In more traditional times, the fine art of leather tanning was one that was passed from father to son, and mother to daughter. The best leather workers on the planet, prior to the industrial revolution, were indigenous native tribes who’d already been honing their craft for centuries.
If you don’t yet understand how the tanning process works from start to finish, read How is Leather Made?
If you’re curious about the various chemicals and natural methods used to tan leather after it’s been properly stripped down and prepared, here are the most common, and some lesser used methods to tan leather.
Chrome-tanned leather
The chrome tanning process is the most popular chemical tanning process in use today. Chromium sulfate offers significant advantages over other methods – most notably faster turnaround time in the tanning process, making the finished product look as natural as possible, increased flexibility, and improving the overall strength and durability of the end product. Chrome tanned leather is also superior when it comes to moisture resistance, and the ability to withstand the rays of the sun.
One drawback is that the chrome tanning process isn’t suitable for leather products that will come in contact with metal, such as gun and knife holsters, tool belts, equine products like saddles and bridles, and other applications that involve metals. The production process is also very toxic and all water used in the process must be meticulously treated to prevent serious health and environmental issues.
Vegetable-tanned leather
Vegetable tanned leather was a mainstay in the tanning industry throughout the 1900s, but is used less and less because of the advantages offered by chrome tanning, including turnaround time and flexibility. Strangely, among industrial leather making processes, vegetable tanning is the least toxic and has much less environmental impact.
Despite its name, vegetable tanning doesn’t really involve the use of vegetables, but rather tannins found in tree bark of various kinds. The tannins bind to the collagen in hides and coat them to prevent water from penetrating and bacteria from breaking them down. Rose tanned leather also falls into this category – pure rose oil is used to tan the hide, leaving a permanent rose fragrance in the leather.
The end result is a much less flexible leather, but also a leather that can be used in many of the applications that chromium tanning isn’t suitable for such as gun holsters and tool belts that come in contact with metal products. The hardness of vegetable tanned leather also makes it perfect for people who like to carve and etch designs into their leather.
Brain tanned leathers
Brain tanned leather is far from the most common type of tanning used, but for those who want a traditionally-tanned leather piece that wasn’t manufactured using modern chemicals or machines, brain tanning is the most desirable option. It’s also expensive due to the talent and time needed to produce the extremely durable and flexible leather this time-honored Native American method offers. You can make any leather product from brain-tanned skins.
The hide is first skinned and stripped of all fat and hair (optional). After several rounds of water soaking, drying, and fat scraping sessions; the hide is wrung and stretched out to be completely dried for a few days. After, a mixture of water and animal brains are applied to the hide and then set out to dry once again (learn more about this traditional tanning method here).
Rawhide
Rawhide is the most simple leather to make. It has limitless uses, but isn’t ideal for clothing items such as coats, bags, or footwear. That’s not to say it’s never used for these purposes, but rawhide is very tough and not very flexible. It’s most used for ornamental pieces, cordage, holsters, and other items that benefit from strong, inflexible leather.
Anyone can make rawhide. Unlike the other methods on this list, rawhide leather isn’t actually tanned at all. The hide is soaked in lime, scraped down until it’s free of all animal fats and hair (though the hair can be left on), and left out in the sun until it’s completely dry (learn more). Commercially produced rawhide is usually made with the aid of chemicals that are used in the pre-tanning process by other methods mentioned.
Less Common Leather Tanning Processes:
Aldehyde-tanned leather
Aldehyde tanning is slowly being phased out due to the dangers it presents to workers and the environment. Aldehydes, most commonly formaldehyde, and other glutaraldehyde and oxazolidine containing compounds are used to penetrate the leather and make it water resistant and flexible.
Chamois leather tanning also falls into this category, though that process uses cod and other fish oils to tan the hide. Aldehyde leather finishes are often labeled “chrome free” and are mostly used to make infant shoes and leather seats and accents in the automotive industry.
Synthetic-tanned leather
The synthetic leather tanning process was developed during WWII when vegetable tannins were in short supply. Laboratory-made synthetic tannins or “syntans” such as melamine are used in place of the natural variety. While vegetable tanning and synthetic tanning use the same principals, synthetic tanned leather has a white finish to it, making it perfect for use in leather products where colored dyes aren’t to be used.
Alum-tanned leather
The alum tanning process is another that’s lumped in with tanning methods, even though the leather isn’t technically tanned at all. Aluminum salts are combined with a binder and protein source (most commonly flour and egg yolks) and applied to the hide to prevent breakdown. Alum tanning isn’t a common method because the finished hide can quickly revert to rawhide form if the leather gets wet – ie., the binding solution wears off when wet.
Conclusion
The information listed above is important to know when it comes time to make a leather purchase. As you’ve learned, some processes, like chrome, vegetable, and brain-tanned leather each have their own distinct advantages. While others like rawhide, aldehyde, synthetic, and alum-tanned leathers are often born out of convenience and/or due to the cost savings they offer to the manufacturer.
