather.

Aldehyde tanning is used to produce some specialty leathers for specific purposes like chamois leather.

Synthetic tanned leather was initially developed due to a tannin shortage in WWII and involves treating the rawhide with various polymers and/or resins.  This produces a leather less prone to stretching but really durable, often used for high-end upholstery (at least before the proliferation of artificial leathers).


Modern day tanneries don't have too much in common with the tanning huts and beamhouses of old.  The basic process may be the same but hides are usually treated en masse in large drums using advanced chemicals rather than being scrubbed down and soaked in less savory animal byproducts.  The livestock market provides a constant source of hides to be tanned and other innovations like artificial leathers and split leather (more commonly called suede) have made leather cheap enough to be a practical material for anyone across the globe.