Vegetable tanning is a traditional method of tanning leather using tannins extracted from plants, such as quebracho, chestnut, or mimosa trees. This process dates back to roughly 6000 BCE, when early civilizations sought a way to make animal skins resistant to weather and decay. Vegetable tanning is considered a higher quality method of leather production, as it results in leather that is more durable and develops a unique patina over time. It is also more sustainable, as it does not rely on synthetic chemicals like chrome or mineral tanning. However, vegetable tanning is a more time-consuming process, taking 28 times longer to produce than synthetic methods. As a result, vegetable-tanned leather makes up only about 10% of all leather available today, and can cost around three times more than synthetic leather.
The process of vegetable tanning begins with the selection of raw hides or skins, which are inspected for quality and defects and any necessary repairs are made. The hides are then soaked in water to soften them and remove any dirt or debris. This process, known as "liming," helps to loosen the hair and epidermis (outer layer of skin) from the underlying dermis (inner layer of skin). The hides are then scraped to remove the remaining hair and epidermis, a process known as "fleshing."
Next, the hides are soaked in a solution of water and sodium sulfide or sodium hydrosulfide to remove any remaining fat and protein. This process, known as "bating," helps to improve the absorbency of the leather and prepare it for tanning. The hides are then rinsed and soaked in a solution of water and an acid, such as citric acid or sulfuric acid, to lower the pH and help to preserve the leather. This process, known as "pickling," helps to prevent the growth of bacteria and other microorganisms during the tanning process.
Finally, the hides are soaked in a solution of water and a tanning agent, such as tannins extracted from oak, mimosa, chestnut, or quebracho trees, to begin the tanning process. The tanning agent reacts with the collagen fibers in the leather, creating a stable and flexible material that is resistant to deterioration.
After the tanning process is complete, the hides are dried in a special environment-controlled room where the humidity and temperature must be just right. If any hide is overdried, it must be taken back to the drums and reconditioned. The finished leather is then inspected, and depending on the final product, the leather can be retanned, dyed, or greased to adjust the elasticity, softness, or color. This process is known as fatliquoring.
Vegetable-tanned leather is known for its two defining qualities: it changes color in the light and develops a rich patina over time, and it becomes more supple with age. These qualities are made possible by the natural properties of the leather, as vegetable tanning leaves the hides in an organic state that allows them to age like skin. Synthetically treated leather, on the other hand, is designed to be uniform in color and consistency and does not develop a patina over time.
In conclusion, vegetable tanning is a traditional and sustainable method of leather production that results in high-quality leather with unique characteristics and a long lifespan. However, it is a more time-consuming and costly process compared to synthetic methods, which is why it makes up a small percentage of all leather available today.