Animal hides are essentially a by-product of a much larger meat and dairy industry. Stripped from the flesh of the animal at the slaughterhouses or abattoir, these rawhides must be conserved, typically within two hours of slaughter, to prevent rotting or putrefaction. Traditionally, this was done by hanging the rawhides out to dry, but today they may also be salted or frozen.
Rawhide dealers purchase these hides, classifying and grading them, before selling them on to the tanneries which are only looking for the highest quality hides, free from creases, marks, scratches, bites, and wrinkles.
Once the hides reach the tanneries, they must be prepared for tanning. This pre-tanning process is known as “beamhouse operations” and involves several steps:
- Soaking. Hides are soaked in clean water to remove salt from curing and rehydrate the hide. Biocides are added to the water to prevent bacterial damage to the skin.
- Liming. Hides are placed in an alkali solution made of lime and sulfur compounds which removes hair from the hides.
- Fleshing. The soaked hides are run through a machine to remove any remnants of flesh still attached to the skin.
- Splitting. The leather hides are split with the more valuable top grain side to be used in high-end products and the bottom split used for suede or split leather.
- Deliming. This process completely removes the lime and neutralizes any residual alkalinity.
- Pickling. Hides are treated with enzymes to break down the proteins and make the hides more receptive to the tanning process. The duration of the pickling has a significant impact on the firmness or softness of the final product.
The hides are they delimed to bring down the pH level and get them ready for tanning: an irreversible process which turns the raw hide into the product we know as leather. How long this takes varies widely depending on the method chosen.
Today, about 90% of leather is chrome tanned, a method developed in the mid-19th century which uses aluminum salts and, unlike vegetable tanning which can take weeks or months, can be completed in a day. This process gives the leather a blue tinge (known as “wet blue”).
Vegetable tanning is a much longer process which involves moving the leather between drums filled with increasingly higher concentrations of solution containing natural tannins. These substances are what give vegetable tanned leather its unique and distinct scent.
Once the leather has been tanned, additional chemical and physical treatments are used to give the leather its final desired characteristics. This may involve a combination of waxing, dying, polishing, buffing, and shaving the hides to ensure a uniform thickness. All of this, along with the tanning process itself, produces leather that is distinct to that particular tannery.