Although the process of turning hide into leather varies with each tanner, some steps are quite similar. First, the hides are cured by immersing them in a vat of salt solution which prevents the hides from deteriorating and preserves them until the tanning process can begin. In this way, a quantity of hides can be stored prior to tanning.
Next comes soaking and cleaning to remove the salt and a lime solution is added to remove hair and excess flesh. After the hides have been cleaned, they are sorted according to the amount of holes and other defects they contain. Upholstery leather requires large pieces of relatively unblemished hide, whereas many leather products, such as shoes and handbags, can utilize smaller pieces of leather. Therefore, upholstery leather is generally the most expensive leather per square foot.
Once it has been determined that a hide is suitable for upholstery, it is split in a process similar to veneering a piece of wood. The outer surface of the animal's hide is separated from the lower layers by slicing it with a long knife as the hide is fed through a series of rollers. This outer portion of the animal's skin is referred to as the "top grain" and is usually 3/64 inches think (about the thickness of the edge of a coin). Quality manufacturers use only top grain leathers because they are by far the strongest and most supple part of the hide and, therefore, best suited for upholstery. The layers of leather underneath the top grain are referred to as "split hides." These are generally thicker and stiffer than the top grain and are often used as less expensive upholstery leathers.
After the top grain leather has been removed it is bathed in a chemical solution which stabilizes the hide and changes it chemical structure so that it will not deteriorate. This is the step in the tanning process that has been so improved in recent years. The tanning procedures used now are permanent and create a product which will last indefinitely. Previous methods employed vegetable derived tanning agents and were not as effective in preserving the leather.
If the leather is to be dyed, it is usually done in a vat as part of the tanning process. Aniline dyes in various colors may be added to the drums at this stage, while the hide is absorbent. If correctly done, aniline dyes are permanent and will not rub off or "crock" after the leather is in use. Dyeing should not be confused with pigmentation of adding color to the surface of the leather. Aniline dyeing lets the color permeate the hide so that it goes all the way through the leather (i.e: the color on the back of the hide is the same as the color on the front).
After the hides have been removed from the tanning vats, the backside of the hide is shaved by passing it through a long, cylindrical blade. This assures an even thickness throughout the hide.
To counteract the drying effects of the previous chemical treatments, the hides are then conditioned to replace natural oils and make the leather more supple. This is an important step in tanning leather because without these natural oils, the hides will not have the same strength and flexibility.
Next, moisture is removed from the hides by a number of methods including vacuum drying or air drying. During the drying process, the hide is stretched to remove wrinkles and create an even surface. Hides will not stretch and return to their original shape if they are not properly pre-stretched during drying. Once the leather has been stretched and dried, it is returned to a soft, supple state by tumbling in a large drum and/or working by hand.
After drying, leathers which have excessive scarring may be sanded to a smooth surface. The leather can then be finished or it may be embossed with an artificial grain. Embossing enables some manufacturers to use less expensive hides. However, the buffing process removes the outer portion (top grain) of the hide which has the most strength and embossing tends to compress the leather and make it stiffer. Full grain leather, meaning that nothing has been done to alter the natural surface of the hide, is preferable. Leather in its natural state is more durable, softer and contains the natural markings which prove it is full grain leather. Do not confuse the terms "full grain" with "top grain." Top grain means the hide is the outer skin of the animal. Full grain leathers are top grain leathers which have a natural surface. They have not been buffed or embossed. Full grain leather is considered to be the premier grade in upholstery leather.
Once leathers have been properly tanned and dried, the color can be imparted to leather by applying a pigmented surface coating or by dyeing the entire hide in a vat soaking process. Many leathers are both dyed and then finished on the surface to achieve the benefits of both processes. Some of the most common methods of finishing leather are:
- Machine finishing — An even coloration is achieved by a series of finish coats applied by a computerized spraying machine. When the desired color has been achieved, a clear protective coating is applied to resist moisture and stains. The sheen of this coating can be adjusted to give a high gloss, satin or matte surface. Machine finishing is one of the least expensive and most common methods used to finish upholstery leather. It provides as durable and lasting a finish as any finishing method available.
- Hand antiqued leather — This process is the same as machine finishing except the final color application is rubbed into the leather by hand. This technique is used to bring out the natural markings in the hide, such as healed scars and barbed wire nicks, and gives the leather a highlighting which can't be achieved by machine spraying. Hand antiqued leathers are generally more expensive than machine sprayed.
- Full aniline dyed leather -- These are leathers which have been aniline dyed in a vat process with no color coating added to the surface. Full aniline dyed leathers are prized for their soft, natural feel. They are the most expensive leathers to produce because of the difficulty in finding the superior quality of hides required to produce the full aniline leather. Full aniline dyed leathers may also be more susceptible to absorbing liquids because of the natural porosity of the hide. But because they don't have a top coat, the leather breathes more easily and is cooler to sit on. European leathers are generally not top-coated.
- Semi-aniline dyed leather — Leathers which have been both dyed throughout and have a finish on the surface are referred to as semi-aniline dyed. This type of leather has become more popular in recent years because it incorporates much of the softness and feel of fully aniline dyed leather with the protective benefits of surface finishing. Semi-aniline dyed leathers can be compared to a painted surface which has been well primed before the finish coating is applied. By dyeing the leather to a base coloration before the final coating is applied, a very even coloration is produced with only a thin layer of finish. Thus, the leather remains softer because it is not necessary to apply a thick finish coating. Following the color coating, a clear protecting finish is applied as on the machine finished leathers. Semi-aniline dyed leathers are available in a wide range of colors because they are not limited to the shades achievable by dyeing alone. Semi-aniline dyed leathers are also desirable because small nicks and scratches are hidden by the base coloration of the hide.