Tanning: Turning Raw Hides into Leather
An animal’s skin is soft, flexible, sturdy and durable. Once the skin is separated from the animal, it is raw hide, with none of those qualities. It’s hard, stiff and decomposes quickly. Tanning (processing the hide with tannins) returns the natural qualities to the hide, and we have leather, the material that turns practical items into beautiful and treasured possessions. We all know leather. Most of us, probably all of us, have seen hides when they are still on the animal. But our guess is that few know about tannins and tanning, so we’re going to give you a quick overview.
Tanning with vegetable matter was the first way that early humans were able to make animal hides suitable for use in protecting their bodies. Today, vegetable-tanned leather is created by skilled craftsmen who use the basic processes that have been around for thousands of years.
The vegetable matter (tannins) used today includes the bark, wood, leaves, fruits and roots of trees. Trees include the chestnut, quebracho, oak, hemlock, mangrove, wattle, sumac and mimosa.
Hides are stretched on frames and immersed in vats with an increasingly concentrated solution of tannins. It usually takes three to four weeks for the tannins to penetrate to the center of the hide. Another method is putting the hides into rotating drums, also with an increasingly concentrated solution, that results in a more flexible leather. This process takes about 36-48 hours.
Vegetable tannins make the leather supple, and the color and texture are more natural looking than some other tanning methods. The natural appearance has a slightly brownish tinge over a beige color, and there is a natural earthy smell.
Brain tanning also goes back to ancient times. It was used by people all over the world tens of thousands of years ago, including early Native Americans.
The fatty tissues (emulsified oils) of the brains of animals (e.g., deer, cattle, buffalo) are worked into the hide, which is then stretched in different directions to soften it. The softening is made permanent by smoking the hide over a low heat. The smoke also gives the leather its color: tan, gray or brown.
Brain tanning is a labor-intensive method because it relies so heavily on the physical manipulation of the hide. But the finished product is strong, soft, durable, warm and washable, which makes it in demand for outdoor clothing.
Fish Oil Tanning
Fish oil (usually cod oil) is another method handed down from our early ancestors. It makes soft, porous and highly absorbent leather that can be wet and dried repeatedly. Fish oil is sprinkled or sprayed onto the hide and pounded in with hammers (now mechanical hammers). The hides are then hung in ovens to “bake” the oil into the fibers.
Most well known among fish oil-tanned products is the chamois, a gentle, non-abrasive material that is used for drying cars, cleaning and polishing household items, and grips on athletic gear. Chamois is also used in fashion leathers because it can be dry cleaned or washed.
Rose-tanned leather is a variation on vegetable tanning and brain tanning. Pure rose oil replaces the vegetables oils and emulsified oils of the brains. It is the most valuable leather in the world, because of the rarity and high cost of rose otto essential oil and the labor-intensive tanning process.
The rich, organic rose oil, precious among essential oils, is extracted from the petals of one of the rarest flowers in the world, the Rose Demascena, that is found only in Kazanlak, Bulgaria.
Chrome tanning was invented in 1858 and was the most momentous change ever in the history of leather. Tanners wanted to expedite the tanning process and save money. Chrome tanning does both. Today, 80% to 90% of the world’s leather supply is chrome-tanned.
Chromium salts stabilize the hide’s fibers, making it resistant to bacteria and undue heat. Hides are also softer and more supple than vegetable-tanned leathers. Like in vegetable tanning, hides are either placed in a vat with an increasingly strong solutions of chromium sulfate or soaked in revolving drums.
A distinguishing trait of chrome-tanned leather is the blue-gray color it picks up from the chromium, and because of that, the leather can be dyed any color of the spectrum.
Horween Tannery continues to use its proprietary Chromexcel® invented by its founder, Isadore Horween, in the early 20th century.
First, synthetic tanning is not to be confused with synthetic leather, such as faux leather, leatherette or vinyl.
Synthetic tanning agents (syntans) were invented during World War II when vegetable tannins were rationed. Syntans act upon the proteins in the hide much like vegetable tannins do. They include aromatic polymers (such as Novolac and Neradol), melamine resin and other amino-functional resins.
Syntans are rarely, if ever, used alone to tan hides. Rather, they are replacement tannins used in tandem with natural tanning agents or auxiliary tannins used in the pretanning, retanning and dying processes.
For more of the “inside story” of leather, please click on Know Your Leather.