This is the second installment in a three part series. The article was written by Shep Hermann of the Hermann Oak Leather Company and initially published by Weaver Leather.
How is leather made? At its core, it is a very simple process: impregnate an animal (or fish or reptile) hide with something that will attach itself to the hide fiber. We can imagine the first discovery by prehistoric man of dropping a hide into a dark pool of water full of tree branches or seed pods, and finding it a year later in a “tanned” and useful condition. Then over thousands of years, tanning developed into a very complex process.
You will find any old tannery located next to running water. Water is the most important “chemical” in the process, as well as what carries away the waste. Our tannery uses 150,000 gallons of water daily, and we are fortunate to be located next to the largest river in North America.
The “wet end” of the tanning process is typically divided into four major segments: Preparation, commonly termed the “Beamhouse” after the old wooden logs used as props for removing hair and flesh with sharp knives; “Tanyard” for vegetable or “Tan drums” for chrome where the actual tanning takes place; “Bleach/Color Fatliquor drums” where the last wet properties are imparted; and the final physical work of dewatering, laying flat, and drying the leather. The subsequent “dry end” of the operation is where the leather is “curried” or finished for various end uses.
Hides typically come to a tannery in a salted state, similar to salted meat. In this manner, they are preserved for up to a year (four years if refrigerated). Alternative preservation techniques are dried hides (common in Africa), and “green” or “fresh” but refrigerated hides (for large-volume tanners close to packing plants). Here in the states, cattle hides have typically been “fleshed” in large machines to clean off the heavy layer of unusable flesh and fat, which makes for faster penetration of chemicals. In Europe, hides are more apt to be “conventional” or unfleshed.
The salting process, for temporary preservation of the hides, is done in one of two ways. The old way was in “salt packs,” where hides were layered on a floor with common salt thrown in between each hide. This process would take a month. The modern method is performed in paddle-driven “raceways” filled with super-saturated brine (salt) solution to give a “brine cured” hide. This takes less than a day.
Tanning is chemistry, and pH is an essential way of measuring it. A hide will arrive at the tannery in a neutral pH state. The pH is gradually raised in the initial “alkali” process, then brought back to neutral, and finally lowered in the “acidic” tanning process.
The first major step is the “Beamhouse.” Like any well-constructed building, this is the cornerstone of the tanning process. The hide is first soaked to remove salt and rewet the hide fibers. The pH is gradually increased from pH 7 to over pH 12 with a combination of lime and sulfides. This causes the hair to break down (similar to the depilatory “Nair”) and the fibers to open up, leading to faster and more thorough tanning. In the old days, this was done in a “lime yard” which softened just the hair root, took a week, and often left fine hairs in the hide. Nowadays lime unhairing is done in “paddle vats” taking two days, or in “drums” taking one day.
Specialty tanners like us will pull the “limed hides” at this point, “scud” the grain,
“flesh” to remove more flesh, and “lime split” those leathers that need to be level in thickness. While this adds cost, it makes for better leather.
The hides then proceed to the “Deliming and Bating” phase, which brings the pH back to neutral. “Bating” is an old tanning term for cleaning out the grain for better visual effect through the use of enzymes. Tanners have used enzymes for centuries through the use of animal dung (one reason for our poor historical reputation). Dr. Rohm and Mr. Haas then synthesized enzymes from an animal pancreas in 1907, and started their German chemical company. Enzymes are now used throughout our world in everything from laundry detergents to pharmaceuticals.
To be continued…
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.