The Leather Process
The process of hides and skins into leather is a fascinating procedure that requires a precise combination of many chemical and mechanical operations. Below you will find cutting instructions for your leather, along with a step-by-step summary of these processing operations as provided by our suppliers.
Follow this step-by-step process to see how tanneries convert a natural resource into leather hides.
Deterioration begins immediately when a cow is killed. After the hides are removed from the carcass, they are salted through and through at the slaughterhouses to prevent decay. After they are salted, 55% of the water in the hide is removed, and the hides are dried for 3 to 6 days. The rawhides are then sold to tanneries.
In order for the tanning process to work properly, the dry-salted hides must be washed free of the salt. This is done by soaking the hides in water to which chemical wetting agents (similar to household detergents) and disinfectants are usually added for 8 to 20 hours, depending on the thickness of the hides. This soaking procedure rehydrates the hides to their original flaccid condition and removes the dirt.
The hair must now be removed from the hides. This is done by soaking the hides in chemicals (depilatory agents), which (a) destroy the hair by attacking the hair root so it will release freely from the hides, (b) loosen the epidermis and (c) remove certain soluble skin proteins that lie within the hide substance without destroying the desirable collagen of the hides.
Excess flesh, fat and muscle must now be removed from the hides. This is done with a fleshing machine.
All of the depilatory chemicals must now be removed from the hides. This is done by washing the hides in ammonium sulfate or ammonium chloride and then clear water in big drums. These chemicals not only clean the depilatory chemicals from the hides, they also adjust the acid-alkaline conditions (pH) to the proper point for receiving the bate — enzymes similar to those found in the digestive system of animals. When bates are applied, they attack and destroy most of the remaining
The hides must be placed in an acid environment (low pH) so they will be ready to accept the tanning materials, because chrome-tanning agents are not soluble under alkaline conditions. This is accomplished by adding salt and acid to the hides. This is a preserving process in itself, and hides can be kept in this state for extended periods of time without any deterioration.
The raw collagen fibers of the hides must be converted into a stable product that is no longer susceptible to rotting. This is done by adding chrome-tanning agents to the hides in a revolving drum. These tanning agents also significantly improve the hide’s dimensional stability, abrasion resistance, resistance to chemicals and to heat, ability to flex innumerable times without breaking, and ability to endure repeated cycles of wetting and drying.
The excess moisture must be removed from the hides. This is done by placing each hide through two large rollers similar to those on a clothes wringer.
The hides must now be split into the desired thickness. Unsplit hides average 5 mm thick. The thickness for upholstery leathers ranges from 0.9 mm to 2.0 mm. The hides are put through a splitting machine that is set to split the hides to the desired thickness. The machine cuts the top-grain off first. Another layer and sometimes two are cut. These layers are called splits.
The thickness of the hides must be made uniform all over the hide. This is done with a shaving machine through which the hides are run. The helical-shaped cutting blades level the overall thickness to exact specifications and open the fiber structure to better receive subsequent chemical processing.
This process is done to impart special end-use properties with other tanning chemicals. The substances used add solidity and body to chrome leather and help minimize variations in the character of the leather that may still exist between different parts of the hide.
As soon as the re-tanning process is completed, aniline dyes, derived primarily from petroleum and added to very hot water, are added to rotating drums to penetrate the hides for desired color.
This is the last of the wet-chemical operations to which the leather will be subjected. Fat-liquoring has the most pronounced effect on how soft a leather will be and it contributes greatly to its tensile strength. The more fat-liquors that are added, the softer the hides will be.
14. SETTING OUT
This operation smoothes and stretches the hide, while compressing and squeezing out the excess moisture. This puts hides in the proper condition for drying.
The hides are stretched across a perforated frame and held in place with clips called toggles. One hide is clipped to each side of the frame. The frames are then slid into channels in drying ovens.
Leather is staked to make it pliable. In combination with the correct fat-liquoring treatment, staking governs the final firmness or softness of the leather.
17. DRY MILLING
The hides are placed in a large dry drum and tumbled until the desired softness is obtained.
This process improves the final appearance of the hides by lightly sanding the surface to remove some of the natural imperfections such as scratches, healed scars, etc. It provides the hide with better cutting yield.
This process applies film-forming materials on the surface of the hide. Here is where layers of pigments are added, if required. This process also adds the protective sealant to the surface.
20. PLATING (EMBOSSING)
This is the final step in the leather process. During this process, heat presses a chosen grain into the surface of the hides.
