Tanning is the process that converts raw animal hides into a structure that retains the properties of the living skin--it is soft and flexible as well as tough and durable, and it allows air and water vapor to pass through it--yet it is chemically stable so it will not putrefy.
Living skin consists of three layers: the epidermis--the dry, thin, outer shield; the dermis--the thick, tough, region laced with nerves and capillaries; and the hypodermis--the deepest section, filled with fat and connective tissue. About 90 percent of the elastic connective tissue in the dermis and hypodermis is a protein called collagen. Collagen molecules are long, chainlike structures that are both highly flexible, to allow the skin to bend in all directions, and difficult to break, to retain the skin's shape and protect against laceration.
After the animal's death, the collagen tends to shrivel and adhere to itself, turning the skin into a dried out, inflexible shell. Tanning alters the chemical structure of the collagen so that the fibers are permanently fixed in an open lattice structure that also retains a degree of flexibility. This is typically done in a series of eight steps:
Step 1: Unhairing. The raw skins are first soaked in an alkali solution, which breaks a chemical bond in the keratin--the chief protein in the hair shafts--while leaving the collagen unharmed. The hairs can them be easily removed from the skins.
Step 2: Liming. Then the skins are placed in a lime solution, which alters the chemistry of the collagen. The fibers separate from each other and "swell" into a more open structure, which exposes the fats, globular proteins and other substances the collagen had been protecting in the living skin. Any remaining hair is removed in this step.
Step 3: Deliming and bating. Bating is a form of enzyme digestion that further opens the collagen structure and removes unwanted fats and other substances. The solution starts out highly basic--pH 9 to 10--but gradually decreases as the process goes on. Eventually, the "swelling" of the collagen structure also diminishes somewhat.
Step 4: Pickling. An acid bath brings the pH down to 3 to 3.5 to make the hides receptive to the tanning solutions.
Step 5: Tanning. The hides are then soaked in a tanning solution, during which a number of chemical substances penetrate the collagen and form new bonds with the proteins, "bridging" them together to stabilize the open structure and turn the skin into leather. Different types of tanning solution can be used, each of which imparts its own character into the final chemical structure of the leather. The two general types of tanned leather you'll encounter most commonly are:
- vegetable based, which are derived from highly tannic plant materials such as oak or hemlock bark. These yield a stiff, hardened structure used in shoe soles and belting as well as equestrian tack.
- chrome based, which uses a solution of chromium salts. These leathers are soft and stretchable and are used in items such as clothing.
These two types of solution can also be used in combination, along with synthetic tanning solutions, to produce products with varying characteristics of both.
Step 6: Neutralizing, dyeing and fat liquoring. Then the leather is removed from the acid and its pH raised to neutral via a mild alkaloid solution, and the material can be dyed. Chromium is especially receptive to forming chemical bonds with various dyes, which enables these types of leather to be produced in a wide range of bright colors. Also at this stage, the leather is soaked in a type of oil that attaches readily to the collagen structure to lubricate the fibers, allowing them to move readily against each other for suppleness in the final product.
Step 7: Drying. Finally, the leather is removed from its last solution and the water is allowed to evaporate out slowly, leaving behind a stable chemical matrix.
Step 8: Finishing. At this stage, the surface may be dyed, coated with and/or treated in any number of ways to create its final color and texture--everything from suedes, which are buffed and abraded to create nap, to shiny patent leathers, which receive a polyurethane or acrylic surface.
Source: "The Chemistry of the Leather Industry," The New Zealand Institute of Chemistry; www.nzic.org.nz/ChemProcesses/animal/5C.pdf.
To read more about leather care and conditioning, see "Guilt-Free Leather Care" in the April 2007 issue of EQUUS magazine.