Lesson #4) Leather Tanning: Veg-Tan vs Chrome-Mercury

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. 

Whether it's done in the ancient earthen vats still in use in Morocco or a steel factory in China, there are two main tanning processes: vegetable and mineral (chemical), as well as some other less commonly-used:

• Vegetable tanning is a time-honored and ancient method using plant tannins such as oak bark and can take as long as a month. The result is more durable.
• Mineral tanning was invented during the Industrial Revolution and uses modern chemicals such as chrome and mercury to speed up the process considerably. The result is more flexible. 
• Other kinds of tanning: animal, oil, and mixed, or synthetic tanning

We'll explain each in more detail: 

(Oak Tanning, Oak Leather, Bark-Tanned)

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 historically it used to be one of the most prevalent types of tanning, now it is considered a specialty. Only a small minority of hides worldwide are vegetable-tanned today. Less than 10% of leather is vegetable-tanned.

Vegetable-tanning can be done in ancient or modern vessels. In Northern Africa, it's still done in traditional earthen pits, while in modern China or the US it's done with modern machinery

Wherever it's done, the process is the same: 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. 

Authentic 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.) If you can access it, enjoy the full experience with Mike Rowe in Season 1, Episode 12 of Dirty Jobs.

(Chrome Tanned, Chrome/Mercury, Chrome Excel)

Mineral-tanning, also called chrome-tanning, uses a modern industrial method of tanning invented during the late 1800s for fast mass production. The tanning process utilizes mineral salts (chemicals and metals) like chromium and mercury instead of plant tannins, then re-introduces shelf-stable oils for flex and suppleness.

90% of leather is chrome-tanned these days. That's because the process is cheaper and faster. 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. 

Mike Rowe also experienced 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 new operations would have trouble getting permission to operate here given the US's more stringent environmental laws. 

Chrome tanneries produce chemical-laden wastewater that pollute the local environment if not properly disposed, treated and cleaned. The vast majority of chemically-tanned leather is done in countries with less stringent environmental laws, particularly China, Bangladesh, and Mexico. In these countries, wastewater treatment may not be required. 

Animal, Oil, Mixed, Synthetic

In the earliest days, tanning was done using other parts of the animal, including the dung and brains. Brain-tanning, or buckskin, 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 in place used for making chamois. (Vegetable-tanning is also as old as these processes.)

There are new modern "mixed" or combined methods of tanning that use both mineral- and vegetable-tanning processes. This way the leather can be labeled and sold as "veg tan" while still taking advantage of cheaper and faster tanning methods. 

Synthetic tanning uses even the most chemical-intensive and industrial processes so that it can be labeled "chrome-free." These may include specialized chemicals like naphthalenesulfonic or glutaraldehyde. Also called Aldehyde-tanning, it formerly used formaldehyde until skin sensitivities developed. "Syntan" leather is often used in car upholstery. It is sensitive to water, and can shrink and harden in sunlight and heat.

Now that the hide is split and tanned, it's ready to be sold to craftsmen and manufacturers to be turned into goods. 

In Lesson #5, we recap everything we've discussed so far to answer the question, "What is the 'Best' Leather?"  Then in Lesson #6, we'll move on to leather finishing.