The Tanning Process
Since the dawn of time, leather has played a vital role in the development of civilization. In its various formats, leather and animal hides have provided for one of the most basic human needs - protection. From animal hides, man has crafted footwear, clothing, belts, containers, weapons, transportation devices, and even armor. In the early days of leather usage, unfortunately, animal skins would rot when left wet or in higher temperatures, and if they were allowed to dry or stored at lower temperatures, they would lose their ability to flex and have limited use. For this reason, attempts to produce leather that was stronger and more flexible eventually began. Whether by accident or by trial and error, man discovered methods of softening and preserving leather by treating animal skins with natural resources. Variations of this process, known as tanning, have been used to produce better quality material throughout history.
While historic methods of softening and preserving leather utilize such naturally occurring materials as smoke, animal viscera, and vegetable solutions, modern day tanning and hide processing is chemically controlled throughout the United States and Europe, regardless of the type of chemicals or ingredients used in the tanning process.
Tanning is the process of combining a substance called Tannin with the hide fibers, allowing the resulting leather to be dried, yet remain flexible. The chemistry of tanning is very complex, and the process was actually discovered by accident and refined through trial and error over thousands of years.
There are two main types of tanning - vegetable tanning and chrome tanning. Though Vegetable Tanning is perhaps the oldest method of tanning, using tannins from organic matter such as tree bark, leaves, and nuts, it is now a specialty type of tanning. In years past, it could take up to a year for each individual hide to be processed, but modern vegetable tanning takes between one and three months. Chrome Tanning uses metal chromium rather than organic tannins and, as a result, it takes far less time. It is also less expensive to produce leather in this way.
The tanning process begins when the hide is removed from the animal, as it is salted as soon as possible. It must remain salted, cool, and moist during the initial curing period, which can last up to six months. During this curing period, approximately 15% of the weight is lost as water drains from the hide. At the tannery, the hide is tumbled in a solution of hydrated lime and water to remove the hair. Next, the hide is tumbled in an acid solution of approximately 5 on the pH scale.
After tanning, some leathers are tumbled in a dye bath to produce a desired color. If not, the hide is taken to a drying room, a ventilated area where the hide is stretched, tacked to panels and hung until dry. Once the hide has dried, it is run through a Skiving Machine, which trims off bumps and blemishes in an effort to achieve the maximum uniform thickness throughout the hide. The final step in the leather-making process is referred to as currying or Slicking, and it is done to create a smooth finish.