How does tanning work? And no, we’re not talking about that orange stuff that comes out of a can. Well, at least, not today. We’re wondering how leather becomes soft, supple, and perfect for shoes. Here’s how your winter boots come to be. We can’t completely tiptoe around the fact that leather is made of animal skin.
Completely untreated, it’s not useful to us, because…it’ll just rot. But simply drying animal skin out gives you inflexible rawhide — like those weird stiff treats you give to dogs. You have to do a lot more than dry skin to turn it into leather. That process is called tanning.
There are several purposes to tanning. It prevents bacteria from munching on the skin, which would cause it to rot. It stabilizes the internal molecular structure, making it flexible and long-lasting, and it improves the skin’s resistance to temperature. But even though people have been making leather since before the dawn of written history, chemists are still trying to understand exactly what’s happening on a molecular scale.
Leather is mostly made of collagen, the main protein component of skin. Collagen is made of a long protein string that’s twisted in a helix, that’s wrapped up with other strands to form a bigger helix, that is bundled together into fibrils.
To make leather, you have to remove everything else from the skin while stabilizing the collagen. And that’s a long process. Animal skins may be preserved in salt before they’re tanned, which dries them out slightly and discourages bacterial growth. When they’re ready to use, the skins have to be soaked in water to wash out the salt and replace the moisture.
Then, you have to get out everything that isn’t collagen. And there are a bunch of steps to do that. All of this is to prep the skin and get it ready for the tanning chemicals. First, any excess hair and fat, and other gunk are removed. The skins are treated at an alkaline pH, which helps get rid of the rest of the hair. Then they’re brought back down to neutral, and treated with enzymes to remove pretty much all of the biological components that aren’t collagen from the skin.
THEN, the pH is brought DOWN into acidic territory and more chemical treatments are added. But, get this: All of this to PRE-tan the hide, basically prepping to turn it into leather, as opposed to…actually doing that. That’s right, we haven’t even gotten to the tanning part yet. Which is the part that does all those things we told you about
— stabilizing the collagen in the leather and making it last.
At this point, leather makers have a few choices about what kinds of chemicals to use to actually tan the leather. The most ancient method of leather tanning uses a class of compounds called polyphenols derived from plants, known as vegetable tanning. There are quite a few different potential chemicals involved in this, based on the type of plants they’re coming from, but they might, for example, look like this. The OHs interact with parts of the collagen. An alternative to polyphenols was developed around the Industrial Revolution. They began using chromium instead — specifically, chromium(III) — and here in the present day, it’s used to make about 90% of all leather. Kind of like the polyphenols, the chromium forms larger complexes that interact with the collagen. It’s faster than vegetable tanning, it smells better, and it’s more versatile. It produces softer, more flexible leather that’s better for garments. You probably don’t want to wear a leather jacket that was vegetable tanned. It’d be pretty stiff.
However, there is a real downside to chromium tanning, which is that certain chromium compounds are suspected to cause cancer — and tanning produces a lot of chromium waste. This is why chemists and leather workers are experimenting with greener, more efficient ways to use chromium, as well as other metals like titanium, which don’t always work as well but are probably safer. It’s not clear exactly what happens to the structure of collagen when it’s treated with plant polyphenols or chromium.
But what we do know is that the physical arrangement of the collagen fibers, and the way they’re linked together, changes to form something sturdy yet flexible. In other words, leather. There are some final steps, in which the leather is dyed and treated to give it the proper finish. One of those is fatliquoring, and yes that’s really what it’s called, which replaces some of the natural oils and fats that were removed during tanning with different oils and fats to basically lubricate the new leather.
Leather takes a long journey to get to you. And even though we’ve been using it for thousands of years, that ancient process still works — with a few more modern chemical twists. Thanks for reading. We’d like to thank Indian Leather Manufacturer Company and the Indian Leather Tannery for letting us use all the awesome leather footage in this article.