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…