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Welcome to Brain Stuff from howstuffworks.com, where smart happens.

Marshall Brain

Hi, I’m Marshall Brain With today’s question, what is leather and where does it come from? Most people in the United States touch a piece of leather every day. Think about it. We find leather in shoes, purses, belts, jackets, sofas, and car seats. If you ride a Harley or a horse, leather is essential. Leather is all around us and has been for thousands of years. Leather is strong, supple, flexible, and long-lasting.

But what is leather and how do people make it? It starts with an animal. Most of the leather that we use today comes from cattle. Leather is made from their skin. Since the beef industry in the United States kills millions of cows every year, there’s plenty of raw material to work with. The first step in making a piece of leather is to cut the skin off the carcass and clean it up. On the inside of the skin, cleaning means the removal of all flesh, fat, and membranes from the skin. On the outside, cleaning means removing all of the hair.

To get rid of the hair, the skin is soaked in a basic solution containing lime. That loosens the hair, which is then scraped off, either by hand or with machines. This clean skin is called rawhide. If you have a dog and you buy your dog a rawhide treat, this is what you’re buying. As you know, if you’ve ever seen a rawhide treat, dry rawhide is nothing like leather. Rawhide is hard, stiff, and coarse. If you soak rawhide in water, it regains its flexibility, but as soon as it dries out, it gets hard again. In order to turn this rawhide into leather, something has to change.

The change that occurs is a modification of the protein and collagen molecules in the skin. The process that effects the change is called tanning. The word tanning comes from one of the chemicals originally used to cause proteins to change, tannins. If you were to go back in time several centuries to Europe or the American colonies, people made and used a lot of leather. They needed it for shoes, saddles, clothing, armor, books, bags, straps, whips. To make this leather, the animal skins were soaked in a tea made from tree bark – for example, the bark of oak trees. The bark contains the tannins.

So they would shred the bark and boil it in water for an hour or so to leech the tannins out of the bark. And then they would soak the animal skins in this tea for many weeks. The tannins in the tea would bind to the proteins and collagen in the skin and change the skin from rawhide into leather. In a modern leather factory, they use chromium salts instead of tannins. And the chromium speeds the process up tremendously. Soaking rawhide in chromium salts takes only hours instead of weeks. But the same thing happens. The chromium salts bind to the proteins and collagen in the skin and create leather from rawhide.

Once tanned, the leather can be dyed, dried, polished, and cut up for use. It’s also graded. Full grain leather is the highest quality. You were looking at the hair side of the natural animal skin. Any cuts, scrapes, or imperfections in the skin show through. Top grain leather is a lower grade, but often looks more uniform. The leather is usually sanded to remove any imperfections and then embossed to recreate the surface like that of full grain leather. Next is split grain, made from leather split off from full grain leather in the process of making the full grain leather and even thickness. It’s the inner skin layers that are sanded and embossed.

Leather has a number of useful properties, but it’s not perfect. For example, leather doesn’t do particularly well when soaked in water. So in the age of plastics, there’s synthetic leather made of flexible vinyl glued to a layer of fabric. Vinyl is the nickname for polyvinylchloride, one of the most common plastics. PVC is strong and durable and it’s water resistant – and it’s cheap. About the half the weight of a piece of vinyl is chlorine, which is inexpensive because it’s easily created from salt. Salt is inexpensive because the ocean is loaded with salt. A gallon of seawater contains about a quarter pound of salt.

The next time you put on a pair of shoes or tighten your belt, think about the leather it contains. Whether it’s natural leather or synthetic leather, it’s an amazing material.

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