After leather has been split at the tannery into full grain, top grain, or other splits, it is tanned. Tanning is the ancient process of removing perishable natural oils from animal hides and preserving it with tannins or other substances for a long life.
In this post we'll explain the most common tanning processes: vegetable-tanning and mineral-tanning, plus briefly, other kinds of tanning.
From ancient earthen vats that are still used in places like modern-day Morocco to modern industrial machines, leather tanning methods have diversified considerably.
It can be hard to tell by looks alone if a leather is quality. Basically, almost any leather can be tanned and finished to look like a different kind of leather, while the fibrous structure and authenticity of the product speak most strongly about the quality of the material. There are a lot of decisions that go into the tanning process to turn the raw skin into a material fit for product manufacturing, whether it's piano parts or suitcases, and each decision is tailored toward the end result: which could be to make it cheap, fast, or good quality, but never all three at once.
Vegetable-tanning (oak tanning, oak leather)
Vegetable-tanning - the use of naturally-occuring tannins in plants, particularly tree barks - is one of the oldest and most consistent methods of tanning and is still in use today. While it used to be one of the most prevalent types of tanning, now it is considered a specialty.
Whether it's in the earthen pits still used in traditional tanning, such as in Northern Africa, or in modern machinery, the hide is immersed in a tannic solution of water, ground plant bark and leaves. Usually the solution contains oak, but it may also use hemlock, birch, chestnut, or other trees. Because of its natural materials, it is considered the most environmentally-friendly method of tanning. It is, unfortunately, also the rarest kind of tanning today. Less than 10% of leather is vegetable-tanned.
True vegetable-tanning can take months to process, with a minimum of one month. The resulting leather is carveable, moldable, and structural, and importantly, it is also natural and breathable with little finishing required.
Vegetable-tanning is still done in the US in multi-generation tanneries. Our featured tannery, Hermann Oaks of St. Louis, MO, has been in business since 1881 and still making leather the same way. You can even take a virtual tour of their factory (see our bonus video below, or even this Dirty Jobs episode).
Mineral-tanning (chrome-tanning, chrome/mercury)
Mineral-tanning, also called chrome-tanning, uses a modern industrial method of tanning invented during the late 1800s for modern machinery and mass-production. The tanning process utilizes mineral salts (chemicals and metals) like chromium and mercury instead of plant tannins, and re-introduces shelf-stable oils for flex and suppleness.
90% of leather is chrome-tanned these days, and that must be in part because the mineral-tanning process can take as little as one day. The resulting leather is soft and has a fabric-like drape, and can be dyed in any color because its natural hue is bleached out in the process (called wet blues). The shoe and fashion industries use this kind of leather the most. Because of its fast processing time, it is less expensive than vegetable-tanned leather. Mike Rowe covered a chrome leather tannery in Dirty Jobs(Season 3, Episode 3).
There are some chemical tanneries in the US, but these are grandfathered-in and generally speaking large operations would have trouble getting permission to operate here given current environmental laws. The vast majority of chemically-tanned leather is done in countries with less stringent environmental laws, particularly China and Mexico.
Other tanning methods
In the earliest days, tanning was done using oils, including the animal's dung and brains. Brain-tanning is a practice still in use in many parts of the world, notably by the Aboriginal people of Canada. Oil tanning is still a particular process used for making chamois. Vegetable-tanning is also as old as these processes.
There are "mixed" or combined methods of tanning that use both mineral- and vegetable-tanning processes, which are related to even more chemical-intensive and industrial processes called "synthetic tanning" that use specialized chemicals like naphthalenesulfonicor glutaraldehyde. "Syntan" leather is often used in car upholstery, and it is sensitive to water, and can shrink and harden in sunlight and heat.
Lesson #4 at the Walnut Workshop
The unique qualities of vegetable-tanned leather make it the perfect choice for leather wraps like our Sew-on Bar Wraps. When "veg-tan" leather gets wet, it becomes flexible, forgiving, and stretchy (easy to install). Once it dries, it hardens into place, molding into the shape of curvy bicycle handlebars.
Now that the hide is split and tanned, it's ready to be sold to craftsmen and manufacturers to be turned into goods. In our studio, we use exclusively full grain, vegetable-tanned cowhide, which we buy by the hide, and finish by hand in our workshop. We've covered a lot of ground, so in the next post we're going summarize the last few lessons, and after that we'll discuss leather finishing.
We're writing a leather learning series, and this is the fourth of eleven posts. In the series, we go through the different kinds of leather and the different ways it is tanned and finished, then talk about how leather as a raw material is turned into products, how to tell good leather products in the store, how those products should be maintained, and what ethical leather choices are.
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