The article below was originally published by the Weaver Leather Company. It was written by Shep Hermann, President of the Hermann Oak Leather Company.
Hermann Oak Leather Company - Tanning Leather: A History and Explanation
Leather. Arguably mankind’s oldest chemical product. It is made from the most basic of ingredients (animal hides and tree bark), could be discovered by accident, and makes one of the most useful products we know. We use it for clothing, shelter, farming, transportation, and many other things necessary throughout history. Every town used to have its butcher and tanner. Genghis Kahn travelled with his own portable tannery as he conquered the eastern world. The largest company on the New York Stock Exchange in 1900 was a leather conglomerate. Leather and its chemicals were stockpiled during WWI as a strategic commodity. Up until the advent of petroleum-based synthetics, leather was the most critical fabric aside from wool.
Turning hides into leather involves a chemical reaction. An animal hide can exist in one of two states: wet, where it will rot; or dry, where it is hard. Tanning is the process by which another substance is combined with the hide fiber, allowing the now-tanned item to be dried (so it will not rot) while staying flexible (and therefore useful).
While the basic concept of making leather is quite simple, in practice it is highly complex. We like to say we make 10 products, with 100 chemicals, in 1,000 individual steps, and 10,000 interactions. Change one thing, and it affects many others. A tanner’s job is to know the most important 200 interactions, and to control these. The process is one of multiple tradeoffs, which is why you will often hear tanners say, “Yes we can do that, but…”
Throughout history, physical goods were the most precious items. As such, traditional processes conserved material at the expense of labor. Nowadays, labor is the more expensive item (we live well), so we conserve labor at the expense of material. Tanning has seen these same changes over the past century. Hide prices, adjusted for inflation, are 25% of what they were in 1900.
There are historically two major types of tanning, plus many minor ones. “Vegetable” tanning uses tannic acid from organic plants like trees and nuts, similar to what makes up coffee or tea. It is the oldest tannage, and was the dominant way of making leather until 1900. While it used to take a year to make vegetable leather 150 years ago, it now takes 6 weeks for leather like ours, and can be done in as little as three days using modern synthetic veg tannage.
“Chrome” tanning was developed in France in the late 1800’s, brought to commercial use in the US, then re-exported to the rest of the world. It is now employed in making over 95% of the world’s leather, as it is far quicker (1 day) and less expensive than vegetable tanning.
Other types of tanning include “tawing” with alum (aluminum) that makes a very strong leather halfway between rawhide and tanned leather (true “Indian Tan” lace leather), tanning with oxidizing oils (“chamois”), and “brain” tanning (a misnomer, since the brain is used to prepare the hide, yet the actual tanning takes place by aldehydes in the smoking process).
Vegetable-tanned leather is firm when dry, yet will accept water, which makes it good for molding into items like saddles, holsters or shoe soles. This acceptance of water allows us to carve and stamp designs into the veg leather. It will also edge well, and is the most compatible with the skin, both important in horse tack.
Chrome-tanned leather is thinner and softer, and while the fibers will not accept water, the fibrous nature of the leather still allows it to breath. This makes it better for clothing, shoe uppers, and upholstery. The edges are difficult to smooth, and are usually rolled inside. Hybrid leathers like Latigo are first chrome tanned, then “retanned” with vegetable.
Shoes are still the dominant use for leather, taking 50% of all worldwide leather. Upholstery for autos and furniture is second. Half of all leather nowadays is tanned in China; they are also the world’s largest shoe maker and exporter, accounting for 85% of all US shoe imports.
So how is leather made? That will be the basis for the rest of this series.