Introduction
Alum tanned leather occupies an interesting position among the various tanning methods, and the reason probably isn’t what you expect. In fact, alum tanned leather isn’t technically tanned leather at all. This is due to the substances used in the manufacturing process; tannic acids are notably absent from this type of leather.
Instead, the hide is submerged in a mixture of water and a compound called alum, as part of a hide treatment technique known as ‘tawing’. An important drawback to the tawing method is that alum tanned leather is not stable in water; if it gets too wet, it will actually revert back to rawhide.
Because of this, using the word ‘tanned’ is a bit of a misnomer, and it would be more appropriate to refer to it as tawed leather. Despite this flaw, alum tanned leather has been used very successfully in items which benefit from sensitive components or for which water exposure isn’t a concern, such as bookbindings, ornamental clothing, and even components of pipe organs.
What is Alum?
A bowl of alum salts ready for use in hide tanning.
If the name doesn’t give it away, alum is a chemical compound that usually contains aluminum. In the context of leather making, it most often refers to potassium aluminum sulfate, a naturally occurring salt which has historically been used in a wide variety of crafts. Besides its prominent role in tawing, alum has appeared as an important ingredient in skills ranging from pickling to papermaking.
Alum is no longer as prevalent as it once was, with a handful of specialized modern compounds replacing many of its uses. Even alum tanned leather is long past its height of popularity, having been largely phased out in favor of chrome tanned leather. That said, alum still plays a part in the creation of traditional textiles and artworks, and it frequently appears in household items like toothpaste, baking powder, and deodorant.
What is Tawing?
Tawing is one of the oldest known tanning methods, preceded only by vegetable tanning. In fact, there exists archaeological evidence of the presence of tawed leather in Egypt over 3000 years ago. Part of the reason for its early importance probably had to do with the simplicity of its production. The original tawing process hardly involved any steps at all, essentially being limited to just mixing alum into water and then submerging a hide in the solution.
By the early Middle Ages, tawing had spread to Europe, and the process was improved somewhat by the addition to the alum solution of a few common items like flour and egg yolk. These ingredients were found to produce a softer and thicker leather, which greatly expanded the possible functions of the leather. As familiarity with the method grew, tawing techniques became increasingly sophisticated.
Unsurprisingly, the modern hide tawing procedure is a lot more involved than its earlier forms. However, it remains on the simpler end of leather treatment techniques, and the central step hasn’t changed; the production of tawed leather still mainly revolves around the pelt’s immersion in the alum solution.
What are the Properties of Alum Tanned Leather?
Vulnerable to water
As mentioned earlier, water and alum tanned leather don’t get along. If the leather is exposed to a liquid for too long, the salts can actually wash out, and it decays into something close to the original rawhide.
Hardness
Leather produced through tawing naturally tends to be very inflexible. As a remedy to this, techniques like hand staking have been a common step in the preparation of tawed leather since medieval times.
Lack of color
Traditional leather dyeing pits in Morocco.
One of the most interesting traits of alum tanned leather comes from something that isn’t present. Normally, the tannic acids used in the hide treatment procedure are responsible for providing the characteristic brown color of leather. Because tawed leather isn’t as reliant on tannic acids as other leathers, this dyeing process doesn’t occur to the same extent. In other words, leather which was tanned with alum salts actually remains white in color.
Personally, we love the traditional appearance of brown leather with some slight modern twists, as seen in our new American Chestnut Collection. But if you prefer leather with a more novel color, then alum tanned leather could be worth exploring; its natural lack of pigment makes it a solid candidate for being dyed.
Is Alum Tanned Leather the Same as Latigo Leather?
There is a historical connection between alum tanned leather and latigo leather, but they are distinct leathers, made in different ways. Latigo leather is noteworthy for using combination tanning, in which multiple tanning processes are used on a single hide. Originally, the production of this unusual type of leather involved treating a hide with alum, which would then undergo the vegetable tanning process. Over time, latigo leather manufacturers have mostly replaced alum with chromium, but there’s still an association between these types of leather, and it’s easy to get them mixed up.
Alum Tanned Leather vs. Chrome Tanned Leather
Alum tanned leather is definitely among the less popular types of leather. The most obvious cause of this would be the spread of chrome tanning. These methods have a few things in common: they both use mineral salts for their primary tanning agent, and they both have a white-gray color, making them good choices for being dyed.
But there are some strong reasons why chrome tanning has mostly replaced alum tanning. First of all, leather tanned with chromium salts is stable in most liquids, which immediately fixes the main problem with using alum. More importantly, chrome tanning is one of the fastest tanning methods out there, taking just a day or two to produce. For mass manufacturers, this is an enormous benefit.
That said, chrome tanned leather has a major flaw of its own. Out of all of the tanning methods, chrome tanning might be the most environmentally damaging. Wastewater and chemical byproducts of chrome tanning are significant sources of pollution, which are often disposed of carelessly. Moreover, chromium salt oxidizes into a carcinogen, easily consumed by livestock in the area. For these reasons, it may be preferable to avoid chrome tanned leather.
Conclusion
It’s easy to get the impression that alum tanned leather is obsolete, but this type of leather still has its niche to fill. There are modern tanning methods that are more practical or accessible, but alum tanned leather has an interesting background and unusual properties that set it apart from more common types. Leather aficionados should keep an eye out for this important piece of leather history.