Leather is a natural product. Follow these guidelines to ensure your leather products remain beautiful:
FOR SPOTS AND SPILLS
Wipe up excess liquids with a soft cloth or sponge. If a spot needs special attention, a damp cloth or sponge soaked in lukewarm, mild soap water can be used. Dry with a woolen cloth and wait until completely dry before using.
DON’T USE HARSH PRODUCTS
NEVER use aggressive substances such as stain removers, cleaning solvents, turpentine, shoe polish, saddle soap, furniture polish, oils, varnish, abrasive cleaners, detergents, ammonia water, or any other type of chemical solution on your leather.
PROTECT YOUR LEATHER
Protect leather with a conditioner designed specifically for the type of leather you select. Because leather is a natural product, it will be affected by direct sunlight and will need periodic conditioning to protect it from drying out. Expect changes, since the appearance of leather will alter as it ages by softening, absorbing body oils, etc., which will give it a high-low look and add to the overall beauty of the hide.
Leather is a natural product, and like anything natural it is not always perfect. It is this natural imperfection that attracts the leather aficionado. Leather will have markings that are considered “nature’s signatures,” and this is what relates to the public that the product is genuine leather.
Leather will have several different types of these natural markings:
These are the result of the cattle being bitten by various types of insects. These bites result in tiny scars.
These are the result of the cattle being scratched in one way or another.
Cattle like to scratch their necks on the rails of fencing. This scratching can result in a neck burn if the cattle scratch too hard or very often.
Cattle sometimes get manure on their hides. Since this is a very irritating substance, it can result in a burn on the hide if the cattle cannot remove the manure soon enough.
Holes are usually man-made. They are the result of removing the skin to gain access to the meat.
BARBED-WIRE OR OPEN-WOUND HEALED SCARS
Sometimes the cattle get their hides caught in barbed wire. This results in a tear in the hide. These healed scars, however, are just as durable as the parts of the hide that aren’t scarred.
Types of Leather
Use this handy glossary to help you identify the types of leathers that best suit your leather needs.
FULL-GRAIN (VERSUS PLATED, EMBOSSED, BUFFED OR CORRECTED LEATHER)
This is the term for the leather surface without an attempt to hide or conceal the natural markings of the animal which it incurred during its lifetime. Life leaves its traces on cattle in the forms of healed scratches, insect bites, blood veins, growth wrinkles, and variation of grain, all of which constitute the character and charm of “Nature’s Signatures.” Full-grain leather can be either pure aniline or semi-aniline (or aniline-plus). It is sometimes plated to produce a special effect or even to enhance an already natural grain, such as a textured appeal versus a smooth appeal.
PLATED, EMBOSSED, BUFFED OR CORRECTED (VERSUS FULL-GRAIN LEATHER)
When full-grain leather has too many of “Nature’s Signatures,” it is buffed or sanded much in the same manner as wood is sanded. The sanding or correcting process levels the high spots of healed scratches, etc., and removes some of the natural grain.
A grain pattern is then embossed into the surface to replace what was lost in the sanding process.
Plated leather is usually aniline-plus (or semi-aniline).
TOP-GRAIN (VERSUS SPLIT LEATHER)
The thickness of all hides before tanning can vary quite a bit. To obtain a uniform thickness for upholstery, the hides are fed through a splitting machine with the grain side up, yielding a grain portion called “top-grain.”
The underneath or flesh layer that is cut off is called a “split.”
SPLIT (VERSUS TOP-GRAIN LEATHER)
This is the flesh on the top and bottom and possesses no natural grain.
The tissue structure is not as strong as top-grain and is generally not recommended for the seating area of upholstery, due to this lack of tissue strength.
FULL OR PURE ANILINE (VERSUS SEMI-ANILINE OR ANILINE-PLUS LEATHER)
It is generally accepted that only five percent of the world’s hide supply is of high enough quality for pure aniline upholstery furniture.
Aniline dyeing is the process of soaking the skins in transparent aniline dyes, which color or shade the skins without obscuring the natural markings or grain character of the leather.
The hides are dyed all the way through, with no pigmented topcoat.
SEMI-ANILINE OR ANILINE-PLUS (VERSUS FULL OR PURE ANILINE LEATHER)
These are terms that define a supplemental step beyond aniline dyeing, which adds a top-coating of highly dispersed pigments and dyes to the surface of aniline-dyed hides.
By dyeing the leather throughout before the final surface coating, a very even coloration can be achieved with only a thin layer of finish and the leather remains softer.